A little back ground history before i get started :

Car was down for about a month until I put my money together to get the new tranny. Get new tranny in and find out there is a problem with the pcm. Get the new pcm and case learn has been completed and changed vin back to my actual vin number. Pcm came out of a 98 Buick regal gs also.

The Issue:

Any ways The problem Im having now is a p0300 code that randomly pops up. I watched the misfires on my scanner (hp tuner) and its only 3/6. Since the car sat for a while for the hell of it I change the plugs and wires which were only about 6 months old. Did not fix the problem. So then I change the coil which I thought fixed it but a few miles down the road it was back. Swapped out another coil just thinking maybe the spare one i had was bad but no luck. I even switched them around and misfire stay on 3/6.

The strangest thing is i dont hear a misfire at all and scanner will not show misfires while at idle. even when im driving normally i may see one or two on those cylinders but i dont feel it and it goes back to zero. If i get on it heavy no misfires at all in those two cylinders. When I do see a large amount of misfire is on the highway in 4th when the tcc is locked. just cruising It will jump to 30 and 40 misfires in those two cylinders but I dont feel it at all and if i give it more gas or get into no misfires at all. It basically only misfires like crazy at light load when im going 50mph + Once the code p0300 is present on the highway it disables the tcc which is pissing me off.

The only thing I havent done yet is change The ICM but I just figured either its bad or good and wouldnt be getting this intermittent problem.
There are no other codes present except for the p0300.

can anyone shed some light on this issue or experienced anything like this?

I've seen my share of bad ICMs hitting only one cylinder. But seeing how it'* effecting 2 on the same coil. I would suspect the terminals from the ICM to the coil pack. If these are corroded, it will give you a miss.

I'd suspect the coil

Swap with another and see if the issue follows. If yes..it'* a $20 coil. Then check ICM, etc etcd.

Bill, he swapped 2 coils and moved a set, still same cylinders.

Also as I went over it today I did two scans and noticed that i dont have misfires until decel then it goes nuts, 55 for #3 and the same or more for #6. and its only a decel from a high speed(65-70) that it goes that high. otherwise you might see a blip here and there on those two cylinders. And only when it goes in the 50'* and above for misfires in those two cylinders does it set the p0300 code and disable the tcc.


I will attach two hp tuner scans when I get home. I tried a ICM used from pull a part . At first it seemed to solve the problem then a few miles down the road it came back but not as bad. Im wondering if i could have got a bad one out of the yard geez. The post are very clean on the one i got from the yard but by the post on the side of the plastic its turned a bit white. just happens to be the same coil spot as the previous. I noticed it when i was putting it in but was on my way to work and didnt have time to go pull another. Just seems odd to have two with same problem.

Im going to try and get antoher one tomorrow and look over the harness and wires tonight. If this doesnt solve it I dont know what is next. JUst thinking that if it were the harness it would be more then just those two cylinders

This is a odd one. Generally misfire is spark related. But with it being on decel has me wondering if it'* leaning our. 3 and 6 do not share anything other then a coil. And it is possible you have a mechanical issue that just happens to be at the same time. So lets step back and look at what you have done and remove that from the list of causes.

Bad spark plug. #3 is good for oil dripping on it. You changed these right?
Plug wires. Again, you changed these.
Coil, you changed and swapped
ICM, you swapped.
Injectors. Might want to try moving them around, it'* really not that hard.
injector plug. Inspect them
Chipped piston, need a bore scope to look inside the cylinder. Every had a damaged plug?
Broken valve spring. Seen this one more then once but only on after market springs.
Burnt valve. Compression test

Seeing you have HPT, have you done a CASE learn? If the crank sensor or PCM has been changed you must do a CASE learn.

Thanks for the quick replies. of what you listed the following 3 i havent tried yet

Injectors. Might want to try moving them around, it'* really not that hard.
injector plug. Inspect them
Chipped piston, need a bore scope to look inside the cylinder. Every had a damaged plug?


I changed the plugs saturday trying to fix this issue and I didnt see any damage to any of the plugs they actually looked very good and were only about .005 or so out from when I changed them in may. But I replaced them anyways along with wires again.

Im going to do a fuel pressure test at the rail and borrow a scope from work along with the things you mentioned. This is a very weird one for me and with all the searching I have not come across anyone with a issue that matches mine. My mom always said I was special but I dont think this is what she had in mind lol.

Take your HPT and run a cylinder balance test. See if those 2 cylinders have a bigger effect on the test.

So I did all the checks except for checking for a chipped piston and everything looked good. swapped injectors no difference. I did however take the original ICM to oreilys to be tested and it failed. The two test it failed were bypass deviation and reference pip. I also did cylinder balance test and everything looked great. I got another ICM from the yard this morning and after work Im going to have it tested and try it. I just find it hard to believe that the one I pulled yesterday would show the same problem just not as bad. Im attaching a picture of the balance test and still have to get those scans up but was crazy busy after work yesterday.

The balance test looked great, #4 is just working a little bit harder but nothing to even worry about. A ICM test will only show a ICM is bad, it can't show it'* not bad. Seen several test as good but are still bad.

have you done a CASE learn?