Once again an intermittent problem frustrates a tech and leads to a comeback. SureTrack community member ProAutoValdosta had a Ford Ranger in the shop that had been in for an intermittent high or erratic. They repaired it but within a week it was back again for the same thing. They posed the question to the community, and with the help of fellow community member sleazen and others, they were able to resolve the issue quickly.
The following Community Fix summarizes the interaction between members of the community that led to the solution.
Vehicle
2004 Ford Ranger 3.0L
Title
Intermittent high or erratic idle, fixed shorted wiring.
Complaint
Customer complains that truck will intermittently have a high or erratic idle.
Diagnostic Steps Performed/Parts Replaced
Truck randomly & intermittently idles extremely high and/or has a wildly fluctuating idle. Randomly sets P0507 – RPM higher than expected and P0511 – IAC circuit malfunction. Verified no vacuum leaks. Inspected wiring harness and plugs, no obvious faults found in the harness. Cleaned and inspected throttle body and original IAC valve. Installed aftermarket IAC valve. Installed OEM Motorcraft IAC valve – worked fine for about a week and then went right back to doing the same thing. IAC duty cycle % is normal. TPS and MAF readings appear normal. This truck does not have an EGR system, and the EVAP system is working normally (passes the service bay tests). Accessing the data menu on my Solus causes the idle to go back to normal (?!?!) so I cannot see what it is doing when it is doing it! I can manually control the IAC valve and it works flawlessly. Tapping on the IAC valve occasionally makes it idle normally. Unplugging the IAC returns the idle to normal and plugging it back in causes it to idle high again. Pinching off the PCV hose does not change the idle speed when it is idling high.
Best Answer from Community Member sleazen
Can we assume your KOER Self Test passed? You may have a PCM or other related circuit failure – the IAC driver (PCM) or circuit (loom) is going to ground, pulling the IAC motor wide open. What happens if you tap on the computer while the symptom is occurring?
Cause
We found a spot in the harness where it had just barely rubbed the firewall on the very back of the engine. Repairing the rubbed harness has fixed the issue. It must have been grounding the IAC circuit somehow.
Correction
Repaired damaged wiring harness. Truck idles and drives as it should.