1992 Mercury Grand Marquis high idle in drive.

[pivichot]

This is for anyone whoever has a sudden high idle in drive. I have the pre 92.5 the one with the AOD transmission. I had recently changed out the IAC valve, TPS, and fuel pressure regulator. At the same time I and changed out the o-rings of the injectors. After the car had a smooth idle . The reason I change these parts is because I didn't want the car to have trouble since I use it to drive to my university. A usual 26 miles round trip. also because it had a jumpy idle. Well afterwards I took it for a test drive and no problem. It ran well until it reached operating temperature. After going over 15 miles per hour I felt the car running too rich. I had painted the outside of the injectors and I thought this must have screwed it up. I ran a key on engine running test and all injectors checked out well. I believe this test would have showed me if any sensors were bad, everything checked out good. So after this I said must be a vacuum leak but then I said it doesn't happen when it is parked or in neutral. So I said it must be related to the transmission. The transmission was a little low on fluid but still on the crosshatch area. I refilled it thinking it was low on fluid. yet it still started to high idle in drive. But now it was when I noticed it would happen at over 15 mph. So i finally decided to disconnect the speed sensor. I did this because I had rebuilt a nissan altima transmission which had a bad speed sensor and caused the car to shift late and the engine run a little weird. After disconnecting it it ran perfect . Perfect idle and great acceleration. This also happened after I rebuilt the AOD transmission. Its been only 1.5 years that I rebuilt the transmission. Probably the speed sensor was causing too much resistance or it was starting to malfunction. It was still telling me the speed but I was noticing everybody accelerating and going faster. Then again in miami there is too much road rage.

[sly]

check for vacuum leaks (especially that huge ass breather hose that goes down behind everything - I had to put radiator clamps on that POS to make it stay put) as that can definitely cause this issue.

[pivichot]

It was running well but it returned. Guess it wasn't the speed sensor. Does anybody know what would cause the car to accelerate by its own? I sprayed with PB blaster the accelerator cable and the spring on the throttle body that hasn't helped. This car has cruise control. Any suggestions?

[sly]

if the cruise cable is adjusted badly, it'll hold the throttle open some. You might try adjusting a little slack in the cruise cable and see if that helps.

[pivichot]

I have adjusted the cruise control didn't help. Disconnected it still didn't help. Over 15 to 20 or 25 to 30 this problem occurs. When I go over 1500 rpm it starts to accelerate and I hear a buzzing noise from the intake. I did a KOER test and it came up with code 412. I have done several KOER tests and all have been inconsistent. The IAC valve has been replaced and I believed it to be the tps and I have gone through 2 and I am sure it is not the TPS. Does anybody know the symptoms if a bad ECU? Also Merry Christmas.

[miamibob]

DTC 412 = Cannot control RPM during KOER self-test high RPM check. . .
KE1 - Check For RPM Drop

Diagnostic Trouble Code (DTC) 412 indicates that during Engine Running Self-Test, engine rpm could not be controlled within the Self-Test upper limit band. . .

Possible causes: . .

Open or shorted circuit. . .
Throttle linkage binding. . .
Improper idle airflow set. . .
Idle Air Control (IAC) solenoid contamination. . .
Items external to Idle Air Control system that could affect engine rpm. . .
Damaged IAC solenoid. . .
Damaged Powertrain Control Module (PCM) . . .
Key "OFF". . .
Connect engine tachometer. . .
Start engine. . .
Disconnect IAC harness connector. . .
Does rpm drop or stall? . .

[pivichot]

If I disconnect the IAC valve the rpms drop but the engine doesn't stall. I just runs quiet. The throttle linkage I have soaked in PB blaster to make sure and I have checked it but it seems to be okay. If I disconnect the IAC valve it won't produce the problem but I get engine pinging. The same with the tps. To remove the pinging I have to have both sensors connected. The car has what people know as the bastard block. This would be a perfect excuse to replace the drivetrain but don't have the money for it now. Could there be a short with the signal that is provided by the pcm to these two sensor that could cause the problem.

[sly]

sounds like it's not anything to do with the mechanical links and has everything to do with a vacuum leak since the sensors seem to be doing their job.

Check for cracks in the large plastic section of the intake tube as well as making sure all hoses on that are connected to everything they're supposed to be.

[pivichot]

Changed out all vacuum hoses and it decreased dramatically. Now I just hear a Humming noise. I talked with Rockauto, since I bought the IAC valve from them, they already have a replacement on its way. I bought a standard motor IAC probably got a defective unit. next time Will buy a Mototrcraft. The only time it somewhat accelerates is when it enters gears say from 1 to 2 when going slow or somewhat hard on the throttle. When it does this apart from hearing the mechanical fan working I hear the Humming buzzing noise. Will update when finished.

[SVT98t]

Yeah, that big bastard hose goes down to the IAC. If that's not hooked up right, everything goes fubar.
I'd take the time to clean out the throttle body and MAF as well.
How many miles are on your GM?

-ryan s.

[pivichot]

Yeah, that big bastard hose goes down to the IAC. If that's not hooked up right, everything goes fubar.
I'd take the time to clean out the throttle body and MAF as well.
How many miles are on your GM?

-ryan s.

The car has unknown mileage. Not lying I have hooked up now a police interceptro odometer and currently the car with me that I have driven it 3000 miles. The car stopped at 85000 . This could be 85000 or 185000. The car was in really bad condition when I got it. I just replaced all vacuum hoses and put clamps on all of them.The car stopped having these surges but only does it about 30 minutes after now. I believe it is the clutch from the mechanical fan. It only does it when it is hot and if I accelerate the engine it goes away. Unless the ECU is going bad. I have invested more than 1200 on the car and now this problem. Now at least I could control it.