I am having a major problem with my 90 f150 4x4 manual 5.0L V8. The idle has crept up from 1100 on Mon. to 2600 on Thurs. I pulled the codes from the EEC -IV which were no help and after checking for air/vacumn leaks I replaced the TP, IAC, MAP with new units.

B4 this problem my A/C clutch was cycling constantly. I adjusted the pressure switch and it seems ok. 20k miles ago I changed the ECT as I broke it by accident. I was thinking about replacing it again but even when the engine is hot the idle stays at 2600.

My only code now is 13 which is RPM unable to reach lower limit.... I am at a loss..

 

Have you checked to see if your IAC (Idle Air Controller) is hung in the open state? This may cause your idle to remain high. Also, crid build up on your thottle plate will also cause the same issue by not closing all the way.

 

Ok! I narrowed it down to either the computer, the wiring harness or a relay SOMEWHEre. WHen I backed out the screw on the computer harness, I started the engine idled fine. I backed it out more engine raced. put it in raced. Turned the key off and on real quick and the idle came down to normal. WHat am I fakeing out to get it back to normal. Does anyone test computers or do I gotta buy one to see if its the harness or computer. Is there a relay in the throtle circuit hanging up?

 

Check for a chaffed wire. It just donned on me that I had the same issue on my 88 Ranger with it idling at 2500 RPMs. I finally found a chaffed wire touching the alternator bracket. When it was shorted out to the bracket, the RPMs woud jump up. I found it by wiggling wires and connectors.

 

I wanted to jump the Wires to the IAC. The red one is easy as it gets 12v from the EEC relay. The other one just goes direcetly to one 10? pin cannon plug then to the computer. How do these cannon plugs come apart?