1991 GMC runs poorly after engine is warm

[willymakeit]

Hello there!

I bought a '91 GMC 1500 automatic with a 4.3 about 1 year ago. Everything seemed fine until about a week ago.

At first, it started to stumble/hesitate every so often. As time wore on it got worse until it flat wouldn't run when I put it into gear without dying. Sometimes it would start ..... sometimes not. I replaced the fuel filter without any improvement. Friend told me to check the Idle Air Control which I ended up replacing. After replacing the IAC it started, idled and ran like a champ (actually better than it ever did before) ........ until the engine obtained normal operating speed. After it warms up it stumbles/hesitates any time you apply pressure to the accelarator. sometime it seems to cough (not really backfire) through the carb, and the engine light sporadically glows. When checking the codes it gives an O2 fault.

OK, now comes the tricky part. I always suspected that the engine had been replaced at one point in time but really don't know that for a fact. Some of the emission systems were removed like the catalytic converter and the air pump. After attempting to check the Oxygen sensor I found it didn't have one. I'm assuming it should have been found underneath just downstream from the exhaust manifold on the passanger side. I found a plug in its place. I'm starting to wonder what else could have been removed if anything.

Should also mention that I also tried to check the EGR valve by letting it idle and lightly pushing the diaphram out until the engine started to die.

Since I'm a greenhorn at being a mechanic I'm not sure where to go from here since the truck seemed to run fine until recently. Hopefully someone can point me in the right direction.

THANKS for any help that can be offered!