1998 4x4 is chugging while under minimal load

[nixon_49]

My 1998 GMC 1500 seems to be having a re-occurring problem with this chugging.

Originally the issue was while on the highway, maintaining a steady speed with minimal throttle the truck would sputter, and lightly backfire out of the exhaust, and chug. but would only do it while lightly on the throttle, the problem will stop if not on the throttle at all or on it heavy.

 

I bought a new distributor cap, new spark plugs, rotor, and had no luck.

 

Problem one day just disappeared, and the truck ran 100% at any throttle amount.

The flipside was that once that happened the truck was hard to start and would stall or want to stall when at a stop and giving it a good shot of throttle and the problem got worse and was finally at the point that it would want to stall almost every time i would start giving it throttle from a stop, it also did the same in reverse and park.

 

I bought a new air filter thinking it may be the problem, it didnt seem to do to much, until a day or 2 after and now it starts fine again and doesnt want to stall when giving it throttle from a stop anymore and now im back to the original problem of it chugging on the highway and almost any time it has a slight bit of throttle

 

ITS DRIVING ME NUTS

PLEASE HELP!

[GMstrong]

Intake manifold gaskets.

[Morgan1986]

maybe fuel pump ?

[nixon_49]

One more thing that I forgot to mention before was that the trucks rpm will also shoot up and down about 400rpm while on the highway for no reason

[grampadirt]

CFI Spider or fuel injection control module

[alz]

Check your plug wires. I had one go bad not long ago. You can check them with a DMM. Not sure what the rsistance should be, can't remember off the top of my head.

Or just swap them all out if they've been on there a while.