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Engine running rough

1991 GMC Safari 150K+. Engine started running rough about 2 weeks ago when we had a cold snap, but it hasn’t recovered as the weather has warmed up. Weird thing happened a couple of weeks ago - when I was accelerating up an on-ramp, the engine started “hiccuping” - almost a backfire, but little ones, and it sounded like it was coming from under the hood, not out of the tailpipe. If I pressed on the gas, it got worse, and if I let off the gas, and reduced speed to about 55, it went away. After driving for about 2 miles, I could accelerate again, but the engine was really rough. It happened again about a week later for a mile or so. What’s going on in my engine? Could it be carbon build-up? I’ve had EGR valve issues before, but even with the high milage, this vehicle has run very well for the last 6 years that I’ve owned it. I’m planning on taking it in to my mechanic next week, but I was curious what could be going on. Thanks for any help!


Yeah, it could be the EGR valve carboned up. But not likely. typically that’ll be felt more at idle than accelerating. It’s normally open during acceleration anyway. And it typically won’t affect your top end.

There’s a whole bunch of possibilities. How long has it been sine the last good tuneup? Are these the original plugs and wires? Original distributor components? Original gas filter? Air filter?

Start with the basics. If those don’t get it fixed then you can start checking things like fuel pressure. If it’s been well maintained and these items are in good shape, start with the fuel pressure.

Or just let the mechanic do the diagnosis.

Thanks. I was going to start with plugs and wires. I hadn’t thought about the fuel pressure or fuel filter. Will report back when I find out more.