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My engine is Idling too fast?

Tiny
CAMMAN
  • MEMBER
  • 2000 GMC C1500
  • V8
  • 2WD
  • AUTOMATIC
  • 120,000 MILES
Engine idles at 600 RPM when cold but as the engine gets hotter the high idle increases to about 1500 RPM. A related problem may be that the AC is not working apparently due to low freon. Will the AC problem cause the engine idle contrls to over compensate?
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Wednesday, July 2nd, 2008 AT 2:50 PM

18 Replies

Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
These are known for the IAC motor needs to be replaced. Here is a guide to help with guides below to show you how on your truck.

https://www.2carpros.com/articles/engine-idles-too-high

#30 in the diagrams Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.
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Wednesday, July 2nd, 2008 AT 3:03 PM
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
  • 1997 GMC C1500
  • V8
  • 4WD
  • AUTOMATIC
  • 180,000 MILES
Just bought this truck. In the beginning it ran at 2500 rpm when idling and 1500 rpm in gear. Put it on the reader at Advance. Had a MAP sensor error and an IAC error, also cylinder five was missing. Replaced the MAP sensor and the idle air control, and plugs wires, and pcv valve. Had codes cleared and drove it home (15 miles)

Now it idles at 1800-2000 rpm and 900 rpm in gear. May still be missing but I'm unsure at this point. Checked the vacuum lines. Does seem to be one missing from a small junction (pure vacuum diag. Switch?) On the passenger side of the intake manifold. It is directly behind the alternator, has a blue plug and two vacuum lines. One goes to the can on the drivers side and the other is missing.

If more info is needed let me know. It has the 350 in it.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
  • 4,749 POSTS
Try spraying carb cleaner at base of intake manifold to see if you have a vacuum leak. Remove oil cap and put hand on top of it and see if you have suction, possible internal intake vacuum leak.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
Alright I will try that when I get home from work tomorrow. If there is suction at the oil fill that will indicate what exactly? When I spray the carb cleaner will I see the fluid being sucked in or will I hear the motor begin to flood as you would if you sprayed it directly into the throttle body? If there is a leak I am assuming we're talking about a new intake gasket or worse to repair. Thanks for your time.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
Alright I will try that when I get home from work tomorrow. If there is suction at the oil fill that will indicate what exactly? When I spray the carb cleaner will I see the fluid being sucked in or will I hear the motor begin to flood as you would if you sprayed it directly into the throttle body? If there is a leak I am assuming we're talking about a new intake gasket or worse to repair. Any thoughts on # 5 misfire? Thanks for your time.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
  • 4,749 POSTS
The suction on oil fill cap would indicate internal vacuum leak from the intake gasket (very common) this also if its on the #5 cylinder will cause the misfire also. Unfortunately on this designed engine with the intake manifold, it has to be replaced on every one of them. They have since redesigned the gasket to prevent premature failure.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
Hello, I checked it out. Spraying it had no effect but when I pulled the oil fill cap she stalled. I put the cap back on, started it back up and removed it again slowly. Once it was unscrewed I could feel the suction holding the cap in place. I quickly removed it and place my palm over it and felt a good deal of suction. You were right. Good job. Now I think I need to pull the manifold and check the gasket and the spacer and replace what is broke. Does this sound like the right course of action and can you offer me any insight as to any issues I may encounter, or things to look for or check? I changed the intake manifold gasket on my other chevy truck last winter(1995 chevy 1/2 ton w/305) the gasket blew and I had coolant everywhere. Pulled it apart, drained, flushed and refilled the oil and no more issues. With the 97 being a vortec will there be much differerance? Your advice is appreciated.
Thank you.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
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Was the 95 a vortec? This job is pretty easy. A little trick of the trade is not to remove the upper plenom, just leave it on, you can access the bolts to remove the lower intake with it on. Be very careful to mark and remark the distributor on this engine because you need a scan tool to set the timing if you dont get it back on real close. The fuel lines at the rear of the intake you should leave on and disconnect from the rear where the lines connect. This is definitely your problem, your lucky most people go round and round replacing a whole bunch of parts before its diagnosed as the intake gasket. Let me know how you make out.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
I'll begin the teardown this weekend. No my 95 chevy wasn't a vortec. I did have an issue with the distributor on the 95. I marked it but when I put it back together I must have had it off a little because it missed badly. Took a little fine tuning but I got her back in time. No timing light needed that time. What is the best way to mark the distributor on the 97 so I don't make the same mistake twice?
Once more you've been a great help.
Thanks
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
  • 4,749 POSTS
Just mark the rotor position now and then mark it when it comes out because the shaft turns when removing. Also just make sure you mark the housing clearly and you will be ok.
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
ABIRCH
  • MEMBER
  • 6 POSTS
Thanks for all your help. Once I pulled off the intake I saw where the gaskets had been replaced before. The person who replaced them in the past did not install them properly. The gasket was crushed on the end bolts and I could unscrew the bolts by hand.
Got the distributor back in the right place the 1st time. You were right on the money.
Now I will finish going through the truck.
Brakes, heater blower, broke stud, hole in the muffler, and so on:)

Have a good one.
Alex Birch
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
  • 4,749 POSTS
Thanks man
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Thursday, January 7th, 2021 AT 12:34 PM (Merged)
Tiny
KYLE WILLIAMS 7707
  • MEMBER
  • 1 POST
  • 1992 GMC C1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 200,000 MILES
Idle is good I can fine the truck up and everything reverse okay, not good but in drive not so good. Like a vacuum line. New throttle position sensor, MAP sensor and mass air flow.
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Thursday, January 7th, 2021 AT 12:35 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,716 POSTS
Hi,

This could be the result of a few different things. If there is a major vacuum leak, I think you would notice it when you rev the engine in park as well.

Other things such as low fuel pressure or even a plugged catalytic converter can cause this.

If you haven't already, first retrieve any diagnostic trouble codes stored in the computer. You don't need a scanner to do it. A short jumper wire or even a paper clip will do it.

Here is a link that explains how it's done:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

I suggest doing that first because it may point us in the direction of the problem. Of you want, here are links that explain how to check fuel pressure and links reviewing common symptoms related to a catalytic converter issue and how to check.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Start with that and let me know the results. Also, if you have already checked these things, let me know.

Take care

Joe
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Thursday, January 7th, 2021 AT 12:35 PM (Merged)
Tiny
CMSGTSMITH
  • MEMBER
  • 10 POSTS
  • 1990 GMC C1500
  • 6 CYL
  • 2WD
  • MANUAL
  • 78,000 MILES
Hi Mike, My truck has a high idle that I have not been able to solve. I have replaced the Idle Air Control Valve (IAC), the Throttle Position Sensor (TPS), PVC valve and the Coolant Temperature Sensor. When driving I take my foot off the accellerator and the truck will continue at about 25 miles per hour speed. I am sending some pictures of the truck's engine.


http://www.2carpros.com/forum/automotive_pictures/46392_1990_GMC_Idle_Air_Control_and_Throttle_Position_Sensor_1.jpg




http://www.2carpros.com/forum/automotive_pictures/46392_1990_GMC_Gas_Throttle_2_1.jpg




http://www.2carpros.com/forum/automotive_pictures/46392_1990_GMC_Motor_View_1.jpg

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Thursday, January 7th, 2021 AT 12:35 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Hi cmsgtsmith, Welcome to 2carpros

Check the fuel pressure should read 9-13psi

these are known for the gasket below the throttle body needs to be replaced. Here is a guide to help with guides below to show you how on your truck.

https://www.2carpros.com/articles/engine-idles-too-high

#30 in the diagrams Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.
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Thursday, January 7th, 2021 AT 12:35 PM (Merged)
Tiny
CJWORLEY14
  • MEMBER
  • 1 POST
  • 1989 GMC C1500
Engine Performance problem
1989 GMC C1500 V8 Two Wheel Drive Automatic

my truck is idling real high and I dont know what it could be
why is it idling high and how could I fix it
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Thursday, January 7th, 2021 AT 12:35 PM (Merged)
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
If this is a TBI unit vehicle, spray choke cleaner around the base of the TBI and see if your idle goes down. If it is replace the gasket at the base of the TBI as thego bad and leak.
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Thursday, January 7th, 2021 AT 12:35 PM (Merged)

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