1990 Cadillac El Dorado Idle Speed Control

Tiny
DUKEY
  • MEMBER
  • 1990 CADILLAC EL DORADO
  • V8
  • 2WD
  • AUTOMATIC
  • 135,000 MILES
When ignition turned on, idle speed control motor pulses about 3 times. I can feel it at the gas pedal. Engine starts rough from cold with low idle speed and no power to drive off. Soon warms up, power is good, but engine still misfires on light throttle. When ignition turned off, engine warm, idle speed control motor pulses about 17 times.

Throttle position switch gives correct voltages, have changed idle speed motor and spark plugs. No vacuum leaks found. Any other ideas?
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Friday, February 5th, 2010 AT 2:27 PM

4 Replies

Tiny
FACTORYJACK
  • EXPERT
The pulsing of the ISC can be normal as it rehomes itself. 1990 saw it's problems with fuel injectors, when warm they should measure 15-17 ohms resistance each. If they have not been replaced, there are probably some shorted. A misfire at light throttle can also be caused by an EGR valve opening prematurely. There is also a idle learn and minimum air rate adjustment that is to be considered if you changed the ISC motor. Usually the symptom for idle not learned is a sail on condition, when the throttle is released to coast the vehicle can still maintain approximately 30 mph before settling down. It will even speed the car up if you just slightly accelerate and let off the gas.
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Saturday, February 6th, 2010 AT 12:57 AM
Tiny
DUKEY
  • MEMBER
Thank you gstacey, your answer was informative. As a follow-up question, why does the ISC motor continue to pulse when I switch off the ignition - surely the ECM knows that the engine is not running? While the ISC is pulsing like that, is the computer sending "more fuel" signals to the injectors? If so, could that explain the rough running when I start the engine from cold?
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Wednesday, February 10th, 2010 AT 12:39 PM
Tiny
FACTORYJACK
  • EXPERT
It does not pulse fuel when the ISC is doing its extend/retract stuff. I don't believe the ISC after key off is causing you any problem. As long as the behavior is fine when running, speeds are ok at start, and no sail on or stall at coast. The roughness after cold soak could be several possibilities, oil coming from leaking valve guides/seals, injectors bleeding off fuel pressure, coolant in the cylinders, for example. Fuel quality can also bring some cold issues.
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Wednesday, February 10th, 2010 AT 10:59 PM
Tiny
CADDYFAN
  • MEMBER
Dukey -- I've got a 1990 Cadillac Deville doing exactly what you described in 2010, right down to the tapping of the idle speed control motor. The engine performs exactly as you described. Were you able to correct the problems? What did you do?
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Friday, April 15th, 2011 AT 8:06 PM

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