Any DEFINITIVE way to test a bad ignition coil WITHOUT a CEL? (1 Viewer)

Joined
Jan 14, 2019
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160
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St. Louis, Missouri
My truck (2000, 243K) has been suffering an intermittent misfire (I think) for about the past week. However, it has thrown no code. It tends to be worse in the morning or otherwise on a cold engine. Every couple of seconds, I get a small "bump" sensation (similar to the AC compressor kicking on). When cold, this will be followed by a noticeable pop in the exhaust, almost like a tiny backfire. This is worse when in gear with the brake depressed. I'll also have some roughness when first accelerating from a stop, or when transitioning from coast or light throttle to heavier throttle. At highway speeds, or with more aggressive throttle, it is not noticeable at all. When hot, it might idle for a minute or more perfectly normally, and then start bumping again.

I've no CEL and scanned it multiple times, confirming no code.

I did inspect all the coils and, based on physical condition alone, I have three suspects. One is a OE Toyota coil that might be original to the truck. The other two are no-name replacements, one of which also appears to be quite old. The other five are either newish Toyota coils or newish Denso coils (one of which I replaced myself in the last two months), so chances are those are not my problem. Plugs have all been changed within the last two months.

I don't like throwing money and parts at a problem without proper diagnosis, but I might just order four Densos from Amazon, replace all three, keep a spare, and chalk it up to preventative maintenance. However, I am wondering if there is a way to definitively test a possibly faulty coil if no CEL is present.
 
Joined
Aug 18, 2019
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690
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St Louis, MO
There is definite way of finding weak coil. I used it to find bad coil in Audi UrS4 when it was misfiring under high boost.


Set gap around 3/8 and check each coil. It should spark without missing. If you don't find one that misses - increase gap to 1/2 or so and go round again.

Weak coil(s) will skip spark and you will see it for sure.
 
Joined
Jan 14, 2019
Messages
160
Location
St. Louis, Missouri
There is definite way of finding weak coil. I used it to find bad coil in Audi UrS4 when it was misfiring under high boost.


Set gap around 3/8 and check each coil. It should spark without missing. If you don't find one that misses - increase gap to 1/2 or so and go round again.

Weak coil(s) will skip spark and you will see it for sure.
Thanks!
 
Joined
Aug 18, 2019
Messages
690
Location
St Louis, MO
Btw, you may find that all of the coils different, some stronger, some weaker.

You don't need to take spark plugs out, but do want to disconnect injectors, I'm not even sure how to do it in LC. Ideally I would remove fuel pump fuse and run truck until it dies, but there is no such fuse in LC, right?

Then just crank using helper for couple cranks and look for spark, keep battery charged so you get about same RPM

I would put stronger coils on a back, under airbox, in those harder to reach spots as they most likely will live longer.
 
Joined
Mar 27, 2012
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1,031
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Garner, NC
Your description is spot on for a failing coil. I don't have a technique to diagnose it, but I would go ahead and get a new coil and get ready to change it out. Just have a code reader & some basic tools handy and get ready because it's only a matter of a little while before it gives up.

BTW, I've driven 70+ miles on a failed coil when my mother was hospitalized and I was leaving the hospital after business hours, so I had to wait until the next day to get a new coil. So, while it's inconvenient and annoying, it won't leave you stranded.
 
Joined
Jan 14, 2019
Messages
160
Location
St. Louis, Missouri
Your description is spot on for a failing coil. I don't have a technique to diagnose it, but I would go ahead and get a new coil and get ready to change it out. Just have a code reader & some basic tools handy and get ready because it's only a matter of a little while before it gives up.

BTW, I've driven 70+ miles on a failed coil when my mother was hospitalized and I was leaving the hospital after business hours, so I had to wait until the next day to get a new coil. So, while it's inconvenient and annoying, it won't leave you stranded.
True... It's funny, I replaced one about 2 months ago. It failed suddenly, completely, and without warning. I was sitting a stop light, the engine stumbled, the CEL came on, and I drove home on 7 cylinders. Pulled the code and replaced the coil.. Interestingly, that was an OE Toyota coil that looked pretty new (I still have it on my workbench).
 
Joined
Apr 14, 2015
Messages
376
Location
Tallahassee, FL
When mine were failing I would keep a new coil in the vehicle, along with a code checker and basic socket set (10mm is all that's needed). I'd deal with the stupid hesitation until it finally threw a code, then check and replace it as soon as possible (usually in a parking lot somewhere).

I did this for all 8, and now I carry another spare cheapie coil in the vehicle just in case.
 
Joined
Nov 4, 2007
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10,567
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Colorado
Very easy to find with tech stream. You simple watch live data!

The coil test is interesting idea. But much more time consuming.
 

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