Pleasant valley

Hey all! New to the site here, I joined to seek some help diagnosing an issue I'm having. I have a 1989 f150 xlt 4x4 regular cab with a 5.8 engine. I got the truck from my grandfather, and it was in rough shape when I got it. I have rebuilt the whole top end, removed the EGR, fresh gaskets, cleaned and painted it, new water pump, belt, IAC, and MAP sensor, and computer. This fixed a great deal of the problems it had but one still lingers. It starts fine, isles well initially, and drives fine most of the time. Every so often I get a lurching after driving at highway speed and the check engine light will come on for short periods of time, but goes away. When this happens when I stop and it's still in gear the idle is high. When I put the truck in park when this occurs, the idle goes significantly higher, is guess around 2200 rpm (I don't have a tach so I don't know for sure.

Im stumped as to what the problem is. I've looked over the vacuum lines and they all seem fine. The throttle doesn't appear to be sticking, and I have replaced the TPS as it was a likely culprit.

What are your guys thoughs? I'm a bit stumped here, and definitely not ruling out something I've already done being a lemon part. Ideas?

PerryB

It sounds like an EGR stuck open but .......
Does that motor have an intake runner door? I had a '91/5.8 and I'm thinking it did, but my memory could be defective. A stuck intake door (in short runner/high speed mode) will behave like you describe.

Pleasant valley

Thanks for the reply. I'll recheck everywhere the egr system used to be for leaks, and look in to the manifold as well. Keep those suggestions coming!