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o2, mass air, misfire codes after new exhaust...help!!

6K views 17 replies 4 participants last post by  CJ_03_2500HD 
#1 ·
I have a 2003 2500HD 6.0 automatic non allison trans. I got exhaust put on at my local exhaust shop last saturday, the jackass who put a new cat on the truck a few years back shoved exhaust pipe inside the cat hitting the o2 sensor and damaging it instead of clamping it or welding it the right way. I got two new cats, pipe, and muffler from the cats back. The OBD2 showed codes for the o2 sensor downstream (which was the bad one) and cat codes. I cleared them on my tuner when I got home which was 5 minutes from the shop. I realized the truck was running a little rough since the hole for the o2 sensor was plugged until I could get one for half the price they wanted and I could put in myself. I had to order the sensor so it took a few days but it only took a day for the truck to run bad enough I didn't drive it for a week. I put the new sensor in, new plugs, and changed the oil yesterday, it still ran terrible. Today I bought new wires since the codes were still there, i also changed out coils 6 & 8 because my friends scanner said they were the two misfiring even after the new plugs and wires. Needless to say its still running bad and i put the tune I originally had back on and it runs better and it showed there were no misfires as it was running but o2 sensor codes for all 4 were showing and mass air code. I cleared the codes and drove it about 3 miles and plugged it back in after and all the codes were back AND about 4 cylinders were taking turns misfiring... Im so fed up with this damn thing and I work on cars for a living and so do my friends that helped me and were all stumped. Our next thought is to take the intake off and valve covers to make sure theres no internal damage from the backfiring. We also re-learned the crank position and it was successful but still no help. Someone PLEASE help with this, Im so frustrated and tired of spending money to rule things out even after testing them showed it should be done. I can't get on this forum too often so please email me with the subject "chevy forum help" my email in cjackman911@gmail.com. Thank you!
 
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#2 ·
What cylinders are misfiring. One bank or two. Are there any fuel trim codes. It's also best not to relearn the crank if there is a fault. It will then try to run correctly to the fault and will then cause issues when you do actually fix the root problem. These trucks were prone to intake gasket leaks when cold, broken manifold exhaust bolts causing unmetered air to enter the exhaust and effect fuel trim. Broken dipstick tubes also unmetered air and bad upstream o2 sensors not setting a code but messing with fuel trim.
 
#3 ·
The misfires are random between all the cylinders but since I put the tune back on it runs better but only at higher RPM's. Before I did the relearn after replacing everything, 6&8 were doing the most of the misfiring more frequently. My buddy had his scanner hooked up yesterday and we went for a drive and there were barely any misses (1 or 2 at a time) in random cylinders on both banks throughout the whole time driving but once we pulled over it got more constant in bank 2. Also the only codes coming up now are the random misfire and for o2 bank 2 upstream. I sprayed around the entire intake system with carb cleaner and the RPM's never jumped, the manifold bolts haven't been touched. The dipstick tube is also fine. Im going to replaced the upstream o2 sensor either way because its original and to eliminate a possible problem. After I replace the sensor should I do another relearn or what do you think I should do to get the truck back in the best running order?
 
#6 ·
have you gotten a new GM MAF sensor yet to replace your current one??? a bad MAF sensor would cause it to run bad, can cause misfires/codes, etc... if you haven't done that yet, then I would get a new GM maf sensor for the truck... do NOT get an aftermarket one, get the GM maf sensor that's for the truck...
 
#7 ·
I havent gotten a new MAF yet, thought about it but they're pretty expensive and I haven't had any codes for it besides one that was old before this issue. Im going to do an injector flush tomorrow and the upstream o2 because it needs to be done either way. If i continue to have this problem after that than I might just try a new MAF. This has been nothing but a damn nightmare...
 
#8 ·
You can unplug mass air flow to see if its causing your driveability issues if you don't have access to a labscope or atleast a good scanner that can graph data. If unplugged it goes to default predetermined values based on input from other sensors.

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#11 ·
it may not run at all if you unplug the MAF... I know my car wouldn't...
 
#13 ·
interesting...

yeah, I know when I played around with my Monte SS a few years back, unplugging the MAF on that car just caused it to die/stop running instantly...
 
#14 ·
Well I did same thing on my 2000 bonneville just about two weeks ago I wad driving around and if I hit bumps it would run rough for a couple seconds so I pulled over and tapped on every sensor I could reach and maf would stumble every time I tapped it so I just unplugged
It til the next monday to replace it at the shop.

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