idle on 98 gmc goes up and down

[jay98gmc]

hello im new to the forum. my concern is this. well first let me thank anyone who

this form and especialy those who try to help!! when i start my truck it starts fine but revs up to maybe 15 rpm. then when it comes down it almost dies out. thhen in idle state it just idles up and down. i have already check and cleand the air idle controle valve. thanks

[1997SierraSLT]

throttle position sensor going bad perhaps?

[jay98gmc]

yes thanks for the tip! as i was waiting to see if i had goten any replys i went and bought a TPS and no change. i cant seem to put my finger on this one thhanks.

[REVYCHEVY55]

Sounds like a vacum leak. First place I would check is gasket under throttle body. They are bad at deteriorating. You can check by spraying starting fluid around that area at the base while its running. If it idles way up that is where your leak is. Next check other vacum hoses especially brake booster hose. You can pull hose out and put a piece of cardboard over end. If this solves it brake booster diaghphram is bad. The way it sounds its the throttle body gasket though. Hope this helps

[Terrick down Under]

G'day jay98gmc, I have just sat down from fixing my problem that was the same as yours. I found 4 small vacuum leaks at the base of the throttle body and 1 on the PCV valve line which made it better but not good enough, then I replaced the TPS and still no change, so I pulled the IAC, small splits on the nose cane, put in a new IAC valve. Took it for a test run of about 10 miles...great idle and no up and down.

Good luck with yours, it is time consuming.

When you get frustrated just think of me down here, it takes 10 days to get any item I need from the USA, that makes swapping out parts individually a very slow process.

[Squirrely]

IAC. that would be my guess