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Ignition coil or Icm?

Old 12-17-2018, 01:10 AM
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Ignition coil or Icm?

I am having a problem with my 90 Camaro with 305 tbi v8 again. Hopefully someone here can tell me if this sounds like the ignition coil or icm.
The engine will run smoothly but once warmed up the car will “miss” at idle periodicallt and when under load like in third or fourth under acceleration it will jerk. Also sometimes (only when warmed up) when you start the car it will fire and immeadiately die. Then you bump the key and it fires right up and runs fine. It will go 70 80 mph down the road just fine.

I replaced the cap and rotor with brand new one. Also bought brand new Delphi wires and platinum spark plugs. Do the ignition coils get weak after time or when warm or does this sound more like an icm problem? Car only has 98k miles on it.

i read online that a bad coil will cause all these issues,but I always thought coils usually just fail completely.
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Old 12-18-2018, 01:08 PM
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Re: Ignition coil or Icm?

Originally Posted by JimRockford View Post
I am having a problem with my 90 Camaro with 305 tbi v8 again. Hopefully someone here can tell me if this sounds like the ignition coil or icm.
The engine will run smoothly but once warmed up the car will “miss” at idle periodicallt and when under load like in third or fourth under acceleration it will jerk. Also sometimes (only when warmed up) when you start the car it will fire and immeadiately die. Then you bump the key and it fires right up and runs fine. It will go 70 80 mph down the road just fine.

I replaced the cap and rotor with brand new one. Also bought brand new Delphi wires and platinum spark plugs. Do the ignition coils get weak after time or when warm or does this sound more like an icm problem? Car only has 98k miles on it.
This could be a dozen different things, it wouldn't have to be the ignition coil or the ignition module. Testing the ignition coil is easy. Testing the module is not. I'd also want to know what the fuel pressure is, and I'd want to connect a scan tool so I could see the data stream.

Originally Posted by JimRockford View Post
i read online that a bad coil will cause all these issues,but I always thought coils usually just fail completely.
Not at all true. Ignition coils can become weak, typically due to faulty internal insulation. I've seen them so bad that the engine will idle, but not run above 2000 rpm under load.

Easily tested with a ~$15 spark tester calibrated for HEI ignition systems. This is the kind I prefer, but there are other styles.




HEI style has recessed center electrode to make a longer spark gap.
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Old 12-18-2018, 03:35 PM
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Re: Ignition coil or Icm?

small cap dist. Have you had a good look at the distributor under the rotor? The metal parts there are infamous for attempting to return to their natural state.

if it's all green/brown under there a new dist will include a new ICM and pick up coil. or it can be removed, cleaned and a new coil and ICM installed.

when the pick up coil fails it usually causes the tach to bounce along with the mis-fire. when the ICM fails it usually just won't run.

test your spark and inspect your distributor.
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Old 12-18-2018, 03:59 PM
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Re: Ignition coil or Icm?

The rust issue can't be overstated. I had a bunch of issues with my L98, until I realized the rust was causing some odd triggering. Replacing the distributor two years ago resolved the problem.
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Old 12-18-2018, 04:02 PM
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Re: Ignition coil or Icm?

Surprisingly, the car was still running with this mess...but badly.

Last edited by Galaxie500XL; 12-18-2018 at 04:18 PM.
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Old 12-19-2018, 12:37 AM
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Re: Ignition coil or Icm?

I can't say for sure that the following applies to f-bodies. My truck is a 1995 GMC Sierra 305 TBI.
Some rotors actually arc out from the rotating contact through the rotor's plastic to the center mounting post of the shaft.
This problem can drive you crazy trying to find it. You need an eye loupe magnifier to see the ultra fine arc line.
You said you replaced the cap & rotor, but how long ago - and with what quality?
This is just something else you might look at.
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Old 12-19-2018, 12:48 AM
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Re: Ignition coil or Icm?

Just get all new ignition components, quick and simple and you eliminate that as a factor. That's the first thing I would do.
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