I have a 91 S-10 4x4with a 4.3L and 190 000 km. on the clock.The truck runs fine most times but I get a bad idle when vehicle warms up, but not always. I have replaced the following: Distributor cap and rotor, Plugs, wires, map sensor,Tps sensor, IAC valve, Coolant temp. sensor, EGR valve, and nothing seems to have fixed my problem. I seem to be thowing parts at it in hopes of fixing it but I have always been able to fix similar problems with these fixes, not this one. Would an exhaust leak at the manifoldcause a rough idle? What about TBI problems, but it should have problems all of the time, right? It never runs poor when it is cold and even the colder the ambient temp. outside is , the better it runs. Any help?
Thanks

 

Is your check engine light on? if it is, plug in a code reader and see what the code is that comes up. Otherwise, it soulds like an O2 Sesor because you only have this problem with the car is heated.

Good Luck.