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Ford Explorer High, Fluctuating Idle

Q. Vincent, Thank you in advance for any help you may be able to offer.

  • 1991 Ford Explorer
  • 4.0 liter Multi Port FI DIS ignition
  • Automatic Transmission (A4LD)
  • 142,000 miles
  • Rear ABS brakes
  • P/S, A/C, Cruise control

Ford Explorer High, Fluctuating Idle

1. Intermittent high idle - idle is smooth at all times, no missing, normal or high. Varies from 1,300 to 2,500 rpm, no pattern. Meaning that sometimes it will idle at 1,300 to 1,400 rpm and at other times it will rocket up to about 2,500 rpm.

Mostly, 90% of the time, it will jump up to 1,300 to 1,400 rpm. Happens dead cold sometimes, sometimes not all day, sometimes numerous times during short trips.

Turning it off and restarting sometimes allows idle to return to normal, about 650 RPM, then again sometimes turning it off and restarting does nothing to help. Sometimes it will self correct from one stop light to the next.

2. Length of problem - 1 year.

3. No error codes. No check engine light. (have checked for error codes numerous times - Have run both KOEO and KOER diagnostics with no problems/error codes found)

4. Unplugging the ISC solenoid brings idle back down to normal every time.

Completely rebuilt top of engine one month ago due to cracked left cylinder head and leaking oil. Block and cylinders in great shape. No discernible ridge in cylinders and crosshatching still very prominent. Engine went back together great. Started and ran fine after burn in on second hit of key, felt good!

Parts replaced/work done:

  • New cylinder heads
  • Felpro gaskets throughout
  • New lifters
  • New timing gears/chain
  • New injectors (Motorcraft)
  • New fuel filter and pressure regulator
  • New plugs and wires
  • New vacuum hoses
  • New TPS
  • New ISC (idle speed control) solenoid
  • New Air temperature sensor
  • New temperature sensor
  • New Mass Air flow sensor
  • Cleaned all sensor and control contacts
  • Wiggled all wires I can get my hands on.
  • Disconnected battery four hours at a time several times.
  • Unplugged and replugged Engine ECM.
  • Performance notes: Timing is spot on, Vacuum is 20.5" and steady as a rock. Have used Mobile 1 10-30 every 3,000 since 9,000 miles. Vehicle has been driven very reasonably and has been garage kept just about it's entire life.

    Vehicle runs like new, smooth as glass with plenty of power. Only problem now is this fast idle problem is still with me. Should last me for another 150,000 if I can get rid of this problem.

    I have all of the Ford Tech manuals for this vehicle including the diagnostic manual for the 1991 year. I have followed all the suggested trouble shooting guides/procedures in the diagnostic manual to no avail thus far.

    My last ditch suspicion is either the Barometric Pressure Sensor (BAP) or the Vehicle Speed Sensor (VSS). But I don't want to spend another $150.00 and come up with nothing again. It seems it has to be an input to the engine ECM telling it to boost the idle speed but then again I don't do this all the time. Any suggestions will be appreciated.

    Thank you,

    A. You have done just about anything I could have suggested from here. I'm just wondering if you have driven it enough, 20 or 25 miles, for the PCM to relearn the new idle values.

    This idle condition may be caused by an incorrect ground circuit in the electrical distribution (EDIS) module connector. Inspect the terminal on the ground circuit wire. And if required, install a new terminal assembly with brass sleeve.

    Locate the ground circuit wire in the module connector, it is a black wire with a white stripe. If the terminal does not have a brass sleeve, replace the ground circuit terminal by splicing in a new terminal assembly. The Ford part number is: F1TZ-14461-A.

    Ford Explorer High, Fluctuating Idle

    One other possibility. There was a change to the Ford Service Manual. Since you probably got yours before the revision was made and new manuals published, you might be adjusting the base idle speed incorrectly. I would go to the Ford dealer and ask them for a copy of TSB 9111B4. That has the idle airflow adjustment procedures revisions.

    The Manifold Pressure/Vacuum (MAP) Sensor is a good possibility but not so much the Vehicle Speed Sensor (VSS).

    Other than that, you will need to run it with a scan tool connected to see what the sensors are doing. And don't assume that since parts are new, they are good.

    Additional Information provided courtesy of AllDATA

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