Really weird idle problem when car is in motion

[Boehlldozer]

I have a 1991 Eagle Talon TSi 2wd that drives great but has a really weird idle problem that has been baffling me and all of the mechanics that I have asked about it or taken it to. The problem is that when the car is sitting still, it seems to idle just fine at the usual 750+-50 rpm, but once the car gets moving while not under power, such as when coasting in neutral or even if somebody just pushes the thing a couple of feet (the problem is the same weather or not the clutch is engaged too), the idle jumps to about 1200 to as high as 2000 rpm until the car comes to a stop, then the idle falls back down to normal. I'm beginning to get suspicious of the speed sensor, since the problem is directly related to the movement of the vehicle itself, and thats the only thing that I know of that could let the ecu know that the car is in motion. does anyone know of anything that could cause this?

[Blackcrow64]

Does it jump to the higher RPM and just sit there until you come to a stop or does it surge the whole time?

[david-b]

Could be a vacuum leak also. Extra air could be getting in places where it's not supposed to and causing the rpms to raise. And that could be anywhere, EGR to PVC so check everything.

[Boehlldozer]

Originally Posted by Blackcrow64

Does it jump to the higher RPM and just sit there until you come to a stop or does it surge the whole time?

It usually just jumps up and sits there till I come to a stop....but every once in a while it will fluctuate, like it will jump up to about 12-1300 RPM for a few seconds, and then jump again up to 1500-2000 rpm and sit there till I come to a stop.

[Boehlldozer]

Originally Posted by david-b

Could be a vacuum leak also. Extra air could be getting in places where it's not supposed to and causing the rpms to raise. And that could be anywhere, EGR to PVC so check everything.

I'm pretty sure its not a run of the mill vacuum leak, I've checked most everything, I even put new fuel injector O-rings, a new O-ring on the idle set screw, and put a new plug on the set screw. The intake manifold gasket is also fairly new as well as the throttle body gaskets and most of the vacuum lines. I've checked the EGR valve for correct operation too, and cleaned it.

I hear a lot about the ecu causing problems in these cars, could that cause it? I've been hesitant to think that its that though because it was replaced shortly before I bought the car, but that was back around 2003....

[Blackcrow64]

I wouldn't think this would be an ecu issue... I mean, I'm not saying its impossible but highly unlikely. I would think it to be something simple like the pcv valve... Or a vacuum leak. I actually can see both of those things causing a problem very similar to this.

[Boehlldozer]

Is there anything besides the speed sensor that tells the ecu if the car is moving? My cruise control doesn't work either (I never really cared thought cause I never use it) could that possibly have something to do with it too?

[steviek]

very unlikely its the cruise but I think its important to mention that the cruise will not work if the speed sensor doesn't work.

[big white bufflo]

could it be a throtle position sensor the will do that

[Bodack]

Originally Posted by big white bufflo

could it be a throtle position sensor the will do that

Probably not.

[Boehlldozer]

I FIXED IT!!!! I'm still not exactly sure what the problem is, but what I did was disconnect the vehicle speed sensor from the ECU. It was a yellow wire with a green stripe that was plugged into the back of the speedometer. I simply removed the wire from the harness, put a cap on it, and tucked it behind a bracket. the problem is COMPLETELY GONE now!!!

I have a theory on what caused it in the first place though. I believe that the cruise control (the dead cruise control) somehow was sending weird readings to the computer or making the computer think it was on when it wasn't, cause it was broken, or something like that.