1990 GMC Sierra help

Been driving my Dad's 1990 GMC Sierra 4x2 5.7l truck in the hopes of solving a problem he has. Any help would be appreciated. Truck has

112,000 miles on it. Truck starts up first try every time. But once started it idles rough and the rpm's rise and fall by about 500 to 1000 rpm's. When I apply throttle the truck hesitates and then catches. Then runs great. But as soon as I stop at a traffic light or anywhere else, the stumble continues and the rpm's go up and down again. Seems worse when the A/C is on. Once the truck is moving, it runs great. Transmission seems to work flawlessly. Now on the highway after a few miles the SES light will come on. This light will stay on until the truck is shut off. Then on the next restart the light will be out. Driving around town the light never comes on. Only on the highway after a few miles. Truck is not throwing a code. At least that's what Autozone says. I have had them use the scanner when the light is on and that's what they tell me. Can anyone help me out with this one? Have you seen this one before? I am a Dodge man but Dad has always been a GM guy. Any help is greatly appreciated.

Bob

Reply to
Bob M
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It's probably a long shot, but I had a 91 Sierra at one time that had similar symptoms. It turned out the three bolts in the top of the throttle body were loose. Mine wasn't so much of a stumble most of the time, as it was a high idle, much like when a TPS fails.

Reply to
Yadayada

I'd try checking the EGR valve by pushing up on the diaphragm from the underside of the round housing. If the truck stumbles no worse at idle when you do this it may need replacing and possibly cleaning out the passages that lead to it through the intake manifold. As for Autozone - if the SES light comes on a code must be registered - at least as far as I know. You can pull them yourself by attaching a jumper wire between the A and B ports of the ALDL under the dash while the ignition is off. Then turn on the ignition - the SES will flash a 1-2, then any codes stored, followed by a 1-2 again. Good luck. Kevin

Reply to
KV

Cool. Thanks I'll try this.

Bob

Reply to
Bob M

ok, thanks.

Bob

Reply to
Bob M

If IRC that was one of the years that the throttle body gasket went bad often. Also as someone else said sounds like it could be the egr is stuck open a bit.

Reply to
Shoe Salesman

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