Fast Idle at Start Up

[Jeepster19]

I am having a fast idle on start up but it soon goes back down to a idle.It has been doing this since I bought my Xj over 2 years ago.Is this normal for Xj's to do this or is this a faulty sensor?

[4.3LXJ]

It depends. How fast for how long. A lot if engines, including my Cheyy will start up a little fast but go back down once the computer is able to determine rpms.

[Jeepster19]

About 5 seconds I am going to say.It's not really a problem really, just wondering if that's normal.It does it most every time,only once in a blue moon it doesnt.

[4.3LXJ]

5 seconds might be a little long. Or it might seem like 5 seconds. But I don't see a problem.

[BlueXJ]

Later XJs may be different but the Renix era ones will do that if the TB is dirty or if the MAP sensor is not satisfied (from a leak) but newer XJs have a different ECU and I am not sure how they react.

[bluedragon436]

I was going to say clean the TB and sensors... and make sure you have no leaks both wtih the TB as well as any vaccum lines that you might have...

[jimmixj]

Port fuel injection does a "Idle-flare" on start up. This is because it is easier for the computer to establish idle coming down to the desired rpm as opposed to pulling the idle up to the number. As long as its within a few seconds that idle stabilizes its normal.