1989 b2200 hesitates at mid-high rpm

Greetings all,

I just picked up a 1989 b2200 from a buddy. The truck idles/run great except when I get to around 3,000 rpm (guessing since it doesn't have a tach), then it starts hesitating and bucking. If I step on the throttle it will get through the hesitation but it does it around the same rpm on every gear. I just changed the plugs, wires, and fuel filter. I put a bottle of sea foam through it as well thinking the gas might be bad. I'm at a loss and not the best of mechanics. Any idea what is going on here?

Thanks!

First thing I'd do was check for vacuum leaks, and then check/set the timing.

If there's a vacuum leak, will make your timing appear squirrelly.

Thanks. I went through and checked every possible vacuum line I could see. Everything seems to be attached appropriately - at least visually. Is there a good way to check for vacuum leaks other than a visual and/or audible inspection? I haven't checked the timing yet as I don't have a timing gun, but my neighbor said I could borrow his. I was thinking that it could be a fuel pump issue but kind of doubt that since it only really happens at mid rpm's.

Vacuum advance on the distributor could be bad. You can test that with the timing light.

With the light connected, start to rev up the motor. If the light starts to jump around the advance is losing its pull.

So here's an update:

I somehow remembered when I had it smogged recently that it didn't pass the "visual" test because a vacuum hose was disconnected. They told me it had to be connected in order to pass. No worries, whatever it takes. They even warned me it may run rough since they weren't sure why the hose was disconnected. I went back today to ask them if they remember which hose they disconnected. The guy that tested it came out and showed me. It was the vacuum line coming from the "Duty Solenoid Valve" (or what's called the EGR control solenoid in the Chilton manual). So, I disconnected the hose at the EGR valve exactly where they reconnected it and drove it away. A lot of the hesitation and bucking was gone, however not completely - it still has a little, but at least I'm getting closer. So I drove it home did a little reading and then testing the EGR valve by putting a little vacuum on the valve. The diaphragm moved fine and it stalled out at idle when I did that (which according to what I've read was supposed to happen).

So with that, any ideas on why it would still have a little hesitation but mostly fixed due to the vacuum line being disconnected. Possible a bad Duty Solenoid Valve? I went down to the parts store to price one out and I had them stumped. They couldn't find the part and had no idea how to. Ideas?

Thanks!

The valve and everything might be working but the flow can still be restricted. The ecu still is looking for a preprogrammed certain amounts. What will happen will it will be leaner/richer than it thought it was going to be at certain rpm/load and vice versa. It will adjust out of parameters but it's still a fight. At lower throttle or load you might not see it because a restriction isn't eveident until you need to make it work, like a fuel problem. When you mash gas harder in and thru the stumble an operating egr closes up anyhow and ecu goes into another set of numbers. Now on top of all that a vacuum leak anywhere of course is going to make a stumble as that is unmetered air as well. I would take everything off and clean, and also look for split vacuum lines not just the ones you guys touched.

Vacuum advance on the distributor could be bad. You can test that with a vacuum pump.

Apply a vacuum to the capsule and see if it leaks down. If it does, it's bad.