[ChevyTech]

It sounds like the truck has more then one problem.

It could have a bad TPS. More then one symptom points to the TPS.


If the ECM does not see a voltage reading on the TPS that shows it is at idle, the ECM will not move the IAC.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.


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TPS testing for TBI


TPS Throttle Position Sensor testing:
Piercing the wires to do testing can cause future problems.
When I can’t use a scan tool, I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)

With the key on and engine off:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

Many people just test it with the engine off using an ohm meter on the terminals of an unplugged sensor to test for a smooth change in resistance as the throttle is moved.