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 1,200 RPM's on cold starts. The idle goes up and down repeatedly, and sometimes shuts off, until it finally warms up to a still too high 1,000 to 1,100 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.
Saturday, December 26th, 2020 AT 1:11 PM