Racing idle that doesnt slow down when warm

DRROBERT

MEMBER
1992 CADILLAC DEVILLE
107,000 MILES

Just received this car and when first received it was fine within two days it has progressed to the car almost accelerating by itself. My mechanic is ill and allows payments over time I dont have funds right now. I dont know what I can do it is my only means of transport I will donate next payday.

7 Replies

JUNKIE2100

MEMBER
62 POSTS

First thing I would look into is the tps, throttle position sensor, just not sure if disconnecting it would drop the throttle or not and mine is already in bad condition I dont really want to go and try to do a test of that, hopefully someone else will know, also I would check into the diagnostic codes and see what the engine says is wrong, grab a pen and paper and follow these instructionsand it will display the codes, make sure you get them all current and history just in case, then bring them here or look them up on this page
if you have a code that is about the tps that is likely your issue, if not just bring the codes back to here and at least we will have a better reference point

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Thursday, September 8th, 2011 AT 5:27 PM

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Check the idle air control valve.

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Thursday, September 8th, 2011 AT 5:43 PM

JUNKIE2100

MEMBER
62 POSTS

Its a cadillac, it has an isc motor and is electronically controlled, there shouldnt be an iac valve

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Thursday, September 8th, 2011 AT 5:51 PM

JUNKIE2100

MEMBER
62 POSTS

This may be helpful, these are the tps(throttle position sensor) and isc(idle speed control) on my car, mind you I have the infamous ht4100 where as you probably have the 4.5 or possible 4.9 but if I remember correctly they all use the same components at that level. I think I pulled my replacement off of a 4.5 and they would be in about the same place if not exactly where you see them in the picture, you will have to remove the air intake but it shouldnt be difficult just make sure to be careful with the tubes connected to the under side of it they are fragile, in the yellow box is the isc it pushes the throttle to where it needs to be, in the red is the connector for the position sensor which tells the car where the throttle is at, it is mounted flush behind the throttle body so its hard to get a good pic, and if it cant get a reading then it could send the throttle ever higher untill it can go no more making the car idle at a very high rpm. And please excuse my ghetto alternator wiring

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Thursday, September 8th, 2011 AT 6:21 PM

JUNKIE2100

MEMBER
62 POSTS

Also the coolant temperature sensors have something to do with idle speed as well, seeing as you said it does it when its warm that might be something to look at, the idle on my car goes nuts when I unplug both sensors

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Thursday, September 8th, 2011 AT 7:17 PM

FACTORYJACK

MECHANIC
4,159 POSTS

There is a nose switch on the ISC pushrod that senses when the throttle cam is at closed throttle, therefore the ECM can control idle. Check this pushrod, is it fully extended? If it is fully extended, lift the throttle slightly off this pushrod, and press on the rod. If it tries(or does) fully retract, the isc is at least functional. If it does not, there may be a fault with the ISC motor. The throttle shaft could also be binding, if this was the case, there may be a gap between the ISC pintle and the throttle cam. Press the throttle cam against the pintle, and hold the pressure to see if the ISC tries to control. If it is binding in the throttle body, you would have to replace it, and more than likely you would be locating a used one. Sometimes you can manipulate the return spring to increase the return pressure, this will also cause more force on the pedal to accelerate. If it is 'sailing on' after an acceleration event, then you may just need to have the minimum air rate, and tps adjusted, along with an idle learn.

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Friday, September 9th, 2011 AT 2:46 AM