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1991 GMC Sierra Idle

Tiny
MURPH2
  • MEMBER
  • 1991 GMC SIERRA
Engine Performance problem
1991 GMC Sierra 6 cyl Two Wheel Drive Automatic

When I start my truck in the morning during the hot weather season the idle speed varies up and down and when I shift into reverse the truck will alternately try to kill and then surge like it wants to run away. I have changed just about every sensor and the egr valve and computer. I had the truck in a shop but they could not find the problem. I have seen similar complaints on Two Car Pros site but the solution was not mentioned. 10
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Tuesday, June 22nd, 2010 AT 10:53 AM

10 Replies

Tiny
MIKE H R
  • MECHANIC
  • 3,094 POSTS
Had that problem with my wife's car, Changed the IAC and fixed the problem, Have you changed the Idle air control valve? You can first remove the two screws and remove the unit then try to clean some of the carbon with carb cleaner. DO NOT MOVE THE PLUNGER IN AND OUT, just spray it and see if that helps. If the idle goes high then you know that is the problem.
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Thursday, June 24th, 2010 AT 9:03 AM
Tiny
MURPH2
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Yes I replaced the IAC and most of the sensors except the Oxygen sensor. Intake gaskets were replaced and throttle body kit installed along with fuel pressure regulator. Mechanic said it appeared to be a computer problem but he could not find it.
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Thursday, June 24th, 2010 AT 9:57 AM
Tiny
MIKE H R
  • MECHANIC
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Has the computer been reset?
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Thursday, June 24th, 2010 AT 10:09 AM
Tiny
MURPH2
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I will have to ask the mechanic. I did not know that the computer on a 91 had to be reset. The computer is a Cardone remanufactured unit and there is no mention of resetting in the istallation instructions. I will ask the mechanic and get back to you.
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Thursday, June 24th, 2010 AT 10:36 AM
Tiny
MURPH2
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Sorry I am a little comfused. I assume you were not talking about programming the computer. I did reset the computer but I replaced the computer in an attempt to solve the idle problem and it made no difference.
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Thursday, June 24th, 2010 AT 2:26 PM
Tiny
MURPH2
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The advice I received so far has not led to a solution to my problem. Still need help.
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Saturday, June 26th, 2010 AT 10:51 AM
Tiny
MIKE H R
  • MECHANIC
  • 3,094 POSTS
What I am saying is disconnect the battery, let it sit for a minute. What happens is that the computer will empty out the stored information. After a minute or two then reattach the battery cable. On newer vehicles the vehicle will have to be driven about 50 miles before the computer is reset. If there was a problem and you had changed a sensor the old sensor information will stay stored in the computer until it is reset.
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Saturday, July 3rd, 2010 AT 3:12 PM
Tiny
MURPH2
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I did reset the computer several times for different reasons. I just replaced the intake manifold gaskets when I replaced a bad heater hose connector and at that time the battery was disconnected for several hours. What I was saying is that resetting the computer did not correct the problem. I replaced the computer specifically in an attempt to solve the idle problem The Battery was disconnected during the process of replaciing the computer and the problem persists.
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Saturday, July 3rd, 2010 AT 7:07 PM
Tiny
MIKE H R
  • MECHANIC
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Sorry to take so long getting back to you, computer problems. With everything you have done with the sierra, and you say it only happens when it is hot out. Check for a vacuum leak. There is a spray that is inexpensive and you can get to spray at a auto store that you spray on the engine. If there is a leak the idle speed will change. I don't remember what the name of the spray is but they should be able to tell you at the store.
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Sunday, July 18th, 2010 AT 3:05 PM
Tiny
MURPH2
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I have replaced intake gaskets and all hoses and like I said I had it in a shop and mechanic could not find the problem. I suspected a very freaky problem and thought that Car Pros may have a report of a like problem I Have the truck back in the shop and if the problem is not found I will look for computer harness problems and connections. I am aware of the test for a vacuum leak and I'm sure the mechanic did those tests also. I want to sell the truck and don't want to stick the new owner with it. Thanks for the reply. I gave up and put it back in the shop. Still will appreciate any advice you might have and I will let you know if the solutiion is found.
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Sunday, July 18th, 2010 AT 3:48 PM

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