Fast idle....

[Carlos Murphy]

My 88 2.9 has developed a very fast idle when cold.... WHY???

 

[Spott]

Either it's trying to warm up and you haven't given it enough time, or the Idle Air Control is stuck open. Clean the IAC with carb cleaner.

Spott

 

[RonD]

Fuel injected engines have a cold start routine in the computer, its a "choke mode".

Foot off the gas pedal
Crank engine
Engine starts and idle should go up to 1,500rpms
Idle should then drop to 1,000rpms with in a few seconds(computer is closing IAC valve a bit)
If it stays up higher unplug the IAC valve on the upper intake, if idle stay high then IAC valve is stuck and needs to be cleaned, it may need to be replaced but clean it first.

After 5 to 10minutes engine should be warmed up, temp gauge almost at 1/2, idle should be at 750rpms now

If temp gauge isn't just below 1/2 then your thermostat may be stuck open and computer is keeping idle high because engine is still cold.

 

[4x4player]

A vacuum leak can also cause a high idle. I had an intake manifold leak drive me crazy because it wasn't consistent. When the engine was at it's warmest the metal would expand and seal the leak. The cooler the engine was the worse it leaked. Turned out to be a hairline crack.