Misfire after working on engine

I replaced the oil pressure switch on an '07 Suburban 5.3L because the oil pressure was showing low once the engine warmed up. That did nothing, then I found out about that little filter/screen under the sensor. I intended to remove and clean the filter then reinstall it. While I was pulling it out, I dropped it back behind the engine and couldn't find it. I checked online and found a few people in other forums that said to just remove the screen as a "fix", so I left it out and put the sensor back in, just to get the vehicle drivable. It started right up and ran fine for me, parked it, and a after sitting for a few hours it started popping while driving with the SES light flashing on the drive home.

Code P0300 was set, no cylinder specific misfire codes were present. I checked the misfire data and cylinder #6 showed tens of thousands of misfires, #5 showed several hundred, but I think it was just picking them up from #6 since #5 is right after #6 in the firing order. I disabled each injector with a scan tool, all changed the idle except #6, it did absolutely nothing. #6 fuel injector had good pulse to it, I connected a fuel pressure gauge and put power/ground to injectors #6 and #4, both dropped the same fuel pressure. I did the "cylinder deactivation" test on the scan tool for the four cylinders that have it, they all did the exact same thing - the engine bogged way down for a couple seconds and then died. Is that normal? I can't say I've ever seen a V8 engine die before when running on 6 or 7 cylinders. I checked compression on cylinder #6, it showed very low, almost zero, but I tried it a few times and it would show just a little compression at times... valves staying shut? I'm thinking when I opened the pressure relief valve on the gauge and the piston was near the bottom of the cylinder may be when it showed some compression. I also switched the coil/wire/plug from #6 to other cylinders and the miss did not follow.

Could having that screen out cause this problem? Having the engine die when deactivating the cylinders makes me wonder if maybe it's commanding all four at the same time, although the test shows each cylinder being controlled separately. Using a MODIS, not a TechII.

I just got done putting in a new filter/screen to see if it would make any difference, and of course it didn't.

Thanks,
Jason

I replaced the oil pressure switch on an '07 Suburban 5.3L because the oil pressure was showing low once the engine warmed up. That did nothing, then I found out about that little filter/screen under the sensor. I intended to remove and clean the filter then reinstall it. While I was pulling it out, I dropped it back behind the engine and couldn't find it. I checked online and found a few people in other forums that said to just remove the screen as a "fix", so I left it out and put the sensor back in, just to get the vehicle drivable. It started right up and ran fine for me, parked it, and a after sitting for a few hours it started popping while driving with the SES light flashing on the drive home.

Code P0300 was set, no cylinder specific misfire codes were present. I checked the misfire data and cylinder #6 showed tens of thousands of misfires, #5 showed several hundred, but I think it was just picking them up from #6 since #5 is right after #6 in the firing order. I disabled each injector with a scan tool, all changed the idle except #6, it did absolutely nothing. #6 fuel injector had good pulse to it, I connected a fuel pressure gauge and put power/ground to injectors #6 and #4, both dropped the same fuel pressure. I did the "cylinder deactivation" test on the scan tool for the four cylinders that have it, they all did the exact same thing - the engine bogged way down for a couple seconds and then died. Is that normal? I can't say I've ever seen a V8 engine die before when running on 6 or 7 cylinders. I checked compression on cylinder #6, it showed very low, almost zero, but I tried it a few times and it would show just a little compression at times... valves staying shut? I'm thinking when I opened the pressure relief valve on the gauge and the piston was near the bottom of the cylinder may be when it showed some compression. I also switched the coil/wire/plug from #6 to other cylinders and the miss did not follow.

Could having that screen out cause this problem? Having the engine die when deactivating the cylinders makes me wonder if maybe it's commanding all four at the same time, although the test shows each cylinder being controlled separately. Using a MODIS, not a TechII.

I just got done putting in a new filter/screen to see if it would make any difference, and of course it didn't.

Thanks,
Jason

this issue sounds like the GM defective AFM engine failures. you are not alone. many have had this type failure. oil does not flow properly then this type failure .

what was the engines oil consumption per 1K miles ? what oil are you using ? dexos spec ? new owner ?

this issue sounds like the GM defective AFM engine failures. you are not alone. many have had this type failure. oil does not flow properly then this type failure .

what was the engines oil consumption per 1K miles ? what oil are you using ? dexos spec ? new owner ?

It's a friend's truck, IIRC he said it uses a couple quarts each oil change, don't remember how frequently he changes it. He uses Royal Purple oil. What exactly is the failure? Does it affect just one cylinder?

OK... got it all figured out. Sorry about the stupid "what is the AFM failure" question. I've worked on hundreds of vehicles with AFM engines and not seen one have this failure, my first assumption was that it was some rare problem. A quick search brought me up to speed on the subject.

An answer, for anyone that finds this thread: The #6 exhaust lifter was stuck in collapsed mode. We pulled the valve cover off and started the engine to determine if it might just be a stuck solenoid or a lifter. The intake rocker arm was moving, so we pulled the head off, replaced the lifter and put it back together.

Another question though... there is a filter/screen before each solenoid with a finer mesh than the AFM filter under the oil pressure sensor, all were carboned up, but none plugged and no chunks. I'm pretty sure that having the AFM filter out couldn't have caused this, is that correct? Just amazingly bad timing for the lifter to fail?