My lovely pride and joy drives superbly but hunts i.e. surges a few hundred rpm when coasting with the throttle closed. Very slight but almost negligible hunts at idle when parked. I am baffled, any help or suggestions gratefully received.

 

#1 is the TPS is out of adjustment, or dying.

Attachment 213576

#2 is the build up of "goo" inside the throttle bodies, clean with a solvent soaked rag.

SETTING THE THROTTLE CAPSTAN AND RODS GRANT.doc

#3 The AAV is sticking, age related and very common.

Attachment 213577

Simple DIY fixes, and again, well known issues.

 

Another possibility is mal-adjusted idle switch, if fitted. Fitted from engine # 64245

 

G’day Grant and “thanks mate” (see I have been to OZ ;-)
I will get busy and troubleshooting based on your list
Before I do there is one other clue that might narrow it down: when I accelerate from low revs there is hesitation before the power comes in but once the revs are up it’s fine.
Once again, many thanks for taking the time to help us “less mechanical Jag lovers”

 

Just anothe thought about the AAV being the problem - there is no difference if the engine is warmed up i.e. cold or hot is the same
Les

 

 

TPS still at the top.

AAV moved down a tad.

The hesitation can be many things, and mistimed engine, AFTER the TPS reset is next on my list.

Trimming the timing on a running V12.pdf

The HOT idle fuel pot INSIDE the ECU may need a tickle or 2. DO NOT mess with this until the engine is at operating temp, please.

Attachment 213610

Stale fuel will burn, but badly and STINK.

 

Me again - do you have any idea why the hunting is less when I change down to 2nd? it’s a North American car with automatic

 

Maybe because you're increasing the revs and don't notice it so much?

 

+ a few to Alex's comments.

Lower gear, same speed, less load, all sorts of fun things alter.

It really is a case of ONE system at a time, careful thinking, and drinking of course, and then a few odd words here and there, and then back to the SYSTEMATIC steps of basic diagnosis.

These are NOT complicated cars, even with all the USA junk attached, and the hunting is really limited to the few systems/items we have already suggested.

A stuck distributor centrifugal advance is anther "to check" item, and about mid way down on that list you have compiled.

 

Thanks again and now time for me to get “stuck in”.

 

The one thing I haven't heard mentioned is a vacuum leak. It depends on the size of the leak, as most fuel injected cars can compensate for slight vacuum leaks, I'm not to familiar with the Jaguar enough to know how susceptible they are to this??? Just a thought.

Jack

 

Thanks Jack, the bushings on the throttle linkage going into the throttle were what I thought would be the problem. I read in the “Experience In A Book - Help for the XJS Owner” by Kirby Palm (740 pages of great info,available online), that these bushings get so hot that they fry and on older models are a major problem. However when I sprayed around them with starter fluid there was no reaction in revs. I also sprayed around the engine and could not get any changes. I have still not ruled out that source of the hunting. Still planning what to do as I do not have my workshop anymore and have limited room and time to do work myself.