SOURCE: 95 GMC Jimmy 4.3l W vin rough idle
My problem was solved when I repaired my 4 wd activation dash switch. The switch was contacting on 2WD & 4HI at the same time. The front differential activation is vacuum activated possibly creating a tug of war that didn't actually engage it but drained off vacuum from the engine? A theory at best but stumbbled across it because I knew the switch wasn't working properly I repaired it and low and behold rough idle disappeared and all is good for 3 weeks running.
SOURCE: 2003 Silverado rough idle, idles good after "check engine" light
Check your o2 sensor voltage, those trucks are real bad for o2 sensors, and yes if one sensor is reading 0mv, the others will start to drop out, and the vehicle will run very, rough. You also may check fuel system pressure, should be almost 60p.s.i. If its alot lower, Id be looking into a fuel pump. If that vehicle is drive-by-wire (no throttle cable), the throttle bodies get very dirty and cause that concern also. Id start by having the vehicle scantested.
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