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Son said the the old Bonneville was misfiring.
I got in the car the next morning and it was fine.
Until it warmed up.
Finally triggered a Service Engine Soon light.

Stopped by my friends shop and hooked up his Snap On Scanner. Only code was P0300.

So I went to cylinder contribution test in the snap on computer and cylinder 3 and 6 were misfiring.

Swapped coil packs around. No changes.

We put this motor in awhile back. When we did. New plugs and wires and all that stuff.

I'm trying to think what would cause 2 different cylinders to misfire.

Tomorrow I'm gonna pull plug 3 and see if Insee any coolant. It hasn't skipped long enough to lose coolant but I have a suspicion it may be the upper plenum.

Not sure how to test the Ignition Control Modules on the car. Never had any issues with one.

Any ideas where to start?
Thanks

3 and 6 are batch fired at the same time off of the same coil, so I would check ignition components, primarily coil and ICM.

As said above, the same coil fires those cylinders. Move the coil around to see if the misfire follows it. If not, I'd look at the ICM.

I am at my friends shop right now. Unfortunately, he doesn't have a lift open.
I can hear a knocking sound that changes with acceleration while in park. It sounds like its beneath the car.

Sometimes its in sync with the engine other times its not.

When I go to the passenger side of the car lay down and look beneath. It sounds like its coming from the catalytic converter.

I suppose it could be coming from the rear of the engine but most definitely sounds like it coming from the cat. The smell from the exhaust isnt your typical rotten egg smell .Hard to describe the smell unless the cat is somehow causing 3 and 6 to misfire and thats what im smelling.

UPDATE but not fixed yet.

I tested the ICM. With one coil pack off. The bottom spade should have 12 volts with Key on Engine off. The upper blade will only throw 12 volts when the engine is cranking. So it appears the ICM is fine as far as telling the coil pack to send a signal to the coil and supplying power.

I pulled #3 and #6 plugs. They are AC Delco. Gapped properly. A very good dry look not wet or soiled. I ran a camera in through the spark plug hole and no signs of coolant getting in there. (Ive had this happen on a 95 Buick Park Ave, hydrolocking the engine.)

Again under the car, as far as I can get with ramps. It still sounds like a knocking type sound from the catalytic converter. I cant pinpoint it if its actually farther back.

With the hood open you cant hear it. It also may me the misfire causing the exhaust to rattle around.

With the engine off. There was this little ticking sound very very light. I tracked it down to something that's plugged into the throttle body. I unplugged it and it stopped. Plugged it back in and it started back ticking again. Key off engine off and you have to put your ear near it. May be a normal thing. Wouldve never paid it any attention if it was running normal.

Sorry guys, I swapped the coils around also. No change on cylinder contribution test. Even installed a good used coil I had lying around

https://easyautodiagnostics.com/gm/3.8L ... ck-tests-1

I would test both coil and ecm

UPDATE FIXED

I don't know how or why but replacing the Ignition Control Module fixed it.

I had tested it every way I could with info found online and it passed.

I live near a junkyard and they let me borrow one and it fixed even with my coils on it.

Thanks for the replies and the help.