2000 GMC 2500 rough idle

[jfthomas327]

Recently purchased a 2000 model GMC 2500 with the 6.0 and around 250k miles. It ran fine when I bought it but as with most 19 year old trucks there where some things not quite right. First order of business was to replace the exhaust (previous owner had cut a hole in the factory muffler). I installed a Dynomax cat back kit that was a pain in the rear. Nothing fit right, pieces had to be modified, and they forgot to add a spot for the o2 sensor. After taking it to an exhaust shop its all welded up nice and tight with a new sensor. A couple days of driving with the new exhaust and now it has developed a miss while at idle and also right off idle under acceleration. I have traced the miss to two cylinders on the passenger side of the motors. When I pulled the plugs the #8 and #4 cylinders showed a lean condition. I replaced the plugs and the miss went away only to reappear a couple days later in the same two cylinders. I know it is most likely lean because of the new free flowing exhaust, however shouldn't the computer compensate for that? And why would it only be two cylinders?

[casey.mikl]

It could be lean from a fuel injector that’s clogged or not flowing correctly. I would change fuel filter and run some injector cleaner through it. If not, replace injectors or have them cleaned. Just an exhaust won’t make the truck go lean. And if it did go lean, it wouldn’t be just 2 cyl. It would be all of them.

 

 

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[jfthomas327]

46 minutes ago, casey.mikl said:

It could be lean from a fuel injector that’s clogged or not flowing correctly. I would change fuel filter and run some injector cleaner through it. If not, replace injectors or have them cleaned. Just an exhaust won’t make the truck go lean. And if it did go lean, it wouldn’t be just 2 cyl. It would be all of them.

 

 

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Yeah that makes sense. Anything other than the individual injectors should affect all cylinders and not just a couple. I have ran a bottle of sea foam through the tank and it didn't change anything. Going to try the sea foam motor treatment that you spray into the intake while the engine is running. If that doesn't help I may be looking at replacing the injectors. Is there anything I could take the injectors out and soak them in?

[casey.mikl]

I don’t know about cleaning them. There may be something. But you have to throw power to them I believe to manipulate the valve inside.


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[davester]

If you still have the cat's installed, they are the main source of backpressure, not the exhaust/muffler system after it.

 

You might consider replacing the in-line fuel filter (inside the frame rail, under where the driver sits).

 

I took out my injectors and soaked them in Seafoam directly, as well as blew out dirt caught on the inlet screens.  I don't know how well it worked, as immediately after this, I accidentally used a bunch of REALLY dirty fuel, and promptly loaded them up with crap again, and bought new injectors off ebay.

 

I believe GM has a setup to inject a cleaning fluid through the pressure sensing port on the fuel rail, but I don't know how effective it might be.