High idle

cordog · December 1, 2001

HI, I have a 1988 silverado stepside with a 5.7 litre V8 throttle body, fuel injected that has 245,000 k. It just started to idle high and no one seems to be able to diagnose the problem.

It idles high first thing in the morning and sometimes slows when restarted.

I have replaced the TPS (throttle positioning sensor) the intake manifold gasket, which had a leak and have been told I should try replaceing the distributor.

Any suggestions would be helpfull.

Thanks Cordog

December 1, 2001

Sounds to me like the choke is not turning itself off. I don't know what the comp uses to determine that high idle is no longer needed, but maybe it's not getting that signal.

I looked in my Chilton's but couldn't come up with anything. Sorry

dougmac · December 1, 2001

Is it posible that there is another vacuum leak? That is where I would start. Perhaps the EGR valve stuck in the open position?

Do you have a check engine light displayed?

BTW, throttle body fuel injection has no choke so don't look for problems there.

Doug

NeXuS1 · December 2, 2001

in my 89 when the air filter got really dirt it idled up.

(oops first read it as 1998 for the year)

December 2, 2001

BTW, throttle body fuel injection has no choke so don't look for problems there.

Doug

Hoping not to look like a fool. I know that. "Choke" is quicker to type than "Fast idle".

dougmac · December 3, 2001

Sorry Friz,

I wasn't trying to make a fool of you and I hope you didn't take it that way. I just thought from your post someone might think TBI had a choke.

Doug

95Yukon · December 11, 2001

The last time this happened to me on my 1990 GMC, it ended up being the PCV valve sticking. It was sucking air in constantly and creating a vacuum leak which raised the idle from 900 to about 1200. I know you probably already checked that, but worth mentioning. Also, sometimes those little rubber elbows which connect to the base of the TBI will get brittle and crack and create a vacuum leak.

Also, did you check the TBI for carbon or blockage when you had it off replacing the gasket? Sometimes they get gummed up internally to the point where the passages are blocked where the vacuum lines connect to it. Carbon can build up on the spot where the idle control valve pintle makes contact, which does not allow it to seal into the seat when it tries to close. Pull it out and take another look. Gumout all the passages and make sure they are flowing and not blocked.

Good luck.