Hi everyone, new to this forum, but i have a real stumper here. 02 3800na Bonneville. Possible relavant info: car had rad hoses replaced and one blew off dramatically. Customer claimed he had to "strong-arm" the car off the road, it quit, but didn't seem to overheat. Not sure if these cars would shut right down in an overheat situation? May have just been from the blast of moisture, not sure.
Repaired rad hoses and the car came back with erratic idle and missing problem. Dug around for quite a while and couldn't find anything apparent with ignition system or injectors. Was misfiring a lot on #3 and idle was not stable. After some time, i chose to do LIM gaskets, suspecting vacuum leak. This settled the idle down, but still misfired, mostly #3, #6 the odd time. Compression test, #3 was way low. Removed rockers, pressured up cylinder and found air escaping badly past intake valve. Seemed strange, but finally pulled head and touched up valves. Had a bit of buildup on intake valve at that time. Reassembled, pretty confident it would all be sorted out. Long test drive, seemed perfect.

Went to pick up customer, after about 40miles car started to misfire on cyl 6. No apparent reason, I can only guess fuel trim changed after the highway trip and leaned out to much for that cyl. Spent more time on it, found cyl 4 and 6 would miss at random times. Changed ign module, swapped plugs, wires, coil packs around. Couldn't put my finger on it. Found a vacuum leak at egr pipe, used orange silicon to seal it up, was leaking where it went into valve, i think i bent it a bit on intake removal. High hopes yet again, but still not fixed! I haven't swapped injectors around, but they seem to be fine as the car often runs very well. No pattern to which cyl will misfire, sometimes its 4, other times 6. It sure doesn't seem like a compression issue, but then #3 never missed a beat after the valve grind(hindsight, I shoulda pulled both heads, oops). I think I have about 130-150psi on rear cyls, not perfect, but not a cause for intermittent misfires i dont think. Any advice? I'm really stuck on this thing. To clarify, zero signs of overheating at any point from what i have seen.

One more note to add to my long winded post... This is only at idle. Car is fine at all throttle positions. About the same in park or gear. If its missing on #6 and i unplug the injector it runs worse.

You've done a lot of work trying to track down an intermittant misfire. Being that 6 seems to be a constant, check to ensure the O2 wiring isn't touching the plug wiring an vice versa. That'll mess with idle quick.

Next up is pull the EGR tube that goes from EGR to LIM. Pull back the retainers and hold it up to the light. Usually you'll get cracking there. BTW RTV isn't a good method for fixing these. They'll get exhaust pressures, which will blow the RTV from fender to fender. .

Give the brake booster check valve a test as well. Seems like everything else is done and you are looking for a vac leak, most likely.