2004 Express 3500 with a 6.0 liter 100,000 miles. The engine misfires on cylinders 2, 4, 6 and 8 when it gets warm after driving it for a little while and of course the check engine light comes on. If you let the van sit overnight, the problem goes away and it drives fine for a short while until the engine is hot. Our GM mechanic says the engine in running lean, (computer code shows this), but he has no idea why and he has no idea why it is misfiring after the engine is hot. He let the van idle for almost 4 hours plus he drove it several times but could not duplicate the problem, (even though this has happened to us several times and we had to be towed 80 miles home). He also says we have good fuel pressure, at lease while he is observing it.
So far, we have had to replace the cat and the muffler. We believe the cause for the cat failure was due to the engine running lean.
This is so frustrating because we really love this van and use it for towing. We would hate to get rid of it. Any ideas would be grateful.

 

Update.
Mechanic performed compression check, good.
Replaced spark plugs.
Replaced wires.
Still happens after driving the van about every 40-75 miles. Engine code is P0300.

 

Does this car have a distributor or a ignition coil pack? A coil pack could cause that kind of problem if it was bad.
Or you could have an exposed wire on the fuel rail for the injectors. This is multiport fuel injection right? I dont know cylinder numbering for Chevys but if all the evens or odds are on one head a fuel rail that is grounded to something or touching something when its hot could cause the FIs not to fire for lack of signal.
Clogged injectors could give you a lean because not enough fuel is sprayed or the jet pattern is wrong and that could cause a misfire. With 100K you are up there. Some guys recommend that SeaFoam.

HAve your guy check for FI firing pulse using NOID lights or it the coil pack is bad.

P0300 is just random misfire no specific cylinder.

 

Check the main wiring harness that goes over the drivers side upper a-arm. Mine had the same kind of issue, I spend forever finding it. The harness sits on the a-arm and rubs. Hope this helps.

 

Thanks for the response to my question. I wanted to let everyone know the final verdict on the van. We just now got it fixed, (I know it's been a while). We took it to a highly recommended local shop a couple of weeks ago and this is what they found.
1) Intake manifild gasket was leaking above #6 cylinder.
2) Injectors were in bad need of cleaning.
3) The passenger side O2 sendors were not talking to the computer.
They fixed all of that and now the van runs great! Took it on our first road trip and we couldn't be happier. Again thanks for the responses.

 

I think #1 was the real problem and 2 and 3 were just make money items.

 

I would believe it to be insulation breakdown on your coils all on bank 2 right side which is common for some or inevitably all to fail at some point. You could try switching the coils from side to side and see if problems now are on cylinders 1357. Let us know

 

@Colin
Look at the date that was posted.