T-Rod

I am new to chevys and I got a good deal on a 96 c2500. It has a crate engine and trans but has been sitting for 3 years before I bought it. It started up for me when I got it and I drove it 200 miles home. It was running rough the whole way almost like a misfire but just seemed to not be running right. When I got back I replaced all plugs, idle air control valve, tps, and mass air flow sensor. It was throwing a code for the maf sensor. I cleared the code after changing all that and it seemed to fix the problem. It was running great that day and still does, but occasionally it will still run rough. It will stutter and sounds like it backfiring right at the manifold. When it’s acting like this if I punch it it will get up and run fine but when I try to ease on the pedal it will run rough like I explained. The next time I start it tho it might run like that or it might run perfect so I’m at a loss as to why it might be doing that. Kind of a long post, sorry but any knowledge or advice on this would be a great help. Thanks!

Gumby22

Does it seem to happen somewhat consistently (at least once every time you drive it)? Does it matter if the air conditioning is on?

Being able to punch it and get it to smooth out is generally indicative of a vacuum leak, so for it to be intermittent helps narrow it down to a controlled vacuum leak - namely the EGR or canister purge valve could be getting stuck open.

The backfiring noise you’re hearing - is it at the exhaust or intake manifold and is it a very clear backfire or is it more of a popping, like it’s running lean?

Try checking codes again - they can be stored even if the CEL isn’t lit.

T-Rod

It’s not very consistent it only drives like that probably once every 5 drives, and I can’t make it do it. It only does It when it wants to. I don’t think the ac affects it I just ride with windows down. The backfiring noise sounds metallic almost like a clanging but it’s in the engine bay it sounds like it’s right at the exhaust manifold. And I’ve checked the codes, the cel is on but it shows no codes when I read it.

Gumby22

Have you noticed if it’s running hot when this happens? Not necessarily overheating, but running hotter than normal.

Without any codes, about all you can do is start making basic checks. Given that it’s intermittent, it could be a loose connector or wire that’s starting to rub through its insulation.

Every sensor or solenoid/actuator you see that has a pink wire on it should have battery voltage when the key is on or the engine is running. Try monitoring the voltage at each component as you wiggle the engine harness and see if it drops/cuts out - if it does, inspect the harness for damage - if you can cause it to cut out in one particular spot, that’s where the damage is.

The CEL is ground side controlled by the PCM and gets its power from the gauges fuse in the driver side fuse panel. A short to ground in the instrument cluster, control circuit or PCM could be causing it to stay on. A quick way to narrow it down is to disconnect C3 (Gray connector) from the PCM and turn the key on. If the CEL is still on after a few seconds, it’s the instrument cluster or the control circuit - if not, it’s the PCM, but then you’ll want to verify all the power and ground feeds to the PCM are good before condemning the PCM as faulty.

T-Rod

It doesnt seem to run any hotter when it does that, temp is pretty consistent. I think you’re right about it being electrical tho because last night my dome light was stuck on so I started the truck and it was idling weird and the dome light was flickering but the button wouldn’t turn it off. I turned the truck off and left the ignition on and it still wouldn’t turn off. I took the key out and back in and turned ignition on again and then the button for the dome light worked. That made me think there must be a loose wire or something in that area, it seemed like somehow the ignition switch and the dome light are related because after I got the dome light off and turned the truck on it was idling normal. I will check all the fuses after work today and try what you said with the PCM and hopefully that gets me somewhere. I’ll keep you updated when I figure out more.