Surging idle

[Daniel Crone <daniel.crone@lexis-nexis.com>]

Knick Curtis wrote thet "I have an 89 XJS that when cold and during warmup when in neutral the idle surges between 700 and 1500 rpm's.  I think its a heat sensor, but the man who works on it says its probably the computer.  It comes and goes, but seems to disappear when both the ambient temperature and engine temps are warm." 

I have a similar problem with my current 89 XJ40.  Although dissimilar cars, I suspect there is some common component/sensor between the two cars that causes the problem?  Perhaps the ECU is misbehaving because of faulty input from bad sensors?  Let me know if you find the answer.

My problem first occurred on a extremely cold day.  I hadn't been driving long, perhaps 3 miles from starting the car when the tach started surging. At stop signs the tach would jump as you have described.  

In my case the surging has disappeared only to be replaced with an annoying problem.  The car now idles around 1000 rpm, whereas before it was 750.  The engine now resists returning to idle speed, descending much too slowly for my tastes.  (In fact, on another cold day the car wouldn't come down from 2200 rpm while driving and I had to turn off the ignition.  Luckily, this problem hasn't reoccurred, and no one posted responses to my note at that time.)

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Follow-Ups:

• Re: Surging idle
• From: Randy Wilson <randy@taylor.infi.net>

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