Questions and Answers

Ford Fluctuating Idle

Q. Hi, I own a 1991 Ford F-150 half ton, with air, has 212,000 km on it. It has been well maintained, and has had little problems with it, however I am trying to pin point a problem that has come up. The vehicle runs high and low idle at times; I have changed plugs, wires, cap, and rotor. I have put a new coolant sensor. I also cleaned the throttle body and intake manifold, and noticed a heavy and sticky coating of carbon. Could my catalytic converter causing this problem??

Any help would be great.
Shawn

A. I was looking through some Ford Technical Bulletins for the symptoms you described. I found one that connected with what you were saying about how dirty the throttle body was. The bulletin described how the Mass Air Flow (MAF) sensor gets dirty and causes a fluctuating idle. Basically they recommend a through cleaning of the intake system and replacing the MAF. This may be the problem with your truck.

There are other things that can cause the fluctuating idle as well. You can have a bad O2 sensor, Idle Speed Control (ISC) or Idle Speed Control Bypass Air valve (ISC-BPA).

The best thing to do is have the system checked with a scan tool to see which of these is responsible for the fluctuating idle.

Information provided courtesy of ALLDATA