My 1994 ford F150 XLT 5.0 has a high Idle problem that just recently showed up. The truck had been acting just fine until one day while hauling off limbs – the idle just jumped up. This is a standard shift truck – so the high idle was very noticeable during the shifts (sorry I don’t Have a tach so I don’t know the rpms) but it defiantly wasn’t racing rpms – by very high for an idle. When the ignition is turned off the and the truck restarted – everything is normal – the idle is where it is supposed to be – until I step on the gas pedal and raise the rpms, the high idle appears (doesn’t matter whether the engine is hot or cold). Sometimes it happens immediately and then sometimes it appears after a few shifts. Even it has disappeared at times when I am driving the truck only to reappear later. Got any Ideas as to the cause? I'm really lovin' this old truck and I want to keep at it's best

 

Check for a vacuum leak. SD motors will idle higher with a leak. Check around the upper and lower plenum interface. Mine did the same as yours. I've had to replace mine twice.

The cpu increases the gas volume because of the change in the MAP reading which has changed by the increased airflow from the leak.

Yes, my first leak was intermittent between plenum passages..

 

Will check today - going to get some carb cleaner to check for Vacuum leaks. I'll let you guys know what I find

 

If you haven't left for the parts store yet, while you are there, ask them to check for any trouble codes. Most parts places will have a code reader. Also, the next time the idle jumps up, pop the hood and check to see if the throttle has returned all the way. You will need to remove the plastic cover first. Mine did something similar and I just needed to clean and lubricate the external linkage.

 

That does not sound like a vacuum leak to me. Classic symptom of a faulty TPS or connector to the TPS.

I found the connector to the TPS on my truck was full of green corrosion cuasing the same type of issue. Scraped it off, sprayed a little contact cleaner on it, then reassembled. Verified the TPS output signal was linear from closed to open throttle with a DVM. Still working good to this day.

It would be a good idea to make sure the throttle is closing completely as well. Sometimes a sticky throttle cable or the throttle body bushing itself can keep the throttle partially open from time to time.

 

Well folks - Here is the offender - Notice the upper wire end - it is missing some of the insulation! Obviously not making a good connection. Time to replace the TPS. Unfortunately this is the model in which you have to remove the whole throttle body to get to the TPS. When I go everything off - the backside of the tubes and the throttle plates where coated with carbon - so I gave it a cleaning as well. I also found some oil reside that had crept into the TPS (maybe another reason for the failure). I installed the new TPS and adjusted it to 1.o volts as per manual. Then bolted everything back up and cranked the engine - normal idle. Took the truck out for a test drive - no problem.




Thanks for the help

 

Thanks for reporting back with the resolution!