High Idle

[SpecialOps]

Following a throttle body cleaning this weekend, my idle engine rpm has significantly increased. Truck runs great otherwise and is much more responsive but the high rpms are a worry...any suggestions on how to fix? Thanks in advance for your help.

[99silveradoz71]

Welcome aboard, what are you driving?

[RyanbabZ71]

what is it idling at? What engine?

 

I know in the summer my 00 5.3 idles at 500 RPM and when it gets colder out it idles at 800-1K until the truck warms up

[SpecialOps]

1990 GMC Sierra, 350 TBI, 5 speed trans.

[SpecialOps]

I do not have a tach, but I would guess around 1000 - 1200.

[99-2door]

I do not have a tach, but I would guess around 1000 - 1200.

Check the IAC valve. Sometimes they stick allowing more air than usual to enter during idle. Sometimes you can clean them with carb cleaner and they will work correctly again, but sometimes you have to replace them.

[redchev]

I would also check to make sure the throttle body bolts are tight. I dont know the specs but my 1994 had a bad base gasket because they loosen over time. It is actually pretty easy to rebuild with a kit from your auto parts store. It is reasonably priced and you can clean all the passages so it will perform like new.

[AZ-Z71]

There is a learn procedure for the IAC. I think I saw it here. Something about letting it idle in drive then holding brake bringing the idle up for 30 sec then turning off.....something like that. I know I have the exact procedure at home.

 

Don't know much about 90 so this is if it has an IAC.

 

It could be possible that when cleaning you accidently pushed the pintal in or something.

[cyclejerk]

cracked torn gasket at upper or lower throtle body. ???

[Whitechevy]

ok, so whats a iac ?

[99-2door]

ok, so whats a iac ?

Idle Air Control

It is a pintle type valve in the side of the throttle body that controls how much air flows in while the throttle is closed (idle)

[JayMan]

The 1990 with TBI has two fuel injectors mounted right on top of the manifold. There is an idle motor but it's not the same as the IAC on the new Vortec motors.

 

It's quite possible the throttle body cleaning actually knocked the carbon loose in the idle air passages and it has jammed the idle air motor. Red Chev also makes a good point in that if the gasket was iffy to start with, the solvents in the cleaner may have created a leak so it will idle high.

 

 

The TBI is a VERY easy unit to service. RedChev is right, get the gasket kit from your parts store, unplug the electrical connectors (They're all different size so they're Goof Proof for folks like me), also check out the idle motor, and clean the passages out.

 

Anybody have a shop manual on how to set the base idle? Some of the early fuel injected and TBI cars still have a base idle set screw.

[SpecialOps]

Fixed it! The upper gasket was torn and not intact, however the fast idle motor plug was not connected all of the way.

 

Problem solved!! Thanks guys!!

[k1500chevy97]

Glad to see you got it fixed

[JayMan]

Fixed it! The upper gasket was torn and not intact, however the fast idle motor plug was not connected all of the way.

 

Problem solved!! Thanks guys!!

 

Yep, check the simple stuff first. Sure wish my 1990 Toyota 4Runner #%@!!!*&$# had been that easy to work on. All's well that ends well!