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'90 DeVille '97 DeVille
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Discussion Starter #1
My '90 Deville with 123K runs great (better low end punch than my NorthStar) but I have a couple funky idle problems. When you slow to a stop sign, it seems like curb idle doesn't occur till after you stop. What gives with this? Also, it "hunts" 3 or 4 times for curb idle before it settles down. Idle Speed Control motor is fairly new. Would it be worthwhile to check Throttle Position Sensor setting? Thanks for your help!
 

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93 & 88 Allante, 88 Eldorado
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mdogg1950 said:
When you slow to a stop sign, it seems like curb idle doesn't occur till after you stop.
My dad's Deville does this also. Mine used to do this but I got tired of it and disconnected the ISC and set the idle at 500 rpm. I've noticed a couple mpg improvement in fuel economy around the city since disconnecting the ISC. Don't know if a new ISC would help this or not or is it just the nature of the beast.
 

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'90 DeVille '97 DeVille
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Discussion Starter #3
Disconnecting ISC may be worth a try. With ISC disconnected, does the cold engine still have fast idle?
 

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93 & 88 Allante, 88 Eldorado
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With the ISC disconnected you do loose the fast idle when engine is cold, A/C is on, turning the steering wheel, etc. I've learned how to keep my foot on the gas when starting the engine to keep it running. After about 15-20 sec. of warming up it will idle fine at 450 rpm. I also make sure to turn off the A/C when coming to a stop light or when driving around town. It takes a little time to get used to, but the extra in town gas mileage as well as the increased brake life makes it worth it.
 

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I'm a Cadillac Fanatic!
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If the idle speed control motor is operating correctly and the closed throttle switch is operating fine inside the ISC (assume it is since it is new..???) then the first thing to do is to clean the throttle body and disconnect the negative battery terminal to reset the idle learn offsets.

Hold the throttle blades wide open and spray the throttle bores beneath the throttle blades with carb cleaner and scrub with an old toothbrush. If the deposits are building up at the blade interface to the throttle bore it may be restricting the throttle movement in that area, blocking off air flow and generally screwing up the idle speed control. There is no reason to disconnect the ISC and fix the idle speed. This provides for no cold start idle and can lead to stalls and other problems as there is no way then for the ECM to compensate for ambient conditions or engine idle load increases. If the idle speed control is operating correctly it should seem "invisible" to the driver and allow the system to control to the lowest idle speed possible for maximum fuel economy. If not, something is amiss and diagnosing and correcting the problem would be the recommended approach rather than disabling the system.
 

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2008 SRX-V8, 1991 Eldorado
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********* said:
If the idle speed control motor is operating correctly and the closed throttle switch is operating fine inside the ISC (assume it is since it is new..???) then the first thing to do is to clean the throttle body and disconnect the negative battery terminal to reset the idle learn offsets.

Hold the throttle blades wide open and spray the throttle bores beneath the throttle blades with carb cleaner and scrub with an old toothbrush. If the deposits are building up at the blade interface to the throttle bore it may be restricting the throttle movement in that area, blocking off air flow and generally screwing up the idle speed control. There is no reason to disconnect the ISC and fix the idle speed. This provides for no cold start idle and can lead to stalls and other problems as there is no way then for the ECM to compensate for ambient conditions or engine idle load increases. If the idle speed control is operating correctly it should seem "invisible" to the driver and allow the system to control to the lowest idle speed possible for maximum fuel economy. If not, something is amiss and diagnosing and correcting the problem would be the recommended approach rather than disabling the system.
I agree, as I had a similar problem with my 91. I had already cleaned the throttle body, changed plugs and wires, set timeing to factory specification, calibrated ISC and TPS all with the engine running fine. The car sat for maybe six weeks without being started or moved. I started to drive the car again and noticed a rough idle when it was warm, while braking for a stop. First thing I checked was the ISC travel (according to the FSM procedure). Although it was within specification, the throttle position (stop) was not where I had set it. Sure enough, the throttle plate was beginning to carbon up again, but not enough to throw a code. This time I removed the throttle body, thoroughly cleaned the underside with a soft brush and carb cleaner. Cleaned the EGR tubes (again, though they were not bad), installed a new throttle body gasket, reassembled. Yes, I again set the ISC to relearn and the problem is gone. I just don't understand why it carboned up again so quickly.

The car has 153K miles on the clock. I have used only Shell premium gasoline in the car since I put the car back on the road (about one year now since I got the car). I haven't driven the car on a daily basis or taken it on any trips greater than 25 miles one way for the last few months. It averages around 13 MPG around town and about 22 on the highway, but highway travel has not been recent. Most any other car at 153K, with a poor maintenance history, would show signs of uneven or low compression and sluggish response. Mine is very responsive and amazes anyone who drives it. I have tuned the engine in this car and replaced only the throttle position sesnsor, and kept the fluids fresh according to factory specification. Everything works as it should. Once you disable an engine control, you are inviting new problems, some may become serious. The ISC is in place for a reason. Manufacturers are always looking for ways to reduce cost, so every part, right down to a plastic hanger is installed for a reason. Bottom line, you need them all in place and working properly to get the best performance from your vehicle. Will the car still run with stuff bypassed or shorted? Maybe yes, but not the way it was designed. The next problem to occur may not be an easy fix if controls have been bypassed.
 
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