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1991 Cadillac Fleetwood 60 Special
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230 Posts
Discussion Starter #1
ISC ratchets after shutting off the car. I want to be proactive and replace it before it dies completely.

Anybody have the GM part#?

1992 Fleetwood Sixty Special
 

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'87 Coupe DeVille 4.1L FWD(New engine '08) 1981 F100 '03FXDL
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879 Posts
AC/DELCO Part # D6589 Check the ISC for a loose plunger. I had one the was acting similar and the plunger was screwing/unscrewing on its' own. A dab of threadlocker cured the problem.
 

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1987 Eldorado
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82 Posts
My ISC was acting erratically. I took it apart and cleaned the contacts inside. Put some dielectric grease in it and reassembled. No problems since then, about a year ago.
 

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1991 Cadillac Fleetwood 60 Special
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230 Posts
Discussion Starter #4
The plunger threads are fine, actually very tight.

Another problem I'm having is that when i come to a stop, I'm fighting against the car. The ISC doesn't return to the normal retracted position until about 5 mph, so I'm braking and the car is still giving gas.

If i let off the gas just above that 5mph mark, the car will accelerate even though my foot isn't on the pedal. Again, the ISC isn't retracting.

Also, every time I start the car, the engine revs to about 1500-2000 for about 2-3 seconds then slowly comes down to a regular idle.

Yesterday I adjusted the TPS to within spec (0.2 with ISC fully retracted, it was at 1.4), but it hasn't made a difference.

The switch on the ISC seems to be functioning: when I retracted the ISC to adjust the TPS, it would stop when I pressed on the plunger.

Are these all symptoms of a bad ISC or something else?

I thought maybe there was a vacuum leak, so i sprayed carb cleaner around all the vacuum tubes with the car running, no change in rpm.
 

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1991 Cadillac Fleetwood 60 Special
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230 Posts
Discussion Starter #6
E13 O2 sensor - I know i have a bad sensor, needs to be replaced.

E26 Throttle switch short - That's the plunger switch on the ISC. But that's because i had it disconnected while the car was running.

No other codes, current or historical.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,738 Posts
Is the TPS adjustable? If so, then its the culprit. A misadjusted TPS (adjusted too rich) will cause the ISC to try to compensate by extending and/or retracting. The ratcheting when shut off is the giveaway. It seems that you have access to the adjustment specs so review the TPS specs. Also cleaning the throttle body with the spray and a toothbrush should be done prior to either a TPS or ISC adjustment. Once done, I believe your car, like my 91 Allante, requires a learned idle procedure. report back.
 

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1991 Cadillac Fleetwood 60 Special
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230 Posts
Discussion Starter #8
As mentioned previously, the TPS has been adjusted to spec. The symptoms are the same before and after adjustment.

TB was also cleaned prior to adjustment, no change.

There is no learn procedure for ISC, only for vehicles equiped with IAC.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,738 Posts
Sorry, overlooked that. Be aware that the learned idle procedure I believe still needs to be done. My 91 4.5 did require it even tho it has no IAC.
 
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