2002 gmc Sierra V8 auto, has a rough idle. Changed both front sensors and all spark plugs and it is still idling rough, in park or in gear, it runs good if your driving. What should I try next?
Faulty EGR Valve.
SOURCE: i have a 1994 gmc safari.It seems to have a smooth idle at times
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!
SOURCE: 2002 gmc sierra starts up runs then starts to idle
GMC's are notorious for fuel pump problems. Not an easy repair. You have to drop the fuel tank to replace. About an $700-800 job at the dealer.
SOURCE: My 2003 GMC Sierra Idles Rough And Cut Off
Before replacing the O2 sensors, I would check the fuel pressure. You may also try to have someone pull a trouble code from the powertrain control module to verify why the engine light is on. Do not replace any parts until this is done, otherwise your going to be guessing at the root cause, and it will get expensive with part replacement.
Testimonial: "Trouble code P0101 {Mass Air Flow}and code P0332 Knock Sensor.Thanks i saved a lot money."
It sounds like you may have low compression in one or more cylinders, suggest you run a compression test, this should always be done when this condition exists and igintion is not source of problem. This engine also can have issues with intake manifold leaks, use a can of carburetor spray and spray around the intake gasket are to cylinder head, if a leak is present then the idle change when you hit the leak.
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