I have a 90 model pickup with a 2.3 carbureted engine that refuses to idle. The carb has been replaced (with 4 different rebuilt carbs)as well as every vacuum hose, the Idle Vacuum Switch & all the tune up parts. The ECM displays no DTC's. It has no vacuum leaks (I have used a "smoke machine" on it. The ignition & valve timing are correct, compression is about 135 in all cylinders. Fuel pressure is about 4 psi. Nothing I have done has helped Has anyone else experienced this type of problem?