1991 F-150 high idle

GOLF-NUT

MEMBER
1991 FORD F-150

The 1991 F150 (V8-5.0L, 2WD, AOD transmission) I gave to my son in October 2005 has idled way too high since I got the transmission rebuilt (just before I gave it to him). The idle speed is around 1200 RPMs on cold starts. The idle goes up and down repeatedly, and sometimes dies off, until it finally warms up to a still too high 1000 to 1100 in park, with the A/C off, and around 850 in drive, with the A/C on. As you know, it is supposed to idle high on cold starts and then wind down steadily until it gets to the proper warmed up idle speed of 850 or so in park with the A/C off, and 650 in drive with the A/C on (the proper settings to be used when setting the engine timing). Other problems that may or may not be related include backfiring sometimes when power is applied, and always pinging with regular gas, forcing me to use premium gas ever since it was new, even though it was supposed to accept regular.

They transmission folks claim they did nothing that would cause the high idle. They and other mechanics say a vacuum leak might cause the idle problems, but they can't find any such leaks.

The vehicle has 438,000+, but the engine was replaced at 297,000 due to oil seal and gasket leaks. The distributor and injectors are original.

I have cleaned out the throttle body and replaced the Idle Air Control (IAC) valve and the Throttle Position Sensor (TPS). I had experienced the up and down idle speed on cold starts at several times in the past, and changing the IAC stopped that problem. But a new IAC did not help either problem this time (the up and down idling on cold starts or the steadily high idle after the engine is warm).

At various times in the past, we have also changed such things as the PCV, EGR, and catalytic converter. In reading the Haynes repair manual for the 1991 F150, I see references to other sensors that measure oxygen, temperature, and even manifold pressure. Could one of these be the problem? Perhaps the main computer itself is bad? I say this because it has never allowed me to use regular gas without pinging, like it was supposed to.

Any advise to help solve this problem will be much appreciated. And, if you can solve this one, you guys really deserve donations.

4 Replies

INDYUKE

MECHANIC
416 POSTS

From what you describe about the idle, it sounds like you have a bad coolant temp sensor. Often when they go bad, they provide a false reading to the computer and the computer never sees the engine as warming up, so all the cold idle and mixture settings are kept in play. This cold enrichment would also cause the backfiring like you describe.
A bad manifold pressure sensor will cause an extremely high idle. Unplug it while the truck is running, your idle speed should go way up. If nothing changes, then it could be bad.

The pinging is a little harder to diagnose. The knock sensor could be bad, but that is unlikely. Two things that you could double check.

The 5.0 has a penchant for ignition crossfiring between cyls #7 and #8. They are next to each other and also fire in sequence. Make sure all spark plug wires, especially #7 and #8 are well seperated.

The truck 5.0 references it's PCV tap exclusively on the #8 intake runner. So that cylinder will always run leaner and hotter than all the rest, and it will have a tendancy to ping. It will also pull small amounts of oil through the PCV system into the #8 cylinder, lowering the overall octane rating and farther contributing to that pinging tendancy. Some guys will go so far as to drill and tap a new hole in the center of the intake so all cylinders pull evenly to solve that problem.

Check that you don't have a partially plugged injector. Let us know.