High idle rpm

I have a technical automotive question for you.

Here we go|

This is for a 1987 Chevrolet Caprice, V-6, 4.3L engine with 85,000 miles.

Upon starting the engine it sores to something around 5000 rpm, then I turn it off.

This will happen time after time. No way to drive this puppy.

I have checked the following:

1. Vacuum hoses for leaks, cracks, etc. none found.

2. Engine wiring grounds for continuity.

3. EGR valve (and cleaned same).

4. MAP sensor with known good unit.

5. Throttle linkage for sticking, all ok.

6. IAC valve & wiring.

7. Fuel injectors checked for operation.

8. Engine ECM for trouble code with Scan unit, no codes found.

9. No trouble code 51 or 52 found showing a defective PROM or CALPAK.

I have installed the following new:

1. Throttle body gasket.

2. Intake manifold gasket.

3. New vacuum hoses to pcv valve and vacuum boost on the power brakes.

4. Remanufactured ECM.

The exhaust ports and intake passages were cleaned on the intake manifold.

The ignition timing has been set temporarily until the engine rpm comes down.

Any suggestions on how I can check the PROM or CALPAK.

Thanks for your help.

If it is really true that the IAC is ok and the wiring is ok, check and adjust the throttle position sensor. Next step would be to get a scan tool on it to see what the IAC is doing and then to see if when you dial it down if it changes the idle. It sounds like either the IAC is totaly disconnected, the TPS is way off or not registering, or the computer is not reading same. That would be an ECM problem though and not the PROM in the ECM.

What is the throttle itself doing when this happens? If it’s not opening, there must be another source of air, like a ruptured diaphragm in the power brake booster.