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Username Post: 1988 S10 Blazer fast idle 4.3        (Topic#232343)
Posts: 50

Loc: St Croix, USVI
Reg: 01-15-02
12-27-09 03:29 AM - Post#1831043    

Been doing this for years and is worse now it's in a warm climate. When started goes to fast idle but doesn't slow down (so just put in gear and drive). Eventually slows down but the idle still is high. Are there any adjustments on the throttle body or some sensor? Thanks

Subject Matter Expert - Senior Member
Posts: 3733
Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
12-27-09 03:53 PM - Post#1831322    
    In response to bjears

First thing you should do is check for vacuum leaks. Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.

On TBI engines it is common for the gasket under the TBI unit to leak vacuum.

Many people spray carb cleaner or WD40 at suspect areas to see if the engine speeds up. I prefer to use a 3 foot long piece of heater hose to listen as I move it around the engine.

Vacuum can also leak through components such as the brake booster, bad PCV valve, or charcoal canister purge valve. I use pliers to pinch off vacuum hoses and listen to see if it slows down.

It is possible that the intake gaskets have failed and it is sucking air from the valley under the intake.

If there are no vacuum leaks the next thing to consider is the IAC.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on. If you have A/C and turn it on while listening carefully you should be able to here the IAC controlling the idle as the A/C clutch engages and disengages.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If you remove the IAC and plug the passage in the TBI unit, the engine should just *barely* be able to idle, with a warm engine and all accessories off. If the engine is not in good running condition it will stall when attempting this. If it idles to fast when doing this there is a vacuum leak somewhere.

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If your vehicle has been altered from stock, let us know about that too.

J C 
Senior Member
Posts: 490

Loc: Orange Co. Calif
Reg: 04-17-04
12-27-09 07:05 PM - Post#1831433    
    In response to bjears

I had a 91 astro van with a fast idle problem. It would idle ok at startup but would idle fast after the first stop. Turned out that the thermostat was stuck full open and not allowing the car to warm up to proper temp. It told the computer that it was cold and made the engine idle fast.

69RSZ28 71 Chevelle 79 Silverado 24K mi.

Senior Chevytalk Moderator
Posts: 24274
Loc: Austin, TX 78748
Reg: 05-09-00
12-27-09 10:27 PM - Post#1831529    
    In response to J C

Is the initial timing set to zero?

'95 Jimmy SLT (Bought @131,814)
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles

Forum Newbie
Posts: 8

Loc: Evansville, IN
Reg: 09-03-10
12-21-10 01:58 PM - Post#2021091    
    In response to J C

My 92 S10 2.8 had the thermostat stuck open also - didn't find the problem until the outside temp got cold and I needed some heat on my feet. Changed thermostat and fixed heat and the idle at the same time.

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