How To Quickly Diagnose The Sneaky Weak Ignition Coil

A quick video to rule out or confirm a weak ignition coil, the symptoms of which present themselves as a fuel delivery issue.
I filmed the spark of a weak coil and a good strong coil which in person you can visually see the difference, but the differences are very subtle and hard to see in the video. So when checking to see if you have spark with a weak coil, you'll see that you do indeed have spark, but under load the spark isn't strong enough to keep up with increased fuel/air mixture and the engine bogs down or quits above idle under load. I'll show how the engine has a hard time starting and running with the weak coil, but immediately starts and runs strong and smooth with the good coil.
I describe the symptoms and behavior of the weak coil, so if you're experiencing these symptoms swap out the coil which is quick and easy to just jumper one in. You'll quickly rule out or confirm a weak coil. If it's not the coil, it's time to check fuel pressure, and fuel filters, or run circuits in the carburetor.

[@GoatPilot04]

Small pro tjp for anyone here looking for help: For any older electronic distributor ignition, long wires w/ single coil, etc, do this at night or in your garage with the lights off. After you get it running, also check under the hood in the dark for any spark/ignition leaks arcing around the engine from bad/damaged wires, loose connections, bad/damaged/loose boots, damaged/burned through coil, etc.

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Yes indeed on checking for HV Leaks in the dark. It's less of an occurrence with modern plug wires. Used to be regular thing that plug wire insulation would break down after a year or so. A spray bottle of water lightly misting the system will really make the electrons put on a show and confirm the plug wire insulation is breaking down causing misfires and skips.

[@mosessich2163]

Thank you for the video! I am just starting to work on my own vehicles since I am only 17. I always appreciate when people share decades of experience!

[@mkdhdinfiniteendlessconten3873]

Hi. This is instantly and easily the most meaningful comment and compliment I've ever received. You will go far and do well in life with your outlook. I'm getting close to having 100 videos now, mostly on DIY/How to across many topics. I'm also working on and currently posting a series of videos from my Route 66 Trip in my 1974 Oldsmobile Cutlass Supreme. I even have How to Make Cold Brewed Coffee, Setting Up a Floyd Rose Bridge on a Guitar, and Changing Batteries in Old iPods and Tablets. While I might be showing "how to" do a particular task, I'm also teaching skills, methods, techniques and basics that apply to many situations you'll encounter. Go to my channel home and poke around, and you should find numerous videos that will help give you a good foundation. Some of my first videos are on doing a budget build of a Ford 302. I just loaded two-part series on the Ford AOD Transmision Rebuild. I hope to be doing an updated engine build video sometime soon. I look forward to seeing and hearing more comments from you. I wish you the best. Ask me anything any time. I'd be interested to know what you might be working on and assist you on your journey. :-)

[@kennethhettig883]

Great way to test a coil, thanks for posting!

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. This is one of my highest viewed videos, and seems to have helped a few folks out which is the whole point. One person who I know personally was experiencing these symptoms watched it, swapped the coil and problem solved. Bottom line, easiest way to test a coil and rule also out a fuel system problem is just swap the coil. Thanks again for watching.

[@eddieconerly1222]

This is it from New Orleans please more content your delivery is awesome your info is great tell us more about your truck or you keep it running good so I can learn from you thanks

[@mkdhdinfiniteendlessconten3873]

Hi Eddie from New Orleans. I think you're my biggest fan. If I had a fan club, you'd be president. ;-) That's a great idea on the truck, so I'm going to do it as soon as the weather warms up with some longer days. I appreciate the compliments regarding my content and how I deliver it. I enjoy doing such things. There's a big cult following for The Bull Nose F-150's. They had a new cool look that was aggressive just like the body style before them, and much cooler than the styling changes they made after this version. As beat up as my truck looks, I get offers for it constantly. It's my daily driver. Except for a brief time, I've owned the truck since 1991 and it played a big role in my life taking my family and I on many adventures and hauling and towing many things many miles. I always tell everyone "I love that truck so much I bought it twice" which is actually true. It even survived a tree falling on it during a hurricane and has the battle scars to show it. I'll be doing body work and panel replacement on the cab, replacing/fixing/upgrading the A/C, building a new cool engine. The interior needs a lot of work. It originally had an awesome cloth bench seat, and the dude I temporarily sold it to replaced it with buckets, that are sort of ok as they open up the center hump, but they need reupholstering. I'm on the hunt for some vintage 70's Thrush Side Pipes for it and will be putting those behind a good set of long tube headers. The relatively new foreign made fuel pump is leaking oil at all the crimps and seams. I'll be doing a video on that in the next few days, so stay tuned. Thanks again.

[@garrettpolzin7235]

I’ve been trying to find a “fuel issue” for 3 weeks. I cannot wait to test my coil I have a pretty good feeling this is issue. You described my exact issue

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Ya, a weak coil that breaks down under load will give the same/similar symptoms as fuel starvation. You can ohm out the coil statically and it will appear to be fine since it's breaking down under load dynamically. Generally if you work on cars, you have known good coils on your shelf. It's a good idea to grab one every now and then when in the junk yard. I have a recent video regarding fuel starvation again with essentially the same symptoms which turned out to be a bad in line fuel pressure regulator. With fuel starvation the engine will often stall if you don't back off quick enough. With a weak coil the engine will stay running and bog down, but rarely stalls and will come back once you let off the gas. Anyhow let me know how it goes and if it was indeed the coil on your vehicle.

[@ryantabussi7180]

extremely helpful and good explanation thank you very much

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Hope the video helped you out.

[@barryallenporter8127]

Good video. I always forget the simple things and overcomplicate issues.

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks, and thanks for taking the time to watch and comment. This problem is easier to diagnose on lower energy points ignition system since the coil output was lower and the plug gap smaller. it was more frequent in older style coils. HEI has a higher output and larger gap that even a weak coil will still jump. The importance of plug gap is you want the largest gap possible that the spark will jump. The larger the gap, the hotter the spark and the better the initial combustion of the compressed fuel/air mixture.

[@melvis2017]

Thanks for the great job very good explanations and the great humor Thanks again for sharing with us all 🇩🇰👍

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Hope the video was helpful and glad you enjoyed my humor. I've got quite a few other videos on a variety of subjects that you might also enjoy. Thanks again.

[@melvis2017]

@@mkdhdinfiniteendlessconten3873 thanks for that plz more vidéos

[@nelsonluke-se2xp]

Thanks for that I'm leaning for coil symptoms.

[@mkdhdinfiniteendlessconten3873]

Hi thanks for watching and taking the time to comment. Ya, it's rare, but a weak coil will taunt you. in 50 years I've probably only had 3 weak coils, and never a dead one. It's quick and easy to rule out if you have these symptoms so it make sense to rule it out or confirm it before messing with fuel system stuff.

[@michealphy4013]

My HONDA CIVIC 1985 . took me 1 month been through 2 garages unable to diagnose the problem . Thanks alot for this information

[@mkdhdinfiniteendlessconten3873]

Wow, an 85 Civic! That's awesome that you're keeping it running. Great simple economic fun car. Glad my video solved your problem and helped you out. Thanks so much for watching and taking the time to comment.

[@eddieconerly1222]

Man this is Eddie in New Orleans this is the second time you didn’t help me I have a 83 4150 you help me with the gas tank and now you help me with the coil cause my truck is doing the same thing that you describe thanks

[@eddieconerly1222]

👍

[@mkdhdinfiniteendlessconten3873]

Hi Eddie. You had me worried there at first due to a typo in your comment: "second time you didn’t help me", but I think that was supposed to be "Second time you did help me." Probably AutoFill filling in the wrong words. Hopefully that's the case, and glad my videos are helpful. That's my goal to educate and help, and eventually gain enough subscribers to have an income from this. Thanks again for watching and taking the time to comment. Stay tuned, lots more content on the way.

[@DG-od4si]

nice video, thanks

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching the video and taking the time to comment. The spark visual and sound difference didn't really show on camera, so was disappointed in that. For real life troubleshooting you're just going to jumper in another one to confirm or rule out the coil. Jumpering in a coil takes just a few minutes. It's less than a minute on one of these slide terminal connections. Takes a few more minutes on a nut terminal connection. The HEI GM coils built into the cap are quick and easy too. If you have spark and similar symptoms to what I describe, jump in a known good coil and confirm a bad coil, or rule it out. If it's bad, then you can deal with that annoying wrap around mounting bracket if it's an intake manifold mounted coil. Otherwise move on and take a closer look at fuel system and carburation.

[@DG-od4si]

@@mkdhdinfiniteendlessconten3873 thanks again. i order that tool and this will help me to find which one of the 6 pencil coils are the sneaky, not working right

[@hammerhead19able]

Thank you so much!!! I think now I don't have to push it into the parking lot by the mall, helping my son.😂😂😂

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Can't tell for sure, but sounds like my video helped you out. Let me know if you have the time. Thanks

[@gregander1241]

I don't know about that spark color issue, however there may be a difference between the two coils in terms of voltage.

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. The spark color is kind of subjective as it has both purple and orange/yellow "aura's". I might have mixed them up when describing. Usually more purple is a higher voltage. Was just using that as a visual aid. Main point is that a weak coil will give fuel system symptoms. Only takes a few minutes to rule out jumping in a another coil. A friend of mine who works on modern computer controlled vehicles but has one older truck was having symptoms like I describe in the video watched this video and solved his problem in minutes after he'd been scratching his head for a while, being used to OBD Scanners and Fault Codes.

[@gustaveiserner6743]

Air gap for ignition testing outside of engine should be 7 mm , third of inch minimum. Air gap testing device available .

[@mkdhdinfiniteendlessconten3873]

Sorry for the delay. Good info. Thanks for adding that to the comments.

[@fitness_with_boobie5240]

I going through this with my 79 cordoba smelt gas but no start and strong crank ignition coil is the cheapest part to replace first before going down the good ol list lol

[@mkdhdinfiniteendlessconten3873]

Hi Thanks for watching and taking the times to comment. Before you spend $ on a coil, do you have spark at all? Verify that first if you don't have a coil laying just around. Go through it logically instead of shotgun trying parts. If it was running and had the same symptoms I'd say go ahead, but you should confirm what you have and don't have for an engine to run. You could have points or electronic ignition issues. A coil, which is actually a step-up transformer rarely totally fails with an open or short, but as I pointed out in this video can break down under load, or have some partially shorted windings. If you have an a multi meter check the coil primary and secondary windings. Smelling gas could mean the needle valve in the carb is stuck open and it's dumping raw gas straight into the cylinders. Is the distributor rotor turning? Do you have compression? If it's a V8 and has original timing set, if the Cam gear was Nylon toothed, and being that old the teeth could have all stripped off. Been building engines for 50 years, and have seen that more than once on old GM engines. Let me know what you find.

[@fitness_with_boobie5240]

@@mkdhdinfiniteendlessconten3873 yea I haven't checked under the cap yet cause it says it's all electric system but if it's points I'm get the good ol pertronix added but she starts right back up now after swapping the coil it the old hardly had any juice in it

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for getting back to me, and confirming it was the coil. Ya, Ive got Pertronix in several of my old school Oldsmobile's. They require straight 12 Volts at the coil and pickup in the distributor. Check that you have 12 volts. Older systems only had 12V at the coil for starting. Once the key went to Run position, the hot to the coil ran through a ballast resistor dropping the running voltage to about 9VDC on the hot side. Chryslers usually had a big ballast resistor on the firewall. Since yours sounds like it's electronic it might be 12VDC at the coil. It will actually say on the coil "Requires External Resistor". Glad it's going and that my video helped you solve your problem.

[@aolegario1]

Hotter is blue/purple, weaker is orange/yellow brah

[@mkdhdinfiniteendlessconten3873]

That seems logical and what I was expecting as I weld and a high voltage arc is generally blueish, but I saw the opposite ehen comparing them. My main goal was to educate that a weak coil will give fuel delivery symptoms, and it's so easy to swap a coil and rule it out to try that first. Thanks for watching and taking the time to comment.

[@whitecracker4012]

going to give this a shot on my 91 maxum

[@gregander1241]

what about spark advance? How do viewers know if he did'nt tinker with that?

[@mkdhdinfiniteendlessconten3873]

Hi. Not sure I understand your comment, but to verify timing and spark advance, just use a timing light and see where your initial timing is and total timing after increasing RPM. Try it with and w/o vacuum hooked up to the advance so you know how much of the timing is from the vacuum. Make sure and plug the vacuum line coming from the source. Probably should never have a total timing at high rpm over 36 degrees. Keep it under that. Again, not sure if I understood your comment. Hope my response was helpful. Also watch Thunderhead289 and Uncle Tony's Garage.

[@vmunoz90]

Spark may cause battery to explode from gasses causing injury or blindness find a better safer way imo

[@mkdhdinfiniteendlessconten3873]

Technically, yes a remote possibility so I'll give you credit for noting that. Being that it's a healthy battery not drawing current in a charging situation there would be no Hydrogen gas present, at best negligible ppm's with the hood open and a breeze blowing it's not a factor. If that coil's spark couldn't light an atomized compressed fuel mixture in the cylinder it's not going to ignite possible trace hydrogen in free air.

[@johnmiley458]

@@mkdhdinfiniteendlessconten3873 That is exactly right. I've seen many batteries explode. It takes certain conditions. . In fact In Vietnam I got qualified EOD. The conditions for a battery explode weren't there.

[@raymondrios7946]

on my 1978 f250 i can remove the spark wires from the distributor without getting a shock with the car on

[@mkdhdinfiniteendlessconten3873]

Hi. I get zapped every time on any car always. To avoid that I wear welding gloves and use insulated plug wire pulling pliers, and pre pull the wires at the cap and stick them back in loosely so I can pull the suspected ones quickly and easily. Otherwise, it's ZAP, for me anyhow. lol I use an infrared thermometer on the exhaust to find cooler cylinders and then know which wires to pull to see if there's a difference, then know where to check compression.

[@raymondrios7946]

sorry i did not finish asking the question Being able to do that will mean that I have spark problems Excuse me, I'm using a translator

[@j3ff3r]

How about if its heating up?

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. The coil will normally be warm, but not hot. If the coil is getting Hot, either the primary winding is developing shorts between windings, or depending on your year and setup, could be getting over voltage. Up until sometime in the 70's if I recall, most starting/ignition circuits put 12 volts to the coil with the key in the start position, and then in the run position the hot to the coil ran through a ballast resistor in one form or another dropping the voltage to 9 Volts to the Coil. My older GM cars have two wires to the coil hot side. One for Start and One For Run. If it's an older car many hands have been messing with it over the years. Often times people have rewired and run straight 12 to the coil not realizing this will burn up coils not intended for straight 12V. My 83 Ford takes straight 12. When I convert points distributors to Pertronix with a Flame Thrower Coil, they get straight 12V. Measure you voltage at the coil from Hot to ground (not across) and see what it is. Check the primary coil resistance with the wires disconnected from the Primary. If I recall, it should be 4-8 Ohms, but I need to check and confirm that value. What is the car/truck in question?

[@jecrpalier]

But good spark always blue. Bad yellow/orange

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Yes, I agree on the spark color. I must have been kicked in the head by a mule when I miss spoke on that. I will at some point do a better companion video with a drill driven distributor and good and bad coil to show that better. Fortunately the main point of the video to rule out the fuel system if you have these symptoms has solved the problem for numerous folks. It's a quick check and most folks that tinker have another coil in their stash. Coils can have shorted windings that will check good with an ohmmeter as the resistance range for "good" on both the primary and secondary windings is a wide range. They'll appear fine at idle and low rpms but under real loads and the higher frequency of a higher rpm will break down over 3,000 rpm and their high voltage output will significantly drop. Seems like a fuel starvation problem (which I also have a video for) but is actually the coil. Again, thanks for watching and taking the time to comment.

[@MikeB707]

Appreciate the effort but in all honestly, this method in no way helps anyone trying to learn how to properly diagnose a no start or stalling issue. Basically you had a theory and some luck on your side. At no time did you actually “test” anything. Fixing problems through a process of elimination doesn’t help anyone and can get pretty expensive if you’re just going to throw parts at it. Sorry if I’m being rude but there are way too many “mechanics” out there not knowing proper diagnostic methods.

[@mkdhdinfiniteendlessconten3873]

Well, I was waiting for you to make and upload the video with all the theory of how a coil works and how to check pointlessly check the coil primary and secondary with an ohmmeter which won't reveal a problem as it's a dynamic breakdown under load, show more testing than I already did, and also run fuel pressure checks, maybe even take the top of the carb and watch it run, etc, but looks like I'll have to make that video too since you haven't. lol Not only did I have a theory, I know the theory and teach it as an instructor with Master Mechanic and A&P Credentials with over 50 years of experience. There is no luck involved. When I replace a part, it's faulty. Period. The video exactly accomplishes what the Title and Description say. The point was to "Quickly Diagnose The Sneaky Weak Ignition Coil" which is exactly what the video does. I explained and demonstrated how similar and the subtle differences in fuel starvation and a coil breaking down under load. Knowing and recognizing those differences I didn't waste time on the fuel system. I have clear filters and can watch the fuel flow. I knew to grab the test coil I already have on hand and do a quick jumper of it to Quickly confirm it was indeed the coil breaking down under load which the only test is to swap it and confirm or rule it out. This is not "throwing parts at it". As for testing, I actually do show a spark comparison, but it's hard to see the difference on camera. Anyhow, after solving the problem in minutes, a few days later I thought it would be nice to make a video and help people out, which if you read the comments you'll see that it has. This video has been viewed just under 18K times in a year and from the people that took the time to leave a "Positive" comment and 226 Likes it certainly has solved this issue for those who took the time to comment. I'm sure there's more. So, you've given me a good idea for another video and since I don't want anyone disappointed I'll go ahead and make that video teaching/demonstrating/showing testing and theory. Maybe even give you a shout out. Stay tuned...

[@MikeB707]

I’m sorry if I pissed you off but my comment remains spot on. Having your video watched 18,000 times or 18,000,000 times doesn’t make it good content. It simply means you passed on your hackery to more unsuspecting people and created more hacks. If you’re in fact a “Master” / “instructor”, you should have taken the time to present a useful/ educational video. Now, because I pissed you off, you’re going to make a new video? That’s great. Just remember to delete the first one after you upload the new one. There’s already enough uneducated “back yard mechanics”out there watching unhelpful content such as yours. Let me know when the new video comes out. If it’s good, I’ll be the first to give it a thumbs up. Thanks for sharing your thoughts.@@mkdhdinfiniteendlessconten3873

[@acecraige6419]

Amazing,,,and,, so ,Borring,😅😅😅😅

[@kevinsummers9449]

Thnx brutha

[@mkdhdinfiniteendlessconten3873]

Hi. Thanks for watching and taking the time to comment. Glad the video helped you out.

[@jballs1118]

Great video sir i have a question i have a 65 mercury comet it starts and it stays idling it but when i start driving after about a block the car dies. Can you give me your opinion. I can't find the problem. I did the carb, plugs, fuel pump , starter, sileniod, distributor. Etc thanks

[@mkdhdinfiniteendlessconten3873]

Thanks for watching. I see you watched my Sneaky Weak Coil video which has solved probs for folks that had that problem, but yours sound more like fuel starvation from sediment in the tank. Since it's a 65, with most likely an original fuel tank and metal fuel line that's been in it since LBJ was president, it's probably full of fine rust that's clogging up any fuel filter and lines. I'd put a clear Glass cleanable fuel filter right before the pump and you'll probably see it fill up with rust. The filter should always be about half full with fuel trickling in. Your symptoms sound like fuel starvation. Filter clogs, engine dies, back flow pressure loosens up the crud and lets fuel come back in. It starts, runs a bit, clogs back up. I have some videos on diagnosing fuel starvation and float issues. Since it's a Comet it should have the same style tank as the Mustang with a big rubber filler neck in the trunk. Take the filler tube out and look down in the tank. Let me know what you find.