Marky Dissod
Full Access Member
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
Full Access Member
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.
Marky Dissod
Full Access Member
Stipulating to posts 28 , 29 , and 30.
For the record:
a new IAC motor made no difference
the smoke-leak test that found no leaks was done with the engine warmed up, and the OE factory intake between the airbox and the throttle body removed
I'm PERFECTLY WILLING to fire the parts derringer (as in cheaper than the parts cannon) at an '02 vehicle, several times in fact.
What pieces / hoses of plastic / rubber would you replace on a 22 year old vehicle if you were a pragmatic pessimist?
Please understand: I'd be willing to pay for a diagnostic AGAIN, IF it would actually find and fix the problem.
But after spending $150 on a diagnostic session that came up with nothing, why NOT fire the parts derringer judiciously?
For the record:
a new IAC motor made no difference
the smoke-leak test that found no leaks was done with the engine warmed up, and the OE factory intake between the airbox and the throttle body removed
I'm PERFECTLY WILLING to fire the parts derringer (as in cheaper than the parts cannon) at an '02 vehicle, several times in fact.
What pieces / hoses of plastic / rubber would you replace on a 22 year old vehicle if you were a pragmatic pessimist?
Please understand: I'd be willing to pay for a diagnostic AGAIN, IF it would actually find and fix the problem.
But after spending $150 on a diagnostic session that came up with nothing, why NOT fire the parts derringer judiciously?
iamdub
Full Access Member
Do you have or have access to a scanner that will show live/real time data?
Did you clean the TB yet?
Marky Dissod
Full Access Member
For at least a week, no access to live real time data.
As for cleaning the throttle body, and advice on what to use and how to use it?
Got several cans of brake cleaner, but want to be SURE it's safe to use on a throttle body / IAC motor / throttle position sensor.
Absolutely. In this case it's possible it isn't able to go from its more open while cranking position, to less open right after it fires. It should never be open enough to flair to 2500 if the tune is stock though. On some gnarly vehicles I like to make them crack ******* cold starts and hop up to like 1500 for a couple seconds before settling down, but a stock truck wont have that parked position. Unless when it's warm the truck has an issue that causes the iac to be much more open than normal, then when he shuts it down and it cools, it sticks in the much more open position.
I still think it's an air leak causing the flair until the truck has a chance to close down the iac valve after start, but checking the iac valve is for sure a good idea.
Pick up a can of throttle body cleaner and use that for the TB.
iamdub
Full Access Member
If you're removing the TB to clean it, you can use pretty much anything. There are no exposed "soft parts" other than the gasket. But, God invented a specific TB cleaner for a reason, so I'd have to recommend that. My hackjob ass would use one of the ~15 cans of brake cleaner I have in stock.
Spray on, let soak, scrub with a toothbrush, repeat. Finish with more spraying and blasts from an air compressor or towel and air dry.
These things aren't as fragile as one might think. When I did my delete, I wanted to clean the oil out of the intake manifold. Just as it came off the engine with the TB, injectors, MAP sensor, etc. still attached, I filled it with foaming engine degreaser, let it sit then used a baby bottle cleaning brush to scrub each of the runners. Then I hosed it out from all angles- through the TB, in each port and through the brake vacuum hose. Foamed and hosed it a few times until the water flushing out from all the ports was clear. Blew it out with high pressure and volume air until no water droplets came out of any holes. I was gentle with the movements of the throttle blade when I opened it. Was slow to push it open to get the sprayer into it and didn't let it slam shut.
Like others have said, you need to get a scanner on it. At warmed up temp you want to see the short and long term fuel trims, the IAC duty cycle, the engine and air intake Temps, and the rpms. There are others that can help further, but those are the important ones to get it almost for sure figured out if you post the info.
Oh, and tps as well to make sure there isn't something wrong with it and it's showing more than 2 percent throttle at idle. If it is reporting more than 2 percent, it will kick it to the part throttle timing map which has higher timing and also throttle adders via the iac that will hang the idle.
