Using Relative Compression to point you in a Direction

Relative Compression Testing and how to set it up. Also the importance of not relying solely on it and that you should continue your testing.
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#diagnostics #Picoscope #autorepair #snapon

[@PineHollowAutoDiagnostics]

Ah good Ole Pentastar... always some weird failure! Possible explanation for high cyl 3 hump: during cyl 2 intake the overall vacuum actually gets worse (closer to atmosphere) and then #3 gets a bigger gulp of air 😅 Would also be interesting to see a running vacuum waveform to see if the cyl 2 INTAKE valves are leaking pressure back into the intake manifold during the compression stroke 👍

[@overload3996]

Also a pules pressure at the dip stick tube to see cylinder rings activity.

[@jeremyah78]

That seems like a plausible theory Ivan. Just crazy that it would double the starter draw, eh!?

[@nathanlittle8945]

Clear flood crank. Won't be any vacuum

[@PineHollowAutoDiagnostics]

@@nathanlittle8945 good point! The 2 other variables are 1) using voltage instead of current and 2) AC coupling on the scope. Would be interesting to see the classic amp clamp with DC coupling capture to compare 😉

[@autodiagyt]

I’ve done a lot of RC test using voltage and amp clamp and the voltage is always a mirror of the current. The only advantage I found to the clamp is better detail in the starter windings and a little less noise. The voltage is so close that it’s not worth the extra time to grab the amp clamp. It gets used for the one test and then back in the box where as the volt leads can be moved to a pulse sensor, crank sensor, or something else. It’s just a more efficient way.

[@ultron6931]

Great diag on the jeep. I had a customer with same exact issue that came to me for a cylinder 2 misfire for a second opinion after plugs and coil. I knew about the cylinder head issue and tried to get the dealership to cover it and got no luck at all. Ended up buying a new fully assembled cylinder head from dealer. It was less than $400 bucks and included the new part number for the updated head.

[@autodiagyt]

Thanks for the info!

[@stevepark5504]

Another way to use voltage for a relative compression test is to connect scope gnd to battery ground, scope positive to starter case. The voltage drop across the ground cable(s) will show the compression peaks in the positive, like an amp clamp. AC or DC coupling will work the same.

[@johnz8210]

Hello fellow Iatn member - I think.

[@vastylebbq5203]

That’s a good idea, Steve. I will have to try it. Thanks

[@JoseSilveira-newhandleforYT]

Unusual case, Jake! Great work plan - it wasn't a cam follower, but you were on the right track. Borescope and oscilloscope were indispensable for the diagnostic.

[@jeremyah78]

Appreciate your honest reflection Jake! I totally get it! I say to people, sometimes experience is not your friend. But that’s fixing cars!

[@ThunderbirdRocket]

This is a gold mine of learning / development for us viewers ! Super glad to be able to make the fix it journey with ‘ya - you have our learning / problem solving skills at the heart of the presentation ! You are a great coach , fantastic mechanic and diagnostic technician ! Looking forward to the next one ! 👊🏼 🔧🔥⚙️💻📈

[@bartscave]

Thanks for this extensive case study!

[@NoName-yr1jv]

Very good diagnosis 👍

[@alrifainidal]

Great channel and video.

[@franciscoxaviergonzalezrom4648]

Wow this was a good one Jake! From my heart thank you!

[@mlstrawxj]

I just got a PIcoScope so thanks for showing the details on how to set up the scope

[@scottleathers8551]

thanks for the insight on the companion cyl point.had a dodge come in today that had a uneven cadence with the engine cranking over.started to go on a chase but remembered that you had this case study.sure enough the companion cyl was the cause and it was a leaking exh valve as well.

[@elservidor5035]

GOOD JOB BOSS I LOVE YOUR VIDEOS!!!!!

[@danman9017]

you did well tks

[@cthorn2467]

Had a 2006 grand Cherokee 4.7L similar issue on #1 cylinder checked the Block with die it tested positive for blown head gasket. Went one step further looked in number one cylinder, and found very small amount of coolant. Sold the Jeep.

[@JBlake-moon-shdo]

Jake I've found electric utility companies have rebates if your shop gets new new lighting. Which in my case took care of flucker and cold start problems. Glad you gotvto the root of this one so quick even though it threw a few curves.

[@autodiagyt]

That wasn’t my shop although my new shop does need new lighting lol

[@lindakeiken2892]

Thanks so much for the visual example. Its easier to under stand.ck,wisc.

[@15066bas]

Yes the amp clamp does work better as mentioned at 6:20. Never had much luck using battery voltage. Amperage pulls relative to voltage drops are higher and detect low compression better. We always see a higher amperage pull after a cylinder with lower compression (no2). When the crank rotation speeds up the next pull will be higher (no3) since you're compressing faster and have less time for leakage around the rings, valves etc. creating higher compression.

[@spelunkerd]

These are fascinating. In a four cylinder boxer engine, mechanical companion cylinders are pretty easy to understand. When one is on compression, the other is on exhaust, so understandably if the exhaust is compromised the companion cylinder needs to work harder to push that opposite cylinder up. In a V6 with 123456 firing order, you get similar pairing of (1,4), (2,5), (3,6). Then theory meets reality, with multiple cylinders involved, complicated by pressure dynamics and harmonics of each exhaust manifold. I find it surprising when everything lines up! The more alarming problem is the false positive, where relative compression is bad but it turns out to be cylinder wash or some other more innocent secondary problem. Do you ever condemn an engine purely on the basis of relative compression?

[@benjaminbrower540]

Compression gauge for the win. New valves sounds like..Nice video Jake

[@paulspeller968]

to be fair i normally go old school and do a mechanical compression test even when the scope tells me . probably because i am an old boy and can’t get used to theses new fangled contraptions 🤣🤣🤣🤣🤣 however that camera is awesome i like that

[@PaulCTownsend]

Test results are interesting and confusing Good job.

[@martinarrieta248]

Grasias

[@stevepark5504]

The cylinders, with low compression could be the intake is not opening. That puts the cylinder under vacuum at “intake close” so the compression is lower. Crankshaft speed is not a factor in a normal cylinder’s pressure. Speed is not a factor in the formula for pressure vs volume change. If the cylinder has a leak the compression can change due to size of leak vs time available(speed) to leak. Apparent TDC can be incorrect due to shifting the peak pressure due to a leak. Having an external sync will show if that is happening. The usual apparent increased pressure in a normal cylinder, that follows a leaking cylinder is caused by the drag(vacuum) of pulling that cylinder to BDC at the same time that the following cylinder is compressing.

[@cullenmiller8170]

Nice video and diagnosis. They always say it’s a Jeep thing. 🤣🤣 I was looking at Jeep Gladiators, but between you and Ivan I was kind of worried about that Pentastar engine.

[@CodycoWeb]

I was told by a Chrysler engineer that the global disable means that the pcm has the ability to shut the injector down on misfires. When the battery is disconnected or cam/crank variation is coded or not run the enumerator hasn’t populated and the pcm isn’t able to do misfire detection and counters won’t update and it can not disable and injector. The enumerator is basically a learning state of the pcm for cam/crank variations basically an algorithm used to detect misfires.

[@mikechiodetti4482]

Pico for the win! Having the proper equipment and knowing how to use it makes the difference. That 3.6 V6 sure has it's share of troubles. I've seen more of those engines on other channels with similar problems. Good one Jake!

[@arthurfricchione8119]

Jake another informative video. Seems like you covered it all. Think you will get any feed back? I will try and find Ivan’s video. He always seems to find no parts required. Lol. Thanks again. Artie 👍

[@autodiagyt]

I’m not sure what the plan is for it yet

[@user-yc9ql4ww6e]

I think the answer to your problem is that as number 2 goes up the compression stroke it loses air mixture through the exhaust valve. As number 2 goes down the expansion stroke it gets to a point where it goes into a vacuum because there is less air mixture in the cylinder. At this point you have two loads on the starter, the vacuum on number two and the compression on number 3 cylinder at the same time. This shows the larger current draw on the graph.

[@autodiagyt]

That’s a good way to look at it. Maybe that’s the issue.

[@mattw1393]

Similar to a Jake brake

[@sheerwillsurvival2064]

Bad exhaust valves and bad heads on the number 2 bank is common. Did the valves on my neighbors. 2012 and did videos 👊🏻👍🏻 . Love your videos. Leak down tester is your friend 👊🏻

[@ricebike]

So his jeep wasn't covered by the extended warranty that was noted on the 2014 tsb?

[@autodiagyt]

Except when you have no air lol.

[@sheerwillsurvival2064]

@@autodiagyt true🤔😁

[@sheerwillsurvival2064]

@@ricebike yup my neighbor called and gave vin and they said not covered. Soooo much BS they know it’s a problem like the oil coolers and roller rockers but don’t care 🤷‍♂️

[@ricebike]

@@sheerwillsurvival2064 yikes... And this engine is in other vehicles besides the jeep😲

[@1MiketheMechanic]

A quick check on these is to raise the rpm above 1400 and see if the misfires continue. If you only feel the misfire at idle or the counts are only at idle then you most likely have a valve sealing problem. I always put a head on as recommended by Stellantis because the price difference is not worth the risk of repeat or worse failure. Lack of compression in cyls 2 & 4 allow 3 to have more momentum and harder time switching direction so there is more pushback on the starter. I could be off my rocker because it is late.

[@ricebike]

At the end of the day, you reached the correct diagnosis Wonder if the customer will have the head replaced /reconditioned by the warranty 😢 Stinks that the problem showed up so late in the jeep

[@autodiagyt]

Mileage not even that high for the age.

[@ricebike]

@@autodiagyt ahhhh, so he pampered it for that long😭 Another commenter said they didn't honor his neighbor 's jeep warranty as well... Boo!

[@curtisroberts9137]

I have seen where the following cylinder after a low compression has high current draw. This seems to be because while the starter does have some momentum, it is also running on lower power so when it does hit the wall on the next cylinder is drops speed quickly and you get another inrush of current to overcome that wall. It's a bit counter intuitive but if you think about how inductors work, like the winding in a starter motor, it resists change of current flow. So when the motor spins fast the current goes low but when it slows down hitting the #3 cylinder, the actual resistance in the winding goes down causing current to spike. If you have a motor that doesnt spin but continue to give it power, it will max out current draw and burn up. Wasn't what I was expecting originally either though. I was thinking valve issues but not on number 2. I've never done RC on ac coupling. I'll have to give that a try and perhaps it will be a better picture. My scope is a cheap MicSig which isnt the best on clear waveforms as it produces a LOT of noise. On another note, have you considered a patreon page or are you sticking with only KZbin memberships for now? Great channel and video.

[@autodiagyt]

I just don’t know what I would do with patron. I’ve wanted to keep all of my content “free” but if I am to hit my targets I need to travel like I want to I might have to do something to bring in extra revenue.

[@curtisroberts9137]

@@autodiagyt You can still keep the content free on youtube. Patreon is just another way for people to donate. Honestly I dont know if it would be any better than the YT memership for your channel, it's just where I'm used to donating. I have a dream similar to yours. My daughter graduates this year and traveling more and doing interesting work is so appealing to. Meantime, I will support you cuz you are putting out fantastic content.

[@GraditelMacedonia]

29:26 My opinion is valves... probably boroscope and comprehensive investigation, and after that leak down test. For last timing...

[@66C10L]

Only 11 minutes in but great explanation on scope set up for an amateur like me

[@memine5029]

Hey Jake next time use a engine stroke overlay on the oscilloscope pattern to verify valve/ stroke events as well as looking at eng vacuum with a transducer , lots of times the over lay shows were the problem is and we assume its the cylinder in the firing over from a relative compression test.been there been BURNT BEFORE. A quick look at the piston stroke chart would indicate that cyl 3 was in expansion/power near TDC and going down so that didnt cause excess draw/load, now looking at cyl 2 it was just ending its power stroke and starting its exhaust stroke while cyl 4 was well in its compression stroke No right or wrong , just thoughts

[@stevemazzarella7212]

That’s how we learn like u said, it was a good find!!

[@cannbudo]

From what I understand many systems use the crank sensor signal to detect misfires. Wonder if monitoring that signal along with a #1 sync. would be of any help in diags like this?

[@fredautos]

Premature Failure of Head Gasket. Im curious weather the dealership will still cover the warranty?? Nice vid jake😊💯

[@CountryBumkin69]

I would have liked to see a cylinder "leak down" test too.

[@autodiagyt]

That requires air. Not something easily available when in a mobile diag van.

[@hikerJohn]

Just how much air is needed? 12 amp - 6 gallon pancake compressors are $100.00 @@autodiagyt

[@autodiagyt]

@@hikerJohn I don’t have room in my van for a compressor. When deciding what goes in my van I really think about what I need day to day and in the full year of having my van this is probably the first time I would of liked to have air and didn’t have it. So it’s not about the money, it’s purely space and usefulness. I’ve thought about putting a 12v compressor and tank mounted under the van but that’s still not high on my priority list.

[@hikerJohn]

@@autodiagyt I hear that, I had the same thought about mounting a compressor and tank on my truck for 20 years and never did it.

[@eddiereichel9354]

So the 3.6L that are driven more aggressively dont have valve seat issues the way that the ones that never get opened up do.

[@davidreed6070]

I can hear a problem. I wonder if a valve is not opening. It acts like a head gasket blown between cylinders.

[@aymenmohamed1134]

👍👍👍

[@kevinobrien3063]

Pico has an auto compression test in Pico Diagnostic software, does all this setup for and gives you a pass fail bar graph I use it all the time

[@HouseCallAutoRepair]

Maybe it's because I don't own a pressure transducer, I always immediately go to a mechanical pressure test, whenever I hear a cadence issue.. Relative, is good for identifying , quickly.

[@autodiagyt]

Usually I would follow up with compression testing but since the cylinder in question is under the intake I just had them take it apart.

[@johnz8210]

That was interesting. I've read that artificial intelligence is or will be used to interpret EKG's, X- Rays, etc - it's supposed to be far superior to humans in that area. Maybe some day we'll load our scope captures into some AI gizmo and it will provide some answers. Correct answers - well, they're always extra.

[@toddw.9845]

2011 to early 2013 had extended warranty for 10 years or 150,000 miles whichever comes first on left cylinder head only. .

[@johnshanderuk4503]

What was the borescope that you are using in the video?

[@autodiagyt]

I've added it to my Amazon store

[@eddiereichel9354]

Also every single one of these engines I have removed the valve cover off of the "plugs" which are bolts that plug the oil galleys are loose. They are 13mm head and right under each camshaft 2 per head. They will be loose.

[@DuaneDonaldson]

Some ECMs will count a misfire if the CKP square wave is rounded off always or more likely intermittently, the ECM will calculate the time from the falling edge to the next rising edge and the rounded corner will be considered the beginning of the falling edge and the time to the next rising edge will be longer than normal and be counted as a misfire, go figure, so be sure to have SQUARE corners on the scope pattern. Now you need a CKP sensor. Isn't Global another word for Generic as in OBDII Global mode on the scanner? The first go around, I could see #2 low compression and from experience I know the next cylinder in firing order will show higher compression even though a few guys still say NO to that extra work for the battery as the good compression in #3 really puts a strain on the starter and slows it down quite a bit. For those reading my Comment here, you can also see that low compression peak is not true TDC so when analyzing a waveform with the 720* rulers, TDC vs exhaust pocket vs intake timing is not exactly correct. SO instead of aligning up TDC to the rulers, align the exhaust pocket EVO to the good EVO to get a better idea of valve timing as opposed to TDC location. This waveform has the intake off and this is not the typical waveform. This waveform is not exacting based on differences in timing or actually cranking speed at each TDC so take that with a grain of salt, also why not to align TDC's. You need to take into account a single-in-block cam, SOHC, DOHC, or quad-cam like this engine where timing chain stretch comes into play also, cranking slow speed is actually the best speed to find timing component problems such as stretch. This is where you compare several pages of the waveform to find a noticeable anomaly. Manual compression tests are almost always done wrong, you don't crank and crank to get the highest possible compression, maybe 4 pumps is good but the very first pump is the most important so in the 21st century, we need to video the gauge. If the first pump is low, you might have already been on the compression stroke part of the way up so you got robbed of a full stroke so do it again to be (more) sure. I am speaking in general here, not about Jake. This is exactly why I believe in Military Maintenance style checklists to follow along until you get to a point leading to another checklist, no mistakes once the lists are up to date as an experienced diagnostician as opposed to a mechanic who is just getting started building a checklist that is going to need updates and forks off on another tangent. Schrodingers Box (youtuber) thought his scientific approach was good but he only went by his own rules which were imperfect. The main problem I see most everywhere, is techs have a system in their head, but they simply throw their own system in the trash and then make up silly excuses when they don't follow their own rules, it's crazy man. We all have memory problems folks, create a fucking system on paper, update it when proven, and follow your own system as though you are a robot because humans are going to fuck it up for sure, I don't complain when we make a mistake, by not following your own system from a known good previous job/video, no excuse, sorry.

[@elservidor5035]

PLEASE CAN YOU TELL ME ABOUT THE SCOPE CAMERA MODEL

[@autodiagyt]

It’s a teslong td500

[@elservidor5035]

Thank you so much

[@richardcranium5839]

is the global disable due to the scan tool being used in manufacturer mode? its disabing the obd2 counters?

[@autodiagyt]

I wouldn’t think so. I rarely look at it in global obd.

[@richardcranium5839]

@@autodiagyt just for the shizz n giggles ya may wanna try it next time

[@ktcarroll4723]

I watch another mechanic yesterday say when you do a RC test you need to separate the pvc system from the system? I’m just a newbie so I know nothing

[@autodiagyt]

That is if doing a intake pulse at the same time

[@wilamyoukhana1109]

Cylinder 4 common problems in 3.6 L caravan

[@gerrymoeller]

What borescope is that you are using?

[@autodiagyt]

It’s a teslong not sure what the model number is.

[@gerrymoeller]

I found it I believe, looks like the TD500 Pro. I like it. Clear picture and articulating lens..(edited)

[@autodiagyt]

@@gerrymoeller td500

[@gerrymoeller]

@autodiagyt yep that's what I meant, fat fingers 😂😂

[@stevestadnik9206]

What kind of camera is that?

[@autodiagyt]

Teslong td500

[@stevejakobsen2765]

That i so weird! Makes no sense to me. I would take out all sparks and crank it by hand AND starter to see if it sticks somewhere.

[@luigigonzalez5062]

Rocker issue?

[@autodiagyt]

That is what I thought at first as well but nope.

[@mnrujiska771]

I don't believe you wrong, it also sounds like compresion was higher in one of them.

[@justinhayward5027]

Put a left cylinder head on it. Didn’t watch the whole thing yet. Valve guide dropped keeping valve open ever so slightly. Do a leak down on cylinder 2.

[@dk4622]

Why not do a leak down test?

[@autodiagyt]

It requires air.

[@dk4622]

@@autodiagyt You need an inverter and compressor in your van. I'm glad I did that. Also if you get a flat you can use an air jack to change it. Not much of an expense and can be easily moved to your next future vehicle. Good video.

[@autodiagyt]

@@dk4622 I have a 3k watt inverter but no room for a compressor. I almost never need air for anything, usually at a shop that has it if I do.

[@dk4622]

@@autodiagyt I don't have a big compressor in my van. 3 gal compressor is just enough to lift my van to change tires or put air in. I don't have a leak down tester. One of my cylinders is at 150 psi and the others are over 180psi. I don't really want to know why. Lol

[@kevinobrien3063]

A leak down test is a lot of mucking about, getting the cylinder on tdc just not required here

[@marcokahny709]

I love how all good mechanics can speak engine language i think we learned it from women 😂

[@scottnusser6232]

I'd be curious to know how much money the customer had to pay to find out what he already knows, trucks broke and don't run right? Old school here, pull the intake, check compression and leak down with mechanical gauges, they don't tell fibs or leave you still guessing. No disrespect but old method would have saved some time. The customer could then opt to put intake back on and the trade that pos in on a real vehicle😂❤❤

[@autodiagyt]

Mechanical gauge and leak down will both give you good results if exhaust valves are not opening and if the spark plugs are accessible without any disassembly I would have followed up with compression but the conversation with the customer was that there needs to be some disassembly for further testing but the most common failure with these engines are failed roller followers so he opted to just getting the it down for inspection. The cost of the diag was much cheaper with them doing the tear down then it would have been if I took the intake off and did compression testing.

[@victormoras1607]

Hidrolock ?

[@autodiagyt]

Nope

[@calholli]

Nope.. I'd just drive it.

[@GregoryBirulkin]

Wasn't that recall for crack cylinder heads?

[@autodiagyt]

I think is more about the valve seat recessing into the head

[@sleathers1551]

WHAT KIND OF BORESCOPE ARE YOU USING?

[@autodiagyt]

Teslong. I put it on my Amazon store