To whom it may concern. i have a 1994 gmc yukon with a 5.7. it seems to run rough at an idle. i have replaced the plugs,fuel filter,air filter. throttle body gasket,wires,aic,egr valve,egr solenoid. it runs fine otherwise. it idles at about 800 rpm in park and about 650 in drive.some times i can feel a miss when the engine is cold and occasionally idles high in the cooler months after it is completly warmed up.you can even see the tensioner pully move back and forth at an idle. the belt and pully have also been replaced. any help would be appreciated

could it be the fuel cut off switch or the cats are going have you run cheap fuel in it. it kills the cats

Check fuel pressure regulator vacuum line for fuel in it. Replace FP regulator if it does.

Rough Running When Cold GMC Yukon

mcdevito75 here, Sounds like a faulty voltage regulator, it"s 1 unit with the alternator, change the alternator, also the alt. supplies juice to the ignition switch, distributor abd plugs, thus a rough idle due to bad alternator.

My brother had a similar issue with his Ford F-150, he needed the fuel injectors replaced. Good Luck!

HI
P0300 is multiple misfires.
P0171 is lean burn on left bank 1 means drivers side
02 sensor is faulty. Your vehicle is over fuelling that is why your are getting P0300 code and the rotten egg smell. Good idea is to replace both 02 sensors.

Im not sure if you got it fixed yet! It could possibly be idle air control valve. But here some more suggestions! I had a 94 Safari van 4.3 liter I was working on for about three days and I finally figured out what was wrong with it. It would run fine for a couple minutes and as soon as I hit the gas and let off it would surge, but anyways I ended up changing the distributer, after the fuel pump and filter, mass air flow sensor, egr valve, idle air control valve, map sensor, and ignition module. So that might sove the problem if you were on the same track with it as I was. Hope this helps ya!

I know that this will sound Stupid. Change the Fuel Cap. If the Seal on the Gas Cap is Bad it will allow Air to get into the Fuel tank and cause your Rough Idle.

i now for a fact that it is the egr valve . carbon balls in side you have to take it of and clean them out