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Oscillating idle - Yukon Denali
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Post Oscillating idle - Yukon Denali 
Often after I first start the 2001 Yukon Denali, the idle will oscillate up and down between 300-500rpm (roughly at 1 Hz). Then after giving it gas for the first time, it will idle ok. It rarely happens other times, such as while waiting at a light. The dealership suggested a full injector cleaning. But if it was the injectors, wouldn't it happen on every idle? I've run injector cleaner through the system a few times, no change. I wonder if it is computer related. Average gas mileage is 15 city/highway.

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Engines are very sensitive to fuel mixture on cold starts. They are trying to keep emissions low, and driveability good, and there is a very fine line to maintain. An out of spec sensor can cause this. Being slightly off on coolant temp sensors, or most likely the air mass sensor will cause the engine to get slightly less fuel than it needs, causing the rough idle. You likely wouldn't notice this under other conditions. I would first attempt a cleaning of the mass air flow sensor and see how the vehicle reacts to that change. You are gonna need some scan tool data to really get much farther with it. A K&N filter that was over oiled or something may have caused the mass air to get a bit dirty, plus its old enough to need cleaning anyways.

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My Dad's 02 Denali does this too. I think we fixed it last time by cleaning the carbon out of the throttle body. I'm guessing the carbon restricts the idle airflow and destabilizes the control system. I'll go out there and clean it right now and see if that fixes the problem.

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Yup, that did it. Get some throttle body cleaner and a rag. Put a brick on the gas pedal and turn on the ignition. Remove the intake duct from the throttle body and spray the cleaner in there around the blade and perimiter of the opening. Wipe it out with a rag and repeat till it's clean. It took a lot of cleaning to get all the stuff out of this one, but it fixed the idle problem.

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Thems drive by wire? I thought they was cable operated..... I can definately see that happening. Wierd that the throttle plates always have carbon buildup on these engines. I haven't seen anything else do that. How does the carbon come OUT of the intake and build up on the throttle plate????

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Beats the hell out of me how the carbon gets on there. My truck with the manual throttle body gets buildup there too. But yeah the Denalis were all drive by wire as far as I know.

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Howdy Dan,

Is that truck stock?

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Thanks pswired, I cleaned the throttle body and it seemed to do the trick.

James, it's stock except for the audio/nav system from Pacific Audio. My fiancee just got an 06 Pontiac GTO.

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saturnstyl wrote:
Thems drive by wire? I thought they was cable operated..... I can definately see that happening. Wierd that the throttle plates always have carbon buildup on these engines. I haven't seen anything else do that. How does the carbon come OUT of the intake and build up on the throttle plate????


If it has traction control it's drive by wire, otherwise cable operated.

The carbon buildup comes from the crankcase ventilation port just downsteam of the throttle blade. Engine manifold vacuum pulls the crankcase fumes through the port at idle, then at higher throttle openings vacuum drops and crankcase pressure blows the oily mist in the passage back towards the open throttle blade.

PS Vitually all engines have some carbon buildup on the backside of the throttle blade. Some are worse than others. The early LS1 truck motors had a problem with metering PCV flow and tended to oil up the blades worse than most. One of GM's tsb fixes was to eliminate the PCV valve in exchange for a small diameter plastic tube.

Edit: Here's more info on PCV operation.
http://www.automedia.com/PCV/Valve/ccr20050101pv/1

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