Rough idle and acceleration

91 S-15 Jimmy. I checked the board and have seen some similar posts but not exactly. Idles rough in gear, not so much when in park or neutral. Bogs down sometimes when accelerating. When the truck hits about 40-45 mph it starts losing power - If I can ease into it, its usually ok but if I have to get on the gas it starts to bog down.
Have done a tune-up and theres no change there.
Service Engine light Does Not come on, Does Not run hot. air filter and Hoses all look ok, Thinking its the cat converter?

fuel filter?

Have since changed the catalytic converter, pcv valve and fuel filter and still have the problem. I cant think of anything that would cause this issue but not make the "service engine soon" light come on?

Have since changed the catalytic converter, pcv valve and fuel filter and still have the problem. I cant think of anything that would cause this issue but not make the "service engine soon" light come on?


Look at your injectors, are they spraying a nice cone or do they have drips?

Have been mixing in some marvel mystery oil over the last couple of weeks with no change in performance but I'll check the injectors.

Unplug the vaccum line going to the EGR valve(on top of EGR), leave it unpluged and take it for a drive, see if this helps.

Thanks, WIll try that with the vaccum line tomorrow and see how it goes.

I Unplugged the vaccum line from the egr valve but it didnt solve the problem.

Did you check vaccum pressure, or did you do compression test on the engine to make sure it is healthy? I mentioned EGR just in case, it was somethinglike that. If all that checks out, I had something similar on my 89, where it bogged down, and just ran like it was out of power, yet idle was great and stuff. I tried everything, and finally I got it, turned out being the coomputer was garbage, not bad enough to cause it not to run, but bad enough to loose power. I went and bought a used computer, put my chip in from my truck, and problem was solved. I don't know if thats your problem, but I thought I would mention this too.

I went through the boards and started making a list of the possibilities for this and figured I'd eliminate them from easiest/least expensive on. Hoses were fine, Someone mentioned in a post that they had a problem with the wiring harness to the IAC. Went out to the truck and basically made sure all the wiring harnesses were secure. Nothing seemed to be loose, nothing seemed to move but guess what ..... Truck is running fine now. 2 days running no problems. wanted to make sure it wasnt a fluke before posting this but seems to be fine.

timing, unplug the brown and black timing wire from the ECM under the glove box and set the timing with a timing gun to o degrees.

Also, what did you tuneup include?