85 turbo - idle going crazy

[mark french]

Idle speed keeps fluctuating for no reason, way overspeeding sometimes. Tried swapping in a new throttle position sensor as this worked on another one of these cars I had once, no change. Also when car is doing this when first cold, the speedometer starts reading off random numbers (??) but this seems to go away shortly. Am not getting any power loss light nor any stored codes. It definitely seems to be the computer telling the idle motor to increase erratically - if I unplug the idle motor, idle stays steady forever until I plug it back in, in fact I'm driving it like this now. It seems to vary between normal and various overspeeding, it is not making it go too low (not stalling). Any suggestion on what to check or change next? Could it be either of the sensors in the distributor? I changed the rotor but thats it.
thanks!...Mark

 

[Steve Stone]

Try the ISC ?

 

[Neil Nelson]

(mark french) wrote:

Idle speed keeps fluctuating for no reason, way overspeeding sometimes. Tried swapping in a new throttle position sensor as this worked on another one of these cars I had once, no change. Also when car is doing this when first cold, the speedometer starts reading off random numbers (??) but this seems to go away shortly. Am not getting any power loss light nor any stored codes. It definitely seems to be the computer telling the idle motor to increase erratically - if I unplug the idle motor, idle stays steady forever until I plug it back in, in fact I'm driving it like this now. It seems to vary between normal and various overspeeding, it is not making it go too low (not stalling). Any suggestion on what to check or change next? Could it be ...

The vehicle speed sensor (distance sensor) is one of the in-puts to the logic module for idle speed control, i.e., the logic module needs to know the difference between closed throttle decel and closed throttle vehicle stopped.

Yes, the distributor pick-ups in these engines have always been problematic, so between that and the distance sensor, there's two things to try.

 

[frenchy]

Neil Nelson wrote the rotor but thats it.

> thanks!...Mark

The vehicle speed sensor (distance sensor) is one of the in-puts to the logic module for idle speed control, i.e., the logic module needs to know the difference between closed throttle decel and closed throttle vehicle stopped.

Ahh that might explain it, and the digital speedo sometimes reading crazy numbers at idle, and fact that it is only raising it above the normal idle and not dropping it below it. I will pick one of those up and try it, thanks...Mark

 

[frenchy]

Before I spend $ on the speed sensor...if I unplug the reference pickup, the idle drops to a steady level. Plug it back in and idle jumps back up and starts hunting up and down again. Does this indicate the problem might actually be that pickup? thanks! Mark

 

[frenchy]

Correction, I am unplugging the sync pickup, not the reference.

 

[frenchy]

AFter changing the distributor pickup I think the problem turned out to be some crud in the throttle body plate and/or orifices. I had scrubbed it while back but apparently some more must have built up. Idle must have been simply adjusting to fact the crud was built up in that area. Cleaned it and idle is steady as a rock now....Frenchy