Is there any common issues on the 3800 that would cause a misfire only after warmup? Im getting a random misfire code with the misfires only coming from #4 and #6. When its happening, I can feel it misfire once or twice with each revolution of the engine, very rhythmic. What should I focus on first? It gets worse with more throttle.

 

I've seen that happen with coil packs before, but that clearly can't be the issue here as theyre on separate coils.

They're adjacent cylinders- so how are the gaskets? Any sings of a crack / leak at the UIM/LIM?

 

Even though it "shouldn't" be the coils, I have seen where one tower on a coil gets weird. You could swap the wires on the same coils just to see if the mis-firing cylinders change (if they do, I would replace the coils). Another possibility is the ICM. I want to say my mom's LeSabre did the same thing before the ICM took a dump. For me the ICM failed hard (while working on diagnosing the problem in my driveway, it got to a "no start" situation).

 

No leaks and no loss of fluid that would indicate a more severe leak.

It does have some blue no-name coil pack for #6 and #4 has been changed also. Kinda looks like the previous owner was trying to diagnose this as well. I dont think it could be coil packs because why would both of them wait till its warmed up to start malfunctioning? Definitely going to test the ICM tho.

Its strange. We first thought this whole misfire problem was due to a bad plug wire on #6 which is where the majority of the misfires occur. It was grounding on the exhaust manifold. Changed it and the car ran perfect for a few drives, maybe 50 miles. Then out of nowhere it started that quick, THUMP THUMP THUMP at idle and wanting to sputter with throttle.

Then I noticed I couldnt go above 30mph and there was a whistle that that increased pitch and volume with RPM. Also had a horrible burning smell. Cat was broken inside and blocking the exhaust. Replaced that and the car ran perfect again for a while. Now its back to thumping and sputtering.

Maybe its better if I find a nice steep cliff to drive this thing off of. Forgot to mention, this is my Firebird. It has the series 3.

Could a failed 1st o2 sensor cause this? I unplugged the downstream o2 and it seemed to improve a bit...

 

So, I believe the f-body has 2 upstream O2 sensors and a single downstream. I am pretty sure the up streams both handle fueling. But I would think you would have an entire side of the engine mis-firing.

As for the coils, I have seen where a car was unlucky enough to have two failed coils (was a 3100 grand am). If you have another 3800/3100/3400 powered car and want to 100% rule out coils, swap them. GM used the same coils on all of those (even some NorthStar Cadillac v8's and 4 cylinder Cavalier). The L67 to my knowledge is the ONLY one with a coil difference.

I'll ask a pretty obvious question, but any codes other than the ones for the mis-firing cylinders?

 

Oh there IS 3 o2 sensors huh? One of them is reading 0 volts then. I thought it was a mistake with my scanner.

There are no codes other than an EGR code but the EGR is no longer there, I blocked it off. The SES light will start flashing sometimes but still doesnt throw a code.

A couple months ago I was getting a random misfire code but that has since not come back after a shop "fixed it".

Then we were told the whole issue was caused by a bad trans. I knew the trans was going out so we had it replaced. Still didnt fix it.

..And of course I have another 3800 powered car. Ill swap all the coils later but after you mentioned the 3 o2 sensors and having that one not function, I think im going to replace that and see what happens.

Wish me luck cause the next course of action to fix this is the cliff method.

 

Kind of sounds like a fueling issue to me. While the car is in open loop, sounds like it's working well but when it goes closed loop and uses data to adjust fueling, you are having issues?

 

Thats how it acts most of the time. Sometimes its missing right at start up after sitting for a day or two. That one o2 is reading 0 volts tho. Sometimes ill get a flicker of .1-.4. Also, looking more closely I see that all cylinders miss except number 2 I think. Its just all of them other than 4 and 6 are very rare.

 

ICM can cause weird failures, check the ICM harness also. Sounds like you're on the right track with swapping coils out to see if it helps.

 

Definitely get the O2 situation resolved. Perhaps that is the root cause.