good idle at start-up high idle when running
 

Hi, A question for you: My 1996 f-150 5.0 idles fine at start-up but the idle stays high while running. I can shut off the engine and restart and the idle is good again until I press the gas pedal. Nothing is binding and the tps is well within limits because it does idle fine until running in gear and then idle won't reset upon stopping until I shut the truck off and restart. Can this be a computer problem?

Welcome to FTE!

Your hypothesis of "tps is well within limits because it does idle fine until running in gear" is flawed. Have you ever measured the TPS output with a DVM key on/engine off?

Based on your symptoms I would say the TPS is "noisy". The computer is always watching for the closed throttle minimum TPS output and adjusts the idle RPM accordingly (ratch algorithm). By shutting off the ignition key and restarting you are resetting the learning cycle for the computer.

Simple test is to remove the connector to the Idle Air Controller (IAC) when the issue occurs. If the idle RPM drops to below normal/engine stalls the issue is electronic and highly suspect is TPS related. If the idle RPM remains high you have a vacuum leak.

Thanks for the fast reply. The tps is set at .67 and high 4.45. The idle will come down when connector pulled. Tps is new.

New does not necessarily mean good. Your symptoms are classic noisy TPS output. The computer is sensing a very low output during the drive cycle then setting a calculated idle RPM based on that erroneous data.

It can be tough to see that wavering output with a digital voltmeter. I had a similar issue on my old 1992 F350 w/5.8L engine. TPS measured good multiple times. Turned to out be corroded TPS connector pins.

Was the replacement TPS a Ford/Motorcraft? Were the connector pins clean? Did you apply some dielectric grease?

OK I see what you are saying. I will clean up those connectors and try to test signal strenth further down the line. I do have a lot of experience with the Ford eec but a little stumped on this. Thanks

Frustrating when a new part is potentially bad out of the box. I assume it was replaced because of this issue?

Problem found
 

I did find the problem when checking the connector. I had a constant 4.4 volts on the tps output wire. The computer is bad. I should have checked both sides of the connector earlier. First time I have had a computer go bad in over 20 years!

If the TPS output is always 4.4 volts that means it is bad. What color wire has this voltage? Are you sure you are not measuring VREF?

The vref voltage is 5 volts. The wire to the tps output is 4.4 coming from the computer without the tps hooked up. remember I had the tps unplugged.

Just verifying what you measured. While VREF is rated at 5 VDC there is a +/-15% variance.

Based on your response I would agree you probably have a bad computer or something else is supplying power, what that would be is a good question. Most likely internal to the 'puter as previously stated.