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.
Constant high revving Viii
- Thread starter GreatVIII
- Start date
Danny351C
LVC Member
Danny351C
LVC Member
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!
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:
germansheperd
Active LVC Member
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.
I wanted to write what happened to me. I bought a 1997 Lincoln Mark VIII LSC. Beautiful low mileage, no rust. I had to drive it home 7 hours. Well a little bit in the idle just suddenly jacked up to 2300 rpm and made it very hard to deal with. The transmission also hunting. Well i figured it out to be the throttle position sensor. When I unplugged it idle went down at least although made the transmission shift bad and wasn't nice. I had to go slow through the mountains anyways. I bought a TPS at a parts store on the way. I was able to get the screws out of the old one which was hard and install it. It seemed to help but still wasn't quite right. Shifting still not great and idle not consistently good. Made it home and started ordering parts I needed and I thought I would buy a new Hitachi idle air control valve. Well that seemed to help after resetting the comuter KAM using the method of jumping the negative to positive while battery disconnected. Seemed to be excellent for a while.
Then the ******* high idle came back!
Like WTH! Made me want to give up. Well I read more and more. Seems the throttle position sensor is very sensitive. Well I was using my scan tool and I actually caught the throttle position sensor PID jump just as the high idle came back. So I knew that was the issue. But I got a new one right? Well I returned it to the store then got another Delphi one off Rockauto. I took the throttle body off, cleaned it, put on the new TPS and reset computer again. It worked but then high idle started coming back and weird shifting again. And then I got a p1125 throttle position sensor intermittent! This was my big clue! But readings looked ok. I was starting to pull my hair out.
Then I went over things in the engine bay. I noticed the throttle position sensor wiring harness had been repaired in the past. Actually spliced twice with 2 pieces of heat shrink on each wire. I wiggled the wires, didn't change anything but I thought I would order a Motorcraft pigtail for the TPS from rockauto. When it came I cut back the old harness repair and had not too much left there to connect to but I was able to get more wire out of the taped part and splice back enough to solder good. The butt connectors that came in the kit were useless to me because you need a special Rotunda tools ratcheting splice tool. But I got a really good soldering connection to all three wires. Heat shrinked them with the kit double walled stuff until the ooze came out and put everything back together. Also reset the kam again and went for a drive. And wouldn't you know it, like a new car! After a day of driving, it idles perfect, and when air conditioning comes on no more flutuating rpms and no bad shifting. Shifts smooth like butter. I never realized how important throttle position sensor is but now I do! So in case anyone has this problem ever, look at the wiring and replace it. It was not expensive for the Motorcraft pigtail, about $15.
Then the ******* high idle came back!
Then I went over things in the engine bay. I noticed the throttle position sensor wiring harness had been repaired in the past. Actually spliced twice with 2 pieces of heat shrink on each wire. I wiggled the wires, didn't change anything but I thought I would order a Motorcraft pigtail for the TPS from rockauto. When it came I cut back the old harness repair and had not too much left there to connect to but I was able to get more wire out of the taped part and splice back enough to solder good. The butt connectors that came in the kit were useless to me because you need a special Rotunda tools ratcheting splice tool. But I got a really good soldering connection to all three wires. Heat shrinked them with the kit double walled stuff until the ooze came out and put everything back together. Also reset the kam again and went for a drive. And wouldn't you know it, like a new car! After a day of driving, it idles perfect, and when air conditioning comes on no more flutuating rpms and no bad shifting. Shifts smooth like butter. I never realized how important throttle position sensor is but now I do! So in case anyone has this problem ever, look at the wiring and replace it. It was not expensive for the Motorcraft pigtail, about $15.
The Motorcraft part number for the throttle position sensor wiring is WPT993. The thought I wanted to convey is that this circuit is very sensitive to extra resistance. With those 2 different butt connector repairs in the line I believe that brought up the resistance in the wiring slightly throwing off the readings coming from the TPS. That wiring has to be good and clean to get the perfect signal. Still running great since the repair by the way. So glad I figured it out. This pigtail also works on Crown Vics and Town Cars and other Ford products of this era.