Marky Dissod
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Not yet, but I'm about to take it off the engine, take the IAC motor & throttle sensor off, and clean the ever-lovin schidt out of everything thrice.
My '02 Tahoe L59 revs WAY too high on cold start ups. Anyone got any ideas?
- Thread starter Marky Dissod
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I'm with the guy that earlier asked about vacuum leaks, specifically intake gaskets. Was a known issue on at least the 01s and I suppose the 02 as well. I experienced the same thing. The colder it is the worse it would get, once it warmed up a bit you would never know...If they are original gaskets just change them.
his has a IAC idle air control valve, which is what controls the idle primarily mechanically anyway, the 00-02 had them with drive by cable, previous years probably did as well as egr, in 03 they went to drive by wire with a fully electronic idle control (single piece unit),
lots of them in junkyards, cheap fix if it is the iac, unless you want to buy a brand new one
in the photo below the one on the left has a iac which bolts on and has what looks like a metal ball inside, the one on the right is drive by wire it also has a iac but it is all electronic
After it warms up to temp and idles properly, unplug the iac while idling, shut the truck off, and let it get cold. Try starting it again while cold with the iac unplugged and see if it still revs right up.
Most likely it won't rev up. I would guess you have a vac leak. When you start an iac equipped vehicle it uses a "parked" position for the iac. When the vehicle is cold, the parked postion is much more open so the vehicle gets the extra air needed to idle when cold. The vehicle always re-sets to the same parked position vs temperature, so if you have a vacuum leak, the truck will always flair when cold until the truck compensates for the high idle by closing the valve. No amount of learning will help because it will always re-set to the same iac park piston vs coolant temp.
A common vacuum leak on this year is the rear valve cover vent drivers side, check to see if that hose is off or split.
Most likely it won't rev up. I would guess you have a vac leak. When you start an iac equipped vehicle it uses a "parked" position for the iac. When the vehicle is cold, the parked postion is much more open so the vehicle gets the extra air needed to idle when cold. The vehicle always re-sets to the same parked position vs temperature, so if you have a vacuum leak, the truck will always flair when cold until the truck compensates for the high idle by closing the valve. No amount of learning will help because it will always re-set to the same iac park piston vs coolant temp.
A common vacuum leak on this year is the rear valve cover vent drivers side, check to see if that hose is off or split.
And this would be super easy to figure out with a scanner and logging the idle air duty cycle. If it's a vac leak, you will see the iac counta drop way down, probably right to 0 with a leak this bad if it is a leak. The iac will close as far as it can, then the truck will start pulling timing to get it down to the desired idle.
PatDTN
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I used to use an unlit propane torch tip around connections that could leak air. Make sure to take a good look at vacuum hoses for cracks or disconnected hoses.
Propane will cause a momentary jump in rpms even with a warm engine.
Propane will cause a momentary jump in rpms even with a warm engine.
rockola1971
Full Access Member
Cold starts require a rich fuel mixture (aka choked). You may not be old enough to remember carburetors and automatic chokes and before them manual chokes. In fuel injection to obtain a "choked" condition the on time is just increased on the injectors. In carbs they just mechanically shut the butterflies until the engine warmed up.
I would pull your IAC and see if the pintle is caked with carbon deposits. Large chunks can get wedged behind the pintle which doesnt allow the pintle to fully travel retracted. Brake cleaner instantly dissolves this carbon. LS engines are notorious for intake manifold gasket leaks so spray around the manifold sealing areas with ether or the like and see if idle jumps. If it does then you found the leak. A bad engine coolant temp sensor could cause your problem too. If PCM doesnt see the engine warming up it will try to run in choke mode too long.
Not trying to be THAT guy, but the more likely culprit would be that the pintle is restricted from closing quick enough after start up. With the added fuel of the cold start enrichment based off the coolant temp adder, the truck would rev high if the pintle was stuck more so outwards away from the seat, than in. It's easy to get that backwards if you picture the the airflow working like a carb by increasing the venturi draw on the nozzle by restricting air flow. In the case of efi, they are two independent functions, so you add more idle air to go along with the extra fuel added via the injector enrichment.
rockola1971
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The carbon restricts or completely stops the pintle from moving depending on where it gets wedged. It can wedge between the pintle and the inner bore of the IAC or behind or in front of the pintle reducing or completely stopping any movement inward and outward. This was a very common problem in the 2.8 V6 engine years ago. And high idle or rough idle was the symptom.
