86 K3500 GMC running very rough

[Roger2561]

Hi folks, I'm new to this forum so please be patient with me. I have a 1986 K3500 GMC 1 Ton with a 350 V-8, Dually, 4X4, 4 speed manual tranny; it runs extremely rough. When first started it runs okay, not great, but okay. As the engine warms it acts like it's going to stall out like a flooded engine. I wanted to clear it out so I thottled it, observing no telltale sign of flooding, like black smoke. Someone told that it may be in the vacuum hoses, check valves, etc... anything to do with the emissions control system. Can someone tell me which valve will most likely cause this problem. It ran smooth and quiet until this happened. Any advise will be appreciated. Thanks, Roger

[Slonaker]

I don't know much about this stuff, but I'll give it a shot.

My first thought would be that your EGR valve is not working right. My limited understanding of how it works is that it allows exhaust gas into the fuel mixture when you are accelerating. I believe that it is supposed to be closed at idle, and that it will create a giant vacuum leak at idle if it is open. If that is the problem, it could be that it is stuck open or that it is hooked up wrong and opening when it shouldn't.

On my '86 305, you can tell what it is doing by reaching behind the diaphragm and operating it manually by pushing on the soft part of the diaphragm. You could do that and see if it is opening as it warms up, and creating an exhaust leak. You could also replace it with a plate of metal and a gasket and see if the problem goes away, or remove it, clean it, and disconnect the lines from it so it would not open.

Another thought is that my EGR valve had a huge vacuum leak when I got it. It had a spacer between it and the intake that was so rusty that it leaked. A lot of wire brushing and a new set of gaskets made it seal well enough to last until I found on on one of my junkyard trips.

Welcome, and good luck.

Slonaker