Recently, my 77k XJS began idle "porposing." That is, at cold start up, the engine starts easily, but the idle jumps up and down from 5k rpm to 20k rpm repeatedly. I can drive the car a quarter of a mile with this idle, and the porposing stops after warm up usually within minutes, and the engine operates perfectly.

What is the problem, and what is the fix?

 

Being the 4.0 motor I suggest trying a search through the xj40 section. It is usually recommended to check all vacuum lines as a vacuum leak will cause this "searching". Hope this helps..

 

Welcome to the forums vpickering,

I've moved your question from General Tech Help to XJS forum. This is the place to post technical questions about your model.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some information about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham

 

Cold startup would negate most of the sensors.

First off, I guess vacuum leak, or filthy MAF sensor, or faulty MAF sensor connector. Take the connector off, clean it, and bend the teeth a little so they make a firm connection and see what happens. You can also spray some MAF cleaner at the sensor and see what happens.

Another guess would be playing with the wires that connect into the TPS on the underside of the throttle body. Mine are starting to go and if I play with them, the car's idle will respond in kind.

Let me know if that does anything.

 

It could be your idle air valve.