Chevy 6.0L rough idle and misfire codes

blazerbrad · 10-31-2018, 12:35 PM

My 2500HD 6.0L suddenly developed a rough idle and random misfire codes (CEL will start flashing). Just started trouble-shooting but any suggestions that anybody has based on previous experience will help.

2003 with 148k on the clock. Don't drive it that much but started it up last Saturday to run into town to fill up the 5 gallon gas cans and topped off the truck fuel tank. I heard what sounded like the engine belt chirp a few times but everything else was fine. Sunday I started it up and had it idling in the driveway while I pulled my trailer around from behind the garage with the tractor. When I pulled up to it you could visually see it was running rough. Hopped in and ran down the street and back and it had a bad bog/hesitation at first and then cleared up. Pulled back in driveway and turned off.

-Bad fuel has crossed my mind, but it came from a heavily used station that I regularly fill up at.
-Looked at the ignition parts and nothing was obviously wrong or loose.
-Found one clamp on the air intake loose (but ducting was still snug) so tightened it up and no change.
-Pulled connector off from each injector and seemed like the engine tone changed with each one, so not an obvious single cylinder issue.
-I have had sporadic MAF codes over the last year or so but no obvious change in performance. I had cleaned the MAF (yes, the proper way with just spraying it with cleaner) awhile back and the code would still pop up.
-Knowing that a bad MAF can cause a rough idle, there have been several codes in the past for it, and it can trigger the random misfire code because of the rough idle, I installed a new MAF.
-No change as it still idles rough.
-Took a short test drive and at medium to heavy throttle is seems to run fine but when you let off the gas it starts running real rough and bogging down for a second.
-Checked the fuel regulator at the rails (typical failure mode is you find fuel in the vacuum line)....no obvious issue

Next steps are to spray carb cleaner around the intake to look for a bad intake gasket or other vacuum leak. Pull the connector at each coil pack and see if I can pinpoint a single cylinder again (versus pulling the injector connector). Start pulling the plug wires and plugs to look for obvious issues. I don't have a setup to check fuel pressure but that should be on the list also.

Any other common problems on this setup that anybody would suggest checking?

blazerbrad · 11-05-2018, 07:56 AM

Got it figured out this weekend. Quick recap of the symptoms...runs rough with an obvious miss at idle, runs okay at mid to full throttle, P0300 Random misfire codes.

It seemed to run okay after a cold start and running in open loop (default sensor values) and the miss only started after warming up and going into closed loop (starts reading O2 values and such). Did another cylinder balance test by disconnecting the coil pack at each cylinder. Disconnecting each of the drivers side made it run worse so they were working, but none of the passenger side bank seemed to make any difference. Watched the O2 sensor on the passenger bank and it wasn't responding the same as the driver's bank. Pulled the passenger side plugs and they were all black and carboned up by running rich.

Basically the passenger side O2 was giving a false lean signal and thus dumping a bunch of fuel which fouled out the plugs at idle, but again no O2 sensor code. Swapped in a new O2 sensor on that side and went ahead and put a new set of plugs in and now runs fine. Plan on swapping out the drivers side O2 also but the parts store only had one in stock.

10-31-2018, 12:35 PM	#1
blazerbrad - CORE Member - Join Date: Feb 2007 Posts: 831 Images: 6	Chevy 6.0L rough idle and misfire codes My 2500HD 6.0L suddenly developed a rough idle and random misfire codes (CEL will start flashing). Just started trouble-shooting but any suggestions that anybody has based on previous experience will help. 2003 with 148k on the clock. Don't drive it that much but started it up last Saturday to run into town to fill up the 5 gallon gas cans and topped off the truck fuel tank. I heard what sounded like the engine belt chirp a few times but everything else was fine. Sunday I started it up and had it idling in the driveway while I pulled my trailer around from behind the garage with the tractor. When I pulled up to it you could visually see it was running rough. Hopped in and ran down the street and back and it had a bad bog/hesitation at first and then cleared up. Pulled back in driveway and turned off. -Bad fuel has crossed my mind, but it came from a heavily used station that I regularly fill up at. -Looked at the ignition parts and nothing was obviously wrong or loose. -Found one clamp on the air intake loose (but ducting was still snug) so tightened it up and no change. -Pulled connector off from each injector and seemed like the engine tone changed with each one, so not an obvious single cylinder issue. -I have had sporadic MAF codes over the last year or so but no obvious change in performance. I had cleaned the MAF (yes, the proper way with just spraying it with cleaner) awhile back and the code would still pop up. -Knowing that a bad MAF can cause a rough idle, there have been several codes in the past for it, and it can trigger the random misfire code because of the rough idle, I installed a new MAF. -No change as it still idles rough. -Took a short test drive and at medium to heavy throttle is seems to run fine but when you let off the gas it starts running real rough and bogging down for a second. -Checked the fuel regulator at the rails (typical failure mode is you find fuel in the vacuum line)....no obvious issue Next steps are to spray carb cleaner around the intake to look for a bad intake gasket or other vacuum leak. Pull the connector at each coil pack and see if I can pinpoint a single cylinder again (versus pulling the injector connector). Start pulling the plug wires and plugs to look for obvious issues. I don't have a setup to check fuel pressure but that should be on the list also. Any other common problems on this setup that anybody would suggest checking?

11-05-2018, 07:56 AM	#2
blazerbrad - CORE Member - Join Date: Feb 2007 Posts: 831 Images: 6	Got it figured out this weekend. Quick recap of the symptoms...runs rough with an obvious miss at idle, runs okay at mid to full throttle, P0300 Random misfire codes. It seemed to run okay after a cold start and running in open loop (default sensor values) and the miss only started after warming up and going into closed loop (starts reading O2 values and such). Did another cylinder balance test by disconnecting the coil pack at each cylinder. Disconnecting each of the drivers side made it run worse so they were working, but none of the passenger side bank seemed to make any difference. Watched the O2 sensor on the passenger bank and it wasn't responding the same as the driver's bank. Pulled the passenger side plugs and they were all black and carboned up by running rich. Basically the passenger side O2 was giving a false lean signal and thus dumping a bunch of fuel which fouled out the plugs at idle, but again no O2 sensor code. Swapped in a new O2 sensor on that side and went ahead and put a new set of plugs in and now runs fine. Plan on swapping out the drivers side O2 also but the parts store only had one in stock.