Constant high revving Viii

Lincoln Mark VIII

  1. GreatVIII

    GreatVIII New LVC Member

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    Hello, fairly new to LVC. This is only my second post. I have a 94 Mark 8. No engine codes. The car recently decides to rev high as if the TPS or throttle is stuck. My tach is not functioning (almost a year) so Im guessing its revving at about 2500 to 3500 rpms. It starts fine, regular rpms and idle is fine. It just eventually revs high while Im driving. It is accompanied by a higher coolant temp (little bit above midpoint) that very slowly rises. Never to the overheating point but possibly if I kept driving. Im assuming that is because of the trans fluid being worked thru the unrested tranny. Sometimes it will roughly drop down to the ideal rpm (700 rpms) upon stopping at a red light, but less often than not. The only fix is to park it maybe an hour, and again everything is normal. Ive checked the throttle cable and gas pedal. No kinks, no slack, not stuck. Throttle body seems fine, no linkage issues, no gunk or buildup preventing butterflies from closing. Intake tube gasket ok also. Unplugged idle air control valve to verify correct operation. Car died when I did so, and I think that means the iac valve is functional. Overall I did a spray test to check for vacuum leaks and found none. So, is this a vehicle speed conrol issue (recall), tranny problem, or something else? Can the TPS be faulty and not throw a code? Any help or tips would be appreciated.
     
  2. Danny351C

    Danny351C LVC Member

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  3. Danny351C

    Danny351C LVC Member

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    The IAC in these cars are a pain in the A$$. Sounds like you have the same problem I used to have. I could never get mine to reset. I have heard people spending alot of money on this problem. But the great news is that it can be fixed cheap!

    The Idle air control valve became very over active in mine. The rpms would hang and take a while to come back down. It would settle down eventually in park. But shifting out of park would send the rpms soaring and having to stand on the brake to hold it. The car would seem to accelerate by itself. It became a safety issue and I'd have to throw it in neutral to slow/stop the car. Dangerous!

    Well, I disconnected the harness to the valve and that settled the high idle problem. Then it idled too low and in cold weather was hard to keep running till it warmed up had to throttle it.

    I thought, there must be too much voltage driving the IAC so lets reduce it a bit and see if it settles down.

    What I did was install a load resistor. The kind for led lights w/heat sink. Its a 50 watt at 6 Ohms. I extended the wires off the harness and wired in the resistor parallel across the two wires before the IAC. I also installed a toggle switch on the NEG (-) side of the harness to test it and be able to engage the IAC manually. Looking at it now, for safety I should have added a fuse on the positive side. But it seems fine.

    Worked like a charm! It idles at about 600 rpms now and dosent sky rocket out of control like before.

    You'll need to use a multi meter to check the polarity and voltage. I had about 11.5 volts going to the IAC now with the resistor its about 3.3 volts (approx). I have the resistor mounted to the metal part of the strut tower area. It heats up very little, but dont attach it to anything that can melt. The toggle switch is just tucked in the engine bay for now. The runs car is so much better.

    The rpms are hardly affected by shifting out of park like before either. The revs come down quickly and the overall sound of the car is better, like an Old standard shift car in a way.

    This was my idea and It worked for me! Very happy. It saved me alot of time, hassle money and worry. As the IAC in the 93-94 maybe the 95 also, is a PAIN to get at and replace. I am NOT responsible if you or any others fry your computer doing this mod.

    Side note: As the colder weather approaches here in Massachusetts, I might try a lower value resistor and see if the idle comes up a tad bit more. Or maybe a
    Voltage potentiometer
    for more adjust-ability. Good luck!
     
    Last edited: Sep 25, 2019
  4. germansheperd

    germansheperd Active LVC Member

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    First thing I’d do is take the IAC off and clean it very well/or replace it then go from there. You can get a new one for $38 from Rock Auto.
     

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