2001 GMC Jimmy IAC Sensor

Tiny
MIK71670
  • MEMBER
  • 2001 GMC JIMMY
  • 157,000 MILES
Well after 2 months of putting a new IAC its says my IAC is bad. Code was cleared and then came back. When I start the car for the first time each day the RPMs go to 3000 I have to put it in gear just to drop the RPMS. I looked at the IAC when it was idling and it was moving when I pushed on the butterfly spring. Also the gasket that goes on top of the throttle body has some chunks missing. Called gm and they don't even carry it. I am wondering if that is causing a vacuum leak. This thing is driving me nuts. HELP!
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Saturday, July 27th, 2013 AT 7:26 PM

6 Replies

Tiny
CARADIODOC
  • EXPERT
Fault codes never say to replace parts or that they're defective. They only indicate the circuit or system that needs further diagnosis. There are a number of different fault codes related to that valve but basically there are three ways for the system to have a problem. The most common is the valve gets gummed up and the motor can't turn the threaded shaft to run the valve in and out. That can set an improper idle speed-related code but usually no code is set. The typical complaint is the idle speed is too low and the engine often stalls at stop lights. You also won't get the nice idle flare-up to 1500 rpm when starting the engine. The second cause is a break in a wire for one of the four coils inside the motor. That is very rare to begin with and it usually requires two wires to be broken before the computer will detect that. These valves really don't fail very often. The third and most common cause is one of the four wires to the motor is broken. If you have a fault code related to an electrical problem, look for a broken wire, or a corroded or stretched terminal in the connector. The motor will not turn at all if one wire / terminal has a bad connection. If the idle speed does change but an electrical code keeps on setting it is due to a momentary break in the circuit due to engine vibration that is being detected by the computer. That only has to occur in one wire for a fraction of a second to set a code.

Next, you can't see the IAC valve move when it's installed. You might be looking at the throttle valve but the engine shouldn't even run with the fresh air tube removed because the air won't be going through the mass air flow sensor which it must do for the Engine Computer to calculate how much fuel is needed.

The IAC valve gives the computer a lot of control over idle speed but it won't run it up to 3,000 rpm and leave it there. It sounds like you already found the cause of the high idle with that vacuum leak. The computer can close the IAC valve completely but that isn't going to overcome a vacuum leak. If a gasket is no longer available, use some RTV gasket sealer to seal the leak. Be sure to let it set up and cure for an hour or two before starting the engine. I used it once as a very effective temporary fix, but when I started the engine right away it got sucked in and disappeared!

Shifting into gear at 3,000 rpm is real hard on the transmission and the engine mounts. Of course that will bring the engine speed down, but then you'll most likely be driving at 35 mph without pushing the accelerator pedal, and that will be hard on the brakes. Concentrate on fixing the vacuum leak first.
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Saturday, July 27th, 2013 AT 7:55 PM
Tiny
MIK71670
  • MEMBER
We took off the air intake tubing with the mass air connected to it and put it to the side. The mass air flow was disconnected. I unplugged both mass air flow and the air temp sensor. The car started up and we shined a light in the back of the throttle body and we could see the tip of the IAC move back and forth. It took a sec for the throttle to come back down after pushing on the butterfly spring. The back of my throttle body there is an oblong opening where we saw it. It was definitely the tip of the IAC. My car does some crazy stuff. One more thing. The secondary air pump under the radiator has a brittle rubber line about 6 inches long, could that also be a leak. I wish I could take a video and show you what we saw last night.
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Saturday, July 27th, 2013 AT 8:11 PM
Tiny
MIK71670
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Tomorrow I will try and send a pic so you can see the tip of the IAC when it moves forward
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Saturday, July 27th, 2013 AT 8:12 PM
Tiny
CARADIODOC
  • EXPERT
Every throttle body I've ever worked on had the IAC on the side where the valve couldn't be seen. I looked at a picture of yours and it looks like the valve is on the bottom and sticking up so I suppose it could be seen. Unplugging the MAF explains why the engine ran with the tube off. The Engine Computer would detect that and inject approximate values based on other criteria to calculate fuel needs as best as possible. You should find the engine won't run with the MAF connected and the tube is off.

The air pump system is not related directly to idle speed or a vacuum leak. He just injects extra air into the exhaust system to mix with any unburned fuel so it will burn in the catalytic converter. If the hose is dry-rotted replacement is all that's needed. If it is burned or melted, look for a failed check valve in that line. That will allow hot exhaust gas to get up there and burn the hose over time.
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Saturday, July 27th, 2013 AT 8:35 PM
Tiny
MIK71670
  • MEMBER
Took off throttle body and gave it a good cleaning and put a NAPA idle air control valve in and it made a hugh difference. Now the only times the speed still goes up is at low speeds. Would adjusting my idle down help it?
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Thursday, August 1st, 2013 AT 5:56 AM
Tiny
CARADIODOC
  • EXPERT
Idle speed is not adjustable when you have an idle speed motor. The Engine Computer pulses the motor's four coils with varying voltages and polarities to precisely position the armature to one of 256 settings. As the armature turns slowly it runs a threaded shaft in and out to adjust the opening of the valve to allow more or less air to bypass the throttle blade. At the same time it increases the injector on-time to add more fuel to go with the extra air. If you did something to open the throttle blade more, the computer will just close the IAC valve more.
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Thursday, August 1st, 2013 AT 12:57 PM

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