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