97 Yukon wants to idle through the brakes

[thezentree]

My 97 Yukon (5.7/4L60E/241) is doing some weird idle stuff as of late. When I start the truck, it revs up to about 2000rpm and then takes 3-5 seconds to settle back down to about 750rpm. Then when I drive, it'll slowly accelerate (without me on the throttle) to 30-35mph through the first two gears. It'll hold first gear to about 2500rpm and then shift hard to 2nd. If I try to brake, it'll try to power through the brakes for a few seconds and then drop to normal rpms. It started doing this at the end of last summer for a few weeks, and stopped when ambient temps dropped. Now that it's warmed up again, it's started again.

Also, when I went outside just a second ago to try to get a better idea of rpms, I noticed that if I turn the ignition to ON and let the fuel pump prime fully before starting the truck, it starts hard as if it's not getting enough fuel. If I don't allow the fuel pump to prime, it'll fire right up. Idle issue is unaffected.

Any ideas?

 

[thezentree]

anyone?

 

[dudeiwin86]

sounds like a vacuum leak.
and if you have vac assisted brakes.... that might explain why the brakes are not holding the idle,
high idle, and running on are also signs of vacuum leak.

i dont know how the brakes are assisted in these trucks (im still a newbie) but this comes from my experience on other platforms

 

[noJeepshere]

Second dudeiwin on this one, and check the idle air control module, it could be going bad.

 

[Cornfield creations]

As previously mentioned, check the IAC. If it's stuck retracted, it will cause a high idle situation, like a vacuum leak.

 

[noJeepshere]

How many miles does your Yukon have? I turned over 153k a few days ago in mine. Last I recall with yours, you had about 220k on it.

 

[k20]

yeh hes got ~230k on it IIRC.

 

[thezentree]

240,xxx and going strong ...ish.


I discovered that once I put it in gear, if I sit on the brakes for a few seconds, the revs will run up to about 900rpm for a second and then drop down to 600rpm or so, and after that, it behaves normally until I turn the engine off.

Anyway, I'll look into the IAC when I get a few minutes tomorrow. Thanks for the help guys.