High Fast Idle

[BvilleStew]

I've owned my 98 lsc since May 2000 w/20,000 miles. Currently 112,000 miles. This past week when first starting, the fast idle jumps to 2,000 rpm for 5-6 seconds before it drops down to 1,500 for 3-4 seconds, back to 2,000 for a couple more seconds, then to 1,000, then 800. I looked for cracked vacuum lines, but found none. Runs great otherwise!
I removed the IAC solenoid and cleaned it with carb cleaner. No help. Ideas?
I'd like to fix it before I put it away for the winter in a couple weeks.

 

[NoLimit95]

Try the TPS (Throttle Position Sensor) and cleaning an old IAC doesn't always fix the issue.

 

[XLRVIII]

Check the two vacuum lines that tie into the drivers side of the intake manifold.
it's a common place for them to leak and it is easily overlooked (Ask Myco)

Mycos car did much the same thing, he went over it a couple of times and found no vacuum leaks,
He brought the car over and within a couple of seconds I found the leak..
BOTH those hoses had collapsed and were leaking.
It made a "hissing sound" when the engine was turned off..
it wasn't hard to find...

 

[BvilleStew]

Fine for now!

Checked all vacuum lines and TPS. Found no problems. Disconnected the battery for a half hour and reconnected it. Cold start the next morning and all is good! It's been fine for three days! Hmmm. How long will that last?
Any more suggestions?

 

[topher5150]

Checked all vacuum lines and TPS. Found no problems. Disconnected the battery for a half hour and reconnected it. Cold start the next morning and all is good! It's been fine for three days! Hmmm. How long will that last?
Any more suggestions?

i had the same thing when i started up my car, also my car would idle at 1200-1500 rpm at the stop light then i would get up to speed and take my foot off the gas and it would drive it self. i disconnected the batt also and it's been fine since