Closed Loop Idle Problems

1993 Chevy 1/2 ton, 5.7 L, 4 WD, 5spd, 167,000 miles:

Once the truck reaches operating temperature it can't maintain an idle and dies. At steady highway speeds, there is a sudden loss of power.

Two different shops have not been able to correct the problem. I was told there were no vacuum leaks on top. They said the engine was in good shape. One shop adjusted the new IAC valve I had installed. The shops said all sensors appear to be properly connected and functioning. A new mass airflow sensor didn't change condition. The intake manifold gaskets were replaced and timing reset. In between shop visits I replaced the stock computer box with a remanufactured unit with no change in problem. Since coming back from the shop, the service engine soon light has NOT come on despite the problems.

Note: The vehicle previously had a Vortech supercharger installed. All supercharger components have been removed and returned back to stock form with the exception of the additional inline fuel pump. I had earlier replaced the stock TBI with a rebuild ported Turbo City version. I also have tri-y headers (with O2 sensor), replacement cat, and low restriction exhaust currently installed.