P0300 Mystery MISFIRE - Difficult to Diagnose! (Intermittent "FISHBITE")

Cylinder MISFIRES are generally straight-forward to diagnose...
But sometimes they're NOT!
This 2008 Pontiac G5 2.2L Ecotec is setting a P0300 - Random Misfire DTC at 244k miles.
It runs a bit rough at IDLE, with an OCCASIONAL "hiccup" on Cyl 4...not a constant misfire.
Under LOAD, everything is smooth and PERFECT!
Obviously it already has brand-new spark plugs and coils.
What else could it be? Injector? Vacuum leak? Bad valve?
I have to pull out ALL THE TRICKS in my diagnostic arsenal to give the customer a 100% conclusive diagnosis. Even improvise a cool exhaust TAILPIPE PRESSURE PULSE test along the way!
IVAN'S PICO WAVEFORMS:
drive.google.com/drive/folder...
PHAD PRESSURE TRANSDUCER:
www.pinehollowdiagnostics.com/...
OTC 3398 Fuel INJECTION Pulse Tester:
www.amazon.com/dp/B0002SR5OO?...
TESLONG NTS500 DUAL CAMERA BORESCOPE:
www.amazon.com/dp/B07SQ4FLYX?...
THINKTOOL PROS: *****TOTAL $150 OFF!!***
www.amazon.com/dp/B08SWH2KP4?...
Enjoy!
Ivan

[@brianelliott6148]

Interesting lil car right lol😂 it was just a lil above my technology forgot to tell ya it had a timing chain on it recently

[@brianelliott6148]

I was thinking it was like timed at the wrong TDC or something

[@PineHollowAutoDiagnostics]

Still running the same?

[@brianelliott6148]

Yeah she's still taking it everyday to work lol and you were right manual gauge was unable catch it

[@shakalpb1164]

I got a Saab 9-3 with a B207 Engine. It looks exactly like your engine. I did the timing Chain myself, and used the WRONG Timing Tool to lock the cams. After that i had the Random Misfire code/random missfires!!

[@wendwllhickey6426]

Could be carbon on the valves spray in tb maf cleaner use whole can it might work😊

[@donerickson1954]

Worn out valves. I had the same issue years ago. I owned a Ford escort that would misfire and not pass smog testing. I had no money to buy parts other than a couple of gaskets, so I bought the head gaskets and proceeded to remove the head myself. I found the valve sealing surfaces were eroded and wouldn't seal effectively. So I did what anyone without cash would do. I got some fine sand, mixed it with grease and oil to make a paste of lapping compound. right on my kitchen table of my apartment I used a drill to "lap" the seats until they sealed. Fwd and rev over and over on all 8 valves. When I put it all back together the engine ran perfectly, no more miss. A true not many parts required fix and the thing passed smog for 6 more years after that.

[@PineHollowAutoDiagnostics]

Amazing! 😎

[@mattcat231]

im going with a weak or broken valve spring, as i had this same exact issue with my son's mother's 08 Cobalt, except it was #1 cylinder. Went after it with basically the same diagnostic tree as you did, then loaded the whole arsenal in the parts cannon, and out of a stroke of luck, her valve cover started leaking, so i pulled it off and found a broken exhaust valve spring. it was only the bottom round of the spring that was broke off, so im guessing that when the valve rotated, it would hit that sweet spot and cause a little leak.

[@PineHollowAutoDiagnostics]

Wow that's crazy! That just might be the problem here!

[@ronaldderooij1774]

Never in my life, I would have thought of that. But it seems so logical.

[@randyr.parker2698]

Yep, I would go with that. Seems like the next logical move. HAS to be some kind of compression issue.

[@ohboy2118]

I personally wouldn't recommend the customer just drove it till it gets worse if you even slightly suspect a broken spring. That is playing Russian roulette with an engine if that valve drops. Any chance you can get the bore scope in the oil filler opening and manipulate it to view the valve springs if you don't want to pull the rocker cover? Bit of a moot point given the last images from in cylinder though because it definitely needs a valve job.

[@carmo9693]

@@PineHollowAutoDiagnostics Does this car have hydraulic lifters?

[@ThunderbirdRocket]

I’m watching this one again and again ! It’s a fascinating case study . You were incredibly efficient & thorough !! Very smart input from your viewers / community too. Learning lots - thanks mate 👊🏼 🔥 !!

[@lenomalaak]

My Ford Focus 2004 got this exact intermittent idle-missing issue. Several mechanics failed to find & resolve the problem, but billed me big amounts for their work & spare-parts change. Finally, one Indian mechanic resolved it easily, finding the issue within seconds of his examination. Some small airflow pipes were damaged and were sucking air in.

[@Charlie1012000]

The pivot point is when you say "I don't know what my next check is" The difference is you persist finding other ways, in difference to many other mechanics.

[@05c50]

Back in the day of the early Ford 302/351 Windsor engine, before check engine lights, before electronic ignition, before PC based diagnostics, intermittant misfires at idle were not uncommon. "Drive it until it gets worse" was quite often the suggestion. "Worse" usually coincided with a tapping noise and a tear down would reveal an excessively worn valve guide. After a few of these, you started to recognize the problem early on. Great video, I'll be interested if this one is ever fully repaired.

[@misterhipster9509]

I'm from back in the day, you treed me on valve guide.

[@dharley189]

Yep. I got in this career in 72 when only electronic component was the radio ‼️

[@misterhipster9509]

@@dharley189 Age thing, I'm 75.

[@gfbemc0256]

@@dharley189 1968 Volkswagen had electronic fuel injection on the Type 3.

[@dharley189]

@@gfbemc0256 Yep. I was 17. Manuals like today hard to come by. Not many in domestic repair understood it.

[@turboponies]

Treasure trove of information. Have a similarly aged GM engine with similar behavior, just not as far along the way in mileage. Had been a head scratcher for a while. Live and learn. Thanks, Ivan

[@ThunderbirdRocket]

Fascinating journey !! Learning tons from you and your incredibly talented viewers !!

[@dsch6367]

Leakdown test will show leaking valves. I'll bet it's low on compression and the leakdown will be above 50%. Ivan, you need to modify your electric transducers so that compressed air can be supplied for an actual leakdown test.

[@PineHollowAutoDiagnostics]

That's one test that might definitely show something interesting 👍

[@reneneron2971]

I would also do a leakdown. You could also check for a leaking fuel injector seal; they dry out with age, and a small leak at an injector seal will have a problem at idle due to maximum vacuum and smallest amount of fuel being added, That said, I suspect the lower compression on cylinder four is just enough to cause it to misfire. Is valve lash adjustable on this car?

[@MY-ml5rp]

Speaking of old school and the leak down test, not one vacuum gauge was hooked up for another test, a lot of knowledge can be found in the bouncing needle. Remember the one on the Sun 1120 (1st scope I ever used), it was like 10 inches in diameter. It was just yesterday, 49 years ago.

[@Garth2011]

@@MY-ml5rp Love those old and BIG gauges that show some activity when its there.

[@scrappy7571]

@@PineHollowAutoDiagnostics The pressure used for the leakdown should be around 50 psi. If 100 psi or more is used, it can seat the valve and make it it seal. We found this out the hard way on the first 2.2 we had with bad guides. We replaced many heads under warranty for this exact problem.

[@kobaltseven]

After owning many GMs, the last of which was an 07 Grand Prix supercharged, I've come to terms that fishbite missfires are somewhat part of their designs. Although not as pronounced, more like a cough here and there.

[@howiej4084]

On a hunch, I would swap the no 4 coil and plug with a different cylinder to try and move the misfire to that cylinder. It would only take a minute, and I have seen new coils that were weak. Thanks for the entertaining videos.

[@hypocrisyrules]

wouldn't that throw a "coil code"?

[@howiej4084]

@@hypocrisyrules Not really.

[@jgeorges3061]

Ivan, u say u appreciate who is watching and I say appreciate ur time and effort to produce such an AWESOME information can not thank you enough for that u rock in my book always thanks for what u do really. cheeeeers from Ontario, CANADA

[@tam1381]

Ivan had a very very similar problem with VW 3 cylinder cars. Compressions all within 10% of each other, cyl misfire count up and eventually the ecu shuts the injector off to protect the Cat. Problem caused by oval valves !! .The valve guides were badly worn causing poor seating of the valves. Guides changed new vales fitted , compression still within 10% of each other but all reading about 10psi higher than before. Now no misfire and cured. Apparently very common on Vw, Skoda and Seat. That was 3 years ago and still no misfires since repair.

[@PineHollowAutoDiagnostics]

Wow never heard of oval valves before!! Must be a fancy Euro feature 😉

[@tam1381]

@@PineHollowAutoDiagnostics Hey it's a VW.

[@wadet73]

This one certainly is a mystery. I enjoyed watching your diag process 👍🏼

[@shawningler]

Had a few of those do that with crappy valve seats. More common was the early Colorado/canyon. Gm had us put stiffer valve springs in thinking that would make them seal. They later campaigned the complete heads! Did many!!! If you get the saab 9-3 ecotech turbo they have crap valves too but show up very low comp. Cold! I enjoy all the scope stuff. Thanks!!!

[@kastooMcFry]

Intresting problem. I have your pressure transducers. They are great at finding problems without having to tear things apart. I have never seen an intermittently leaking/not opening/closing valve but your explanation of the valve rotating and hitting a bad spot makes perfect sense. Thanks, Ivan!

[@mkilptrick]

That tailpipe gadget was a surprise for me. Always learning something new here. Thanks Ivan. I come here for sanity yet some of the cars you work on have insane problems.

[@robertheim352]

Great collection of data and good hypothesis of the culprit. No obvious flaw does track for such a subtle symptom, but you did the homework.

[@boilertech44]

I have this same situation going on with my 2.4L 2011 Equinox. It has a rough idle, miss fires, and sometimes spits and sputters, but runs down the highway nicely. I've been waiting for this video for a while!

[@albutterfield5965]

Good morning from Texas

[@Zeus-wl2pl]

Good morning from Pennsylvania

[@thatdudeca]

Good morning from new brunswick Canada

[@TheBry_Guy]

g'mornin (well afternoon now), from Ontario, Canada.

[@emilpavlov9441]

I have seen something like this with a bad injector rubber seal to the cylinder head.And it shows on idle because the throttle is closed and there Is higher vacuum.A good test is to smoke test the in cylinder with closed valves to check the injector.All your other approaches were great.Love to watch your videos.

[@kennedysgarage3281]

I must say that you in depth test & methodical approach is excellent Ivan. Fantastic in-depth testing, it is the simplest jobs that can sometimes be the hardest to solve. It would be interesting if it was just a valve spring as said 🤞🤞🤞 Very cool video 💪💪

[@mikeaho4143]

Interesting case study Ivan. If that was in my shop I'd agree to tell the customer to keep running it until it gets worse. I also agree with that many miles it's not terrible looking on the bore scope. Thanks for sharing !

[@chrissmith7655]

Hi Ivan, as soon as you said valve is suspect, I thought he will use camera or do leak down, BUT not until the end. i was shouting CAM! LEAK DOWN! all the way until you finally did it, hahaha. Many thanks for all your uploads, from Nr Liverpool UK.

[@AP9311]

Wow, you covered every single bit of the diagnosis! But I've heard about cobalt have breakage on springs or the valve rotated not always hitting that sweet spot. At least it's more comprehensive diagnosis, great video Ivan!

[@bulygoat]

I put my vote in for the hydraulic lash adjusters. They get worn and spongy at low oil pressure at idle. You bump it just off idle, say 100 rpm and they clear up because the oil pressure comes up and allows the valves to open fully. This engine has the same lash adjusters as the GM 3.6 that has a couple bulletins out about them.

[@W.Khairi]

Excellent point

[@Kris-82]

yep, question is also if it's clean or clogged or if oil is the right one.

[@imwild1]

Good point. As the engine warms up the oil gets thinner and makes the oil pressure lower at idle

[@abdouallahuniverse5689]

It is like 99 percent, that the problem, they should try to change engine oil, and filter with the right one recommended for that car, and see if the problem resolved, because most mechanic shops put bad quality oil or some of them put one kind of oil for all cars, i saw that Myself, the mechanic put on my car 10w40, and my car recommend 5w30, when i told him what are you doing, he said your car is old now, and 5w30 for brand new cars, your car now need 10w40, they are crazy, they take opportunity that the customer has no knowledge about cars, that why I change car engine oil myself, best oil and best filter, Second issue could be hydraulic valve lash problem, and need to be changed.

[@garybowles9062]

One last check; a slow leak of water past the head gasket; open radiator cap to depressure the coolant; and see if missout continues. The weak valve spring suggestion, is a good one also.

[@RussellBooth1977]

It looks like a piece of carbon has stuck to the valve seats. When I did a small automotive industrial course (it was a prerequisite to becoming a diesel mechanic ,I did that because the automotive service course was full) at tech as a part of my higher school certificate one of my teachers spoke about how he cleaned the carbon off the exhaust valves in his old Ford Escort. It would've been loud but he used to tip water down the exhaust ports while revving the crap out of the engine to break up the carbon on the valve seats on the valves & in the cylinder head after the exhaust manifold was removed !

[@josephwash109]

I'd likely pull the valve cover for an inspection of the valve train. I've seen others comment about a broken valve spring, but it could also be a problem with one of the cam followers, such as a flat spot or damaged needle bearings. If an intake valve isn't fully opening, it partially starves the cylinder of the intake charge, and a misfire results.

[@gilbertsanchezz3107]

Would replacing the engine be a simpler fix for something like this? I’m having very similar issues with my Honda Accord j series.

[@hypocrisyrules]

scope the intake and exhaust valve thru the 4th spark plug hole. After that, do a cylinder leak down test with 30 PSI. If you hear leak from the throttle housing, it is an intake valve leak; If you hear leak from oil dip stick tube or. oil fill hole, it is a piston ring leak. If you hear a leak from the exhaust, then it is an exhaust valve leak. It does not matter.............car isn't worth persuing a fix regardless.

[@cowgirlscan2792]

I love it when you say the words Bernie Thompson ❤

[@Frank-pv1hp]

Hey Ivan, most GM's have an injector balance test under special functions. Makes it much faster to check than tying into each injector

[@EngineerLewis]

Thanks for persuing this misfire Ivan. I also have a misfire from 1 of my 6 cylinders and have come to the same conclusion - just drive it until it get a lot worse. The car is not worth more than $2500 so I don't want to drop lots of cash into fixing it. 👍

[@nasnema]

I so like the lack of abridging the thought process. Excellent walk through.

[@cullenmiller8170]

Nice video and diagnosis. That one is definitely a head scratcher.

[@donniejohnson7499]

Hello Ivan. Amazingl troubleshooting skills. Great video. Thanks !!

[@richardnilsen8950]

1. Leave everything hooked up so it runs 2.take valve cover off 3.build a dam out of cardboard around the top of the head so the oil dont go all over everything 4.start it 5.smoke the intake 6.watch the valves 7.look for smoke on Compression stroke

[@JPAutoService]

Look for smoke where?

[@richardnilsen8950]

If the valve doesent seat Or a spring is broken compression will smoke Out under the spring

[@kerrylewis2581]

I'm impressed by the condition of this engine with close to a quarter million miles. The previous owner must have properly maintained and most likely put a lot of highway miles on that little four-cylinder. It's not worth the cost of removing the head, but I sure would like to see how it would run after a valve job.

[@brianelliott6148]

No they didn't it sat for 3 years b4 I got it I got it it was a no crank no start and she didn't change the oil in it for a year

[@kerrylewis2581]

It sat for three years, but it still had over 240,000

[@briandetrick2688]

i have a 95 chevy pickup 400.000 miles work it every day

[@brianelliott6148]

Yeah kerrylewis I got it from my dad's wife

[@JohnDoesGarage]

Great information gathering. You did prove beyond a shadow of a doubt it was compression, and my school of thought is there is more than one contributing factor. Between worn rings, pitted valves and seats, it could be all cylinders are on the cusp of misfiring and it doesn't take much of a drop for a misfire to occur. As you said it could be when a larger pit in a valve aligns with a larger pit in the seat there is enough of a drop in compression to cause a misfire. Maybe at idle there is a valve that is hanging up a little that does not happen at higher RPM's. A leak down test that has already been suggested would be another test, but it would not change the recommendation. Bottom line is as you said, run it until it either becomes undrivable or until the salt completely devours the car, which is a distinct possibility where you live.

[@r.weaver3769]

I remember something like this, and it had the double valve spring, the outer spring was broken, I'm assuming rotation would seal sometimes, but that wouldn't account for running fine when cold or at speed.

[@hpelisr]

I would try a sea foam clean job, for the valves.

[@tschuuuls486]

Other issue it could be: Messed up spray pattern on the injector. Sometimes injectors flow fairly fine but they don't atomize the fuel well enough anymore. Air fuel mixture is not ideal in lean burn mode at idle.

[@W.Khairi]

Cool

[@dustcommander100]

My belief is that as a condition worsens (compression, mixture, etc) you reach a point where an undesired event like a miss will occur intermittently until the condition worsens to the point the undesired event becomes consistent. Interestingly enough, my 1947 Chevy does this same thing, and no amount of idle mixture adjustment makes it go away. It is original, with one carb and one coil for all 6 cylinders - but who-knows-how-many miles. I haven't invested much effort in diagnosing it, because it has so much piston blow-by. So I'm already doing as you suggest, and driving it.

[@keithanderson7007]

Ivan it might have been interesting to see the KV and sparkline for cylinder 4. I didn't see that in your library of waveforms. Max Vanderbrink's seminar indicated when there is a valve seating issue the KV would behave like an inchworm up and down in an inconsistent manner. A broken or weak spring would be more consistent. You might also have seen more hash in the sparkline when it misfired.

[@thomasdemarco617]

thinking ceramic valve seat moving or cracked, GM used to make top cylinder cleaner or perhaps seafoam to try to clean a slight hardened carbon deposit on one of those seats, air in that cylinder may help if you hear some type of leaking .loved your tenacity

[@brianw8963]

Really interesting case. Actually impressed with the mileage and decent condition, though it does appear that a valve job is due. The intermittent aspect is strange, especially when at one point 4 was not missing but 2 other cylinders were. Interesting! 👍👍🇺🇸🇺🇸

[@W.Khairi]

Probably it was cyl4 all the time, It could be false misfire data

[@frankvoynar6427]

Good morning all, woke up this morning and my brain was in overdrive after watching this vid last nite. Back in my analog day's found this almost exact issue. Short story, very high mileage, vacuum gauge diag, engine out cyl, heads off..... Very very worn valve guides. End of story

[@rogeraddy6846]

Possibility the low compression is right at the edge of causing a misfire. Normal variability in compression causing some of the cycles to be below the threshold. I would expect it to get worse over time as more cycles fall below the threshold. Always look forward to your videos as you are one of the most intelligent auto technicians I've seen on KZbin. Have a great day.

[@ednewell1847]

I had deja vu watching this. I ran into the same exact issue on that 2.2 engine in an HHR. Followed almost the identical diag process as yours and came out scratching my head. I suspected a valve sealing issue. The customer did not want to spend any more on the vehicle so I never did see it through.

[@Garth2011]

That one is pretty clean inside the cylinder for 200,000+ miles. I'd do a valve job on that one for sure since its an easy teardown and the rest of the car looks good enough to support the $$.

[@CodycoWeb]

We had lots of issues with carbon buildup on intake valves causing this. Good hot cleaning with GM Top engine clean and let it sit 1/2 hour in middle of cleaning helped them many times.

[@garymoore3159]

I have a 2002 Ford Explorer Sport Trac w/4.0 SOHC engine that does EXACTLY what this Pontiac engine is doing. Engine does not use oil. Only misses on idles. My mechanic has pulled out his last few hairs trying to sort this out. Thank you very much for your tenacity to follow this rabbit down the hole!

[@hypocrisyrules]

do a smoke test and /or cylinder leak down test.............

[@thomasmartin2950]

20+ years ago Honda, (and others), had carbon build up problems on the valves. I believe there was a TSB stating a crank no start condition related to valve sealing/carbon issues. I encountered one, and fed it plenty of carb cleaner while cranking cranking. I finally got it started, blew it out, and apparently solved the problem. A BG, or similar injector/valve cleaner, or GM TOP ENGINE cleaner is definitely cheap enough, and worth a shot. THAT, would be my next step.

[@atikovi1]

Those coils look like $10 Ebay specials. They may not be THE cause of the problem but may contribute to it, especially if the customer said it was running fine 2 weeks ago. Dealing with an issue on a 2004 Saturn Ion with the same engine and 209K that may occasionally run rough and miss just on the first cold start of the day for 10-20 seconds. Was watching your 7 year old video, P0300 Misfire case study: 09 Chevy Cobalt, for ideas.

[@mikesabin8568]

Well they 100% are not the cause of the problem. The problem came about before they started putting parts on the car. This was said at the beginning of the video

[@frankvoynar6427]

Oh an w by the way, thank you Ivan. IMO UR KING OF THE HILL ALWAYS SO INTERESTING AND FOOD FOR THOUGHT. THANKX FRANKO

[@pantherplatform]

I'm about to take a road trip to look at a rare 1990 Lincoln Town Car that has the 5.0 but it's the first year of the new body style that got the 4.6 in 91 before the crown vic and grand marquis got it in 92. It's the first year of a new body style but everything else is what they had left on the shelf.

[@noelcastle3986]

Interesting case study ,im going with broken valve spring and maybe intermittant valve sticking due to poor valve spring tension maybe some gumming in the valve guide. Shame we will probably never know but I agree its way past money pit time for that car ,drooping head linings , transmission issues , ac issues suspension issues its not using much oil but oil leaks etc coming up . Drive it till it dies then off to the junk yard.

[@gmtech2012]

I would have loved to see a good secondary ignition pattern of this misfire. This is one of the best opportunities to learn from.

[@halvorsonmx]

Great job as always. I was definitely following you there because I know that when I worked for a Chrysler dealership back in the day we would replace cylinder heads for an intermittent valve sealing issue causing a misfire. Anyway, out of curiosity and I'd be curious as to what you think. But if you had set up a secondary ignition probe on that cylinder, I would guess that when it misfired, if there was a cylinder leakage issue it would cause the burn line to go crazy because of all of the turbulence.

[@frankoattes]

I’ve had this before with the valve guides causing the valve to stick when the car got warm only on idle. It was a known problem with Fords ztec engines in the 2000 ‘s over here in the uk. They were replacing all the valve guides and valves under warranty. It was a pain trying to diagnose this without the tools that you have at your place, like the pico. It was guess work and a lot of unpaid hours diagnosing. In the end up we took a head off and sent it to the engineering shop who found the guides to be out of tolerance. Once the guides were changed out and new valves put in the issue never came back. It’s trying to explain this to a customer that it’s only a guess at this point until it’s stripped and confirmed, and if it’s not the valve it’s more diagnosis. I hope you get an end result, I would love to see a part 2!

[@theCMW100]

Try sticking exhaust valve guide in 1 and 4. This was my experience with at 2010 Cobalt. After going crazy, I found that there was a quiet recall on the cylinder head of that engine, only up to 100,00 miles. my engine was already 125,000. I pulled the head and sent it to a machine shop. The engine ran trouble free for the next 75,000 mile. after that the car went to the great junk yard in the sky.

[@petar443]

I would try injector swaptronics. And spark plug/coil swaptronics too,even if they are new. Also scope the crankcase pressure and cooling system with the transducer. Also would be great to see the ignition waveform at the moment of misfire. I doubt its broken valve spring bottom coil. We saw misfires on other 2 cylinders. This head needs service due to millage. In nowdays where scopes and meters are the first choice for diagnostics ,i would do leakdown test also. That could have pinpointed the problem.😊

[@jamesbruno5896]

Great Diag Ivan!

[@roxanneabbott8424]

I'm with you Ivan, a real head scratcher!!

[@ronnorman1367]

You're going to pay $1,500 to $2,000 to rip apart that engine to do maybe just a lap on a valve but you're the honest type you're going to replace everything most guys would just slap a valve and send her running

[@davidgrisco1939]

I had a similar misfire on GM 4.2l inline 6 with about 280k miles. I tried checking for intake manifold leak with a spray and found nothing. Since the valve cover gasket was leaking oil, i decided to replace the cover and intake gaskets. Turned out the intake gasket WAS leaking and the misfire went away. Ran smooth as new. I'm guessing that Pontiac has a lifter and/or valve spring issue.

[@truracer20]

@18:07 I would look at crank and cam signals as well as the cylinder 4 injector and coil wave forms.

[@rolandgutierrez813]

You have some awesome videos !

[@huntnfishnuts5880]

I have a very similar situation I'm dealing with now on a Chevy truck with a 6.0 v 8, intermittent miss on one and five cylinder, clears up off idle, switched plug, coil, wire, done fuel injector Ballance test, was as all equal, compression was 160 psi, checked cylinder leakage was well below 10 percent, checked fuel quality, had no fluctuation on a vacume gauge on the manafold, smoke tested the intake with no leakage from the intake manafold, at this point I can only think it has excessive carbon on the back of the intake valve soaking fuel causing a partial combustion or a cracked or weak valve spring, time for more digging I think, like that borascope picture 👍

[@aciddiver1978]

I had a 8 valve engine that had this symptom. No vacuum leaks, nothing. Ran fine over 1.200 rpms.

[@antonisantonis655]

awesome video !!, continue the good job!!

[@demcomp]

I work in the method of... What do you need for ignition. You need Air, you need fuel, and you need spark. If you have fuel, and you have spark, then the problem is air. Because what is a Misfire? Poorly timed ignition, or just failed ignition entirely. If you test for spark, and it's consistent, then you can rule out that part of the ignition triangle. If you test and find you have proper fuel, then you can rule out that part of the triangle. So you're left with air... Either the exhaust valve is sticking and letting enough air out that you lose stoichiometry, therefore no ignition, or late.. or the intake valve is sticking and not letting enough air in during the intake stroke. @17:58

[@Micko350]

Ivan you finally got to it in the end - Carbon Deposits etc... To get a definitive answer you would need to spin the Engine over so all Valves are closed on the Cylinder then stick a Smoke Machine in that Plug Hole(not literally but yeah a hose!) & wait for a Smokin' Pipe! (excluding Leakdown of course so if it's bad just pull/loosen Exhaust Manifold Bolts🤔

[@AKChryslertech]

I would try an induction system cleaning but otherwise, just keep driving it. If it is valvetrain related, its not bad enough for me to justify opening it up unless the customer wants it fixed that bad.

[@bobsmith1101]

Did we do a Leak Down test? Love the videos and your patient approach.

[@somerandomguy3868]

I'm thinking dirty injector, but really all things considered it runs pretty good

[@robertrobbins5456]

I have the same problem with my '08 Avalanche, except it's on cylinder 7. Dealer said it was a "ghost" code. Changed plug, plug wire, coil, and fuel injector. They also did a compression test and told it had "perfect" compression. Still doing the same thing....

[@philh9238]

Surprised he didn’t amp clamp the injectors to see if the pintle humps were in the exact place to spot a weak solenoid in the injector.

[@pcmfuse4692]

This car has a few known engine issues. One of them is related to the harmonic balancer bolt backing off, causing the crankshaft pulley to bounce against the keyway. I would check said bolt to see if it is tight. If it is not corrected in time, the keyway will break. Which will throw off the timing, bend push rods and valves. Please let me know if you looked and what you found.

[@user-hr5dc4ly3q]

My advice check with DC spark plug tester and then on engine current and then check coils with coil tester This first test you should start with

[@ScottDLR]

I would use a compression checker to apply air pressure to the questionable cyl's at TDC and listen for air flowing at the exhaust pipe.

[@neilprice2714]

Due to running ok from cold and milage of vehicle. My guess from your data would be a sticking valve. Excessive carbon build up on valve stem and or tight guide. So yes, compression related fault.👍

[@headpainter1]

So basically keep running it but add some seafoam to the gas and some Marvel Mystery Oil to the oil and hopefully get that loosen back up. Or tear the top end of apart and do a valve job and new lifters.

[@DavidJohnson-id1js]

intake vales and or exhaust valves sticking in the guide when hot the trail blazer heads had a problem with that i fixed a lot of them back in the day working for chevy

[@Sandmansa]

A very puzzling case study indeed. A compression difference of over 10 psi would cause a slight misfire. But a consistent loss of compression should show a much more consistent misfire. And what's with the misfire showing up on other cylinders? That gave me a WTH moment so I can see why you're struggling a little with this one. Now while a valve can rotate ever so slightly during a cycle, it will be very difficult to prove your theory without getting your hands dirty. I do feel that you are on the right track though after seeing those valve seats. Good gravy, they are burned! But I still would have liked to have seen the results of a leak down test. I am really hoping to see what happens next.

[@OzFrog48Z]

If you suspect a sealing problem, maybe an old fashioned cylinder leakdown tester would help. You could compare the leakdown percent with a good cylinder. Listen for air coming from tail pipe. I still take mine out of the antique drawer every once in a while.

[@jamesofallthings3684]

I'd assume he's aware of a leak down test and would perform one if he felt it was necessary.

[@willV6F1]

Had this exact same fault and symptoms with a Renault 1.2 petrol. The exhaust valve clearances were too tight. Once reset, it was fine

[@ekimbrough1413]

@ 27.20...There's either a weak gasket around number four, OR a micro crack/fracture in the head or block in that area.

[@advancedleveldiagnostics]

Great diag, Thanks!

[@JoelAutomotiveInaction]

I have found mechanical issues doing relative compression, but you can find more information than relative compression on an absolute vacuum sensor , those small leaks are very visible on intake pulses , very intermittent your case but tha tool is power full

[@robertoruiz7069]

Great video IVAN,as always.Why are you so sure the valves rotate? Without springs designed to do just that?I ask because a long time ago i worked on a friend's car a 305 GM Firebird that had spring retainers on the exhaust valves only that were called valve rotators.That was my 1st ex-poser to that system and it made perfect sense at the time.What you said about the valves haveing a spot where they leak makes sense too.my 1st thought was a sticky fuel injector.or a wire that's arcing.i wish you had pulled all the plugs and coils and compared them side by side.What if one coil has leakage of elec? I'm sure YOUR viewers like myself,want you to dig further to find the issue.Out of 1,000 cars you found the cause and this one eludes you???? how can to sleep???haha or eat?? Newspaper headlines>>>Chevy Cobalt defeats IVAN THE GREAT

[@hypocrisyrules]

scanner would have thrown a coil or injector code. When you can hear a misfire coming from the exhaust pipe, it is generally an exhaust valve problem.

[@leomcalla29]

Ok, so think it's a valve sealing problem. Along with the tail pipe pressure also take a vacuum pressure from intake and a ignition sync. Then use the overlay and that will tell all you need to know.

[@codyarizona4925]

I agree needs valve job, But on a old junker like that just give it a small vacuum leak to up the idle rpm a bit haha.

[@mikefoehr235]

I am amazed this car made it to this mileage.

[@user-nh8nz2ve6o]

Had a intermittent misfire caused by hydraulic valve adjusters sticking open only at a set rpm . Caused by lack of oil changes. Hope this might help .

[@baxrok2.]

The G8 that beat PHAD? No way. You'll figure it out if not by yourself then with the help of the PHAD community. Thanks Ivan!

[@deeznutz3494]

For what it’s worth I’d have for sure done a compression and leakage test on all cylinders to get a full health report on the engine. Especially considering it’s age, It’s very possible to be leaking from any combination of valves, rings, head gasket leaking into the cooling system or even cylinder to cylinder, and I’ve see it happen

[@mikejr550]

I'd be willing to bet that it has tight valve clearance and the pitting is made worse because of that. Also those exhaust valves look like they have been running really hot. When we had valve issues with the early to mid 2000's Subaru turbo engines this is exactly what we would find. If continued to be driven they would eventually burn big parts of the exhaust valves up. Nice Diag work, it's humbling getting your butt kicked once in a while, makes you think hard and dig really deep. I would have loved to see what your valve clearances looked like.

[@PineHollowAutoDiagnostics]

Hydraulic automatic adjustment, so no issues with valve clearances like Honda or Subaru 👍

[@mikejr550]

@@PineHollowAutoDiagnostics I figured because I was asking myself why you didn't consider it. Makes sense now. I didn't hear any ticking either.

[@wwdiesel]

Just a thought; how about one of the exhaust valves sticking in the valve guide intermittently? Seen this years ago on an engine that only showed a misfire (skip) at idle. Very weird and unusual problem! Looks like you covered or ruled out all the other possibilities.

[@johnbelwell2461]

Had the EXACT same problem on a euro spec GM car with the pre-Ecotec engine for like a week until yesterday! It was driving me crazy, went from changing the fuel pump and filter, map, tps, injectors, idle valve, put premium gas, runned auxilary wires from the sensors to the ECU as this appeared when i was installing a standalone on the harness. Ended up being the vacuum hose on the fuel pressure regulator that i had changed recently with a new one. When the hose was on my hands inspecting it was appearing alright, when slightly bent when installed the hole was turning into an oval and had a vacuum leak.

[@johnbelwell2461]

If you try it out you need to wait for the ECU to adapt the fueling for idling after taking it for a ride or shut the engine off and restart it after removing the battery power for a few seconds. Maybe aswell the membrane of the regulator is perforated in which case it would draw excess fuel through the vacuum hose and make it struggle at idle.

[@autoacoustics4938]

Could be a leaking head gasket cos why does it misfire only when its hot , did see some rust marks at the top of the cylinder .