High idle runs rough and stalls

Tiny
BETH GREINER
  • MEMBER
  • 1992 CHEVROLET TRUCK
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 88,000 MILES
When the truck is started it runs at a high idle, and stalls when put in gear. Tuning it up today. Have read about the IAC. Any ideas welcome! Thanks in advance!
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Sunday, March 19th, 2017 AT 8:27 AM

8 Replies

Tiny
CARADIODOC
  • EXPERT
High idle speed without a corresponding increase in power is usually caused by a vacuum leak. Start by pinching off each vacuum hose attached to the intake manifold to see if idle speed drops. If you find one that does, follow it to its branches and pinch off each one, one at a time, to narrow down the location of the leak. You might also try running a small stream of water over the joints between the intake manifold and cylinder heads while the engine is still cold, and running. If there is a leaking gasket, you might get lucky and see the water getting sucked in.
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Monday, March 20th, 2017 AT 3:55 PM
Tiny
BETH GREINER
  • MEMBER
Thanks as I said tuned it up today started sounded great except for a clicking noise shut it off the cap was not tight screwed it all the way down now it won't start! UGH.
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Tuesday, March 21st, 2017 AT 3:51 PM
Tiny
CARADIODOC
  • EXPERT
You didn't mention that before. It's apparent you know where to look because you only changed one thing, then the engine wouldn't run. Inspect the rotor and the inside of the distributor cap first to see if they were rubbing and something got damaged.
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Tuesday, March 21st, 2017 AT 5:49 PM
Tiny
BETH GREINER
  • MEMBER
Back to the same problem! Could the coolant temp sensor be it?
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Wednesday, March 22nd, 2017 AT 3:12 PM
Tiny
CARADIODOC
  • EXPERT
Very doubtful. The Engine Computer uses readings from the coolant temperature sensor to know when to turn on the electric radiator fan, (for vehicles that use them), when to engage the transmission's lock-up torque converter, and to a small extend, to increase idle speed when the engine is cold and the fuel doesn't vaporize well. (It needs to be a vapor to burn. Liquid fuel won't burn). It is extremely rare for a temperature sensor to fail because there's just one component inside them. Much more trouble is caused by corroded connector terminals and wiring problems.

If a coolant temperature sensor were to read a temperature lower than actual, but smooth and steady, the symptom would be slightly high idle speed, but engine power would be normal. With a vacuum leak, you don't get extra power to match the higher speed. (A mechanic would make this observation by how far he has to push the accelerator pedal to get the vehicle moving).
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Wednesday, March 22nd, 2017 AT 6:50 PM
Tiny
BETH GREINER
  • MEMBER
Understood. But you have to push the pedal to the floor in order to get it to go anywhere. Ignition control module? Oxygen sensor? Ahhhhhhhhhhh? Lol
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Wednesday, March 29th, 2017 AT 5:54 PM
Tiny
HMAC300
  • EXPERT
Just to add here these tbi unit trucks used to blow the gasket at abase of tbi and cause a vacuum leak leading to high idle, rough running etc. Usually at back or front. Check with carb cleaner if rpm changes it's more than likely the problem.
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Thursday, March 30th, 2017 AT 6:05 AM
Tiny
CARADIODOC
  • EXPERT
Thanks brother HMAC300.
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Thursday, March 30th, 2017 AT 4:13 PM

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