Please help. I have brought my truck into 3 shops. Nobody has fixed the problem. I have a 1990 GMC Sierra 350 manual tranny. New engine, alternator, radiator, throttlebody, injectors, IAC, TPS, cap/rotor, coil, distributor, etc. The problem that I am having, is that after the engine warms up, the idling is erratic. RPMs go from 650 to 900 then 550 and sounds like it is going to die, but never does. It then repeats the cycle. When it's cold, it runs great. When you disconnect the timing connector, it runs at a constant RPM. The smog guy says that it runs great over 1000 RPMs, but under, it is burning way too much gas. Please help. OH, I forgot to say, there are no codes. Last thing I'm going to try is a new ECM with Hypertech PROM. If anyone has any ideas, please let me know.

 

Temperature sensor.