Coil going bad... how to tell which one?

M

M Darrah

Registered
I've got an intermittent miss on my 98 that only happens when the engine is hot. At first I thought it was just a bad boot, but every time I've had a bad boot in the past, the cylinder just misses completely. This is just an occasional miss. All of the boots had been replaced maybe three years ago. Furthermore, we pulled all of the coils out and the boots all look new. I couldn't see a single crack or problem of any kind. However, two or three coils themselves have hairline cracks along the side of the raised "rib" that sits under the connector. So I'm guessing I have a bad coil and not just a bad boot. Five of the coils appear to be original, three have been replaced at one point (all three newer ones are on the driver's side).

Is there any way to tell which coil is bad? Can I leave the cover off of the coils and look for sparks at night? I really don't want to replace all of the coils right now.
 
If you have a code reader, pull the codes...it should tell you which cylinder is missing....

1-2-3-4 on passenger side ---- 5-6-7-8 on drivers side. BTW...if no code readre, Auto Zone does it at NC!
 
budpytko said:
If you have a code reader, pull the codes...it should tell you which cylinder is missing....

1-2-3-4 on passenger side ---- 5-6-7-8 on drivers side. BTW...if no code readre, Auto Zone does it at NC!
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Not always... You should disconnect the coil and see if it makes the idle worse... if it doesn't then that is your trouble maker.
 
Mark, without getting shocked or anything else, you can disconnect the primary wire to each coil individually while the car is running. That should help you isolate the gremlin. I have a history of bad insulators, most of which look brand new.
 
whiplash15 said:
Mark, without getting shocked or anything else, you can disconnect the primary wire to each coil individually while the car is running. That should help you isolate the gremlin. I have a history of bad insulators, most of which look brand new.
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Isn't that what I said :p
 
Well, if you have a cracked case on two or three coils, you really should replace them. If they're not bad already, it's only a matter of time for them to short to ground and/or let moisture in.

And guys, what ever happened to resistance checks? Measure the resistance of both the primary and secondary circuits on each coil and compare to specs. If you're out of range, there's your problem.

Disconnecting an individual coil won't work all of the time either, being that this is an intermittent problem. The idle could get worse by disconnecting each coil individually and you could still have a bad coil.

By the way, there's no primary wire to be disconnected. The primary windings are inside the coil. The wires outside of them are 12v power and PCM-controlled ground/trigger, which power the primary windings.
 
AssFoe said:
And guys, what ever happened to resistance checks? Measure the resistance of both the primary and secondary circuits on each coil and compare to specs. If you're out of range, there's your problem.
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That would require removing parts....lol! Why not just unplug the electrical connector to each coil one at a time to see which one doesn't make the car run worse? :)

AssFoe said:
Disconnecting an individual coil won't work all of the time either, being that this is an intermittent problem.
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This is true.

AssFoe said:
By the way, there's no primary wire to be disconnected. The primary windings are inside the coil.
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I think he was referring to a Gen 1?
 
Well, I bought exactly ONE coil and swapped it out in each cylinder. No joy. Could I have 2 (or more) bad ones? As for the cracks, they're very hard to see, and seem to be only on the surface of the coil itself, not the boot. Can such a tiny hairline crack cause misfires?

I had a friend's (Trixie's) son hook their code reader up and it returned no codes at all. Would Autozone's be able to get codes that a home reader wouldn't catch?

I guess my next step is to replace the plugs themselves. I've got a set of 764s that I've never installed. Am I correct that the gap should be .054, torqued at around 130 in/lb?

If that doesn't do the trick, I guess I'll just take the damn thing to Pep Boys and have them tell me what's wrong with it ($84 diagnostic fee) and then drive it back to Trixie's and do it myself. I don't have my own garage so tinkering around with it for hours isn't an option.
 
Well...you could have damaged one of the boots when removing the coils. Did you check for tears/cracks in the boots?

A spark plug usually causes a CONSTANT misfire where as a coil/boot/wire will usually give you a misfire only under load.
 
J said:
Well...you could have damaged one of the boots when removing the coils. Did you check for tears/cracks in the boots?

A spark plug usually causes a CONSTANT misfire where as a coil/boot/wire will usually give you a misfire only under load.
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Yeah I checked all the boots, and they all look good. They're all soft and pliable and no cracks that I could see.
 
M Darrah said:
Yeah I checked all the boots, and they all look good. They're all soft and pliable and no cracks that I could see.
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Soft has little to do with anything. Mine were all soft and the car only had 40k miles on em... I am not saying yours are cracked but you actually need to bend the boots in either direction a bit to surface any hairline cracks that may not be visible otherwise.

That said it sounds like you need new coils ;)
 
maxmk8 said:
Soft has little to do with anything. Mine were all soft and the car only had 40k miles on em... I am not saying yours are cracked but you actually need to bend the boots in either direction a bit to surface any hairline cracks that may not be visible otherwise.

That said it sounds like you need new coils ;)
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I did that (bent the boots), not one of them had a crack.

I'm just not very keen on spending $400 on 8 coils.
 
I have 8 used coils with 47K miles on them if you're interested in buying a few.

Also...you didn't say if you have a constant miss or only under load.
 
It seemed to be only under load, from what I heard/understand.

What about water/oil in the plug area? I could have sworn I had the only under load issue when I had water in the back plug?

The boots really did look fine.
 
J said:
I have 8 used coils with 47K miles on them if you're interested in buying a few.

Also...you didn't say if you have a constant miss or only under load.
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It's very intermittent, and mostly under load. When idling in neutral, it runs smooth. I'll get a few misses (maybe once every five to ten seconds) when I put it in gear, but it's worst when accelerating. And ONLY after the engine gets hot. It usually runs fine when cold.

Three coils have been replaced over the years (before I learned you could get the boots separately), the other five appear to be original. ALL of the boots were replaced about three years ago, which works out to about 25-30,000 miles (in the six years I've owned it, I've put less than 50,000 miles on it).

Before I bite the bullet and buy new coils, is there anything else that can have this effect? MAF? Dirty injectors? If I end up spending $250 on five new coils I'm gonna be one pissed mofo if it doesn't fix it.
 
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Generic code readers may not list all codes. Each manufacturer has their own specific codes - some for specific models. If you've got a miss under load, chances are it will cause a code but it may not be a generic code.

You can let it get worse and it will eventually throw a code but likely not tell you which cylinder - so your best bet would be to find a FLM dealer who would be willing to run diagnostics and tell you the results.

Around here it's easy to spot a miss under load because of the hilly terrain. In the flatlands it may not be so noticeable except under heavy throttle? I suspect if there are hairline cracks, that's your problem. Any water or oil in the plug wells?
 
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