You have too much air coming into the engine, probably due to a faulty AAV - Auxiliary Air Valve

More air intake increases engine speed...which the AAV is designed to accomplish for cold idling so the engine won't stall

If too much air comes in the engine speed goes too high and the ECU -- fuel injector computer-- cuts off the fuel injectors until the engine slows to xxx-level...at which point they're turned back on and the whole cycle repeats

But....

Best to post in the specific XJS section so more of the right people see your posting. Lots of smart people there

Cheers
DD

 

 

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

Graham

 

 

Hi Cajaglady

As has been already said, its the Auxilliary Air Valve that is the cause of the problem, which is causing the Engine Revs to 'hunt' up and down till the Engine Warms up

This often happens on a Car that hasn't been used in a while or has been left standing and in the Scheme of things its no big deal, as once the Car is in regular use, it often puts itself right without doing anything

Which is probably just as well as they cost Megabucks! about £200 (UK) plus the labour for a Shop to fit it, which is very easy to do, as its only a couple of bolts

Though if you want to get a Shop to fit a New One, then it could well be a very expensive day out, so personally I would live with it for at least a while, as once the Car is in regular use, there is every chance that you may find the 'hunting' goes away

Don't let them touch that Adjusting Screw/Bolt or you could forever have problems with the idle speed

 

There is a bolt on the side shown in the photo in the post above, that is the adjustment. Try turning the bolt in and see what happens. If the bolt is all the way in, then the AAV needs to be replaced. It's a simple job to do.

What is happening is when it is cold the engine is getting too much air and at 1500 RPM the computer cuts the fuel when it is in N or Park and the throttle is closed. When the fuel is shut off the RPM falls, then the fuel comes back on and the speed increases and the loop repeats.

Inside the AAV is a piston coupled to a wax capsule. As the engine warms, the wax expands and pushed the piston upward and closes off the air path. It's an aluminium piston in an aluminium bore, and over time and with moisture it can corrode and not move. If the piston is stuck at the bottom of the travel ( the cold position) that lets in the most air and gives the symptoms you see. Unfortunately, it can't be repaired, opening it up destroys it.

However, as it goes away it sounds like the piston is moving, so just adjust the amount of air with the bolt and see what happens.

 

Why on earth not? That's what the screw is there for.

 

Because if the AAV Piston frees itself off, as they often do then the RPM will return to how it was originally set by the adjustment screw, then you will know if the AAV is then working correctly, instead of having to constantly adjust it

 

I've never had an AAV fix itself. The wax capsule does wear over time and it doesn't push the piston as far as it should to close off the air port, so the high idle remains. Thus a trim with the bolt brings the idle speed into the proper range.

In the meantime, it's hard on the transmission to engage drive with a high idle, it puts a lot of heat into the fluid and can burn the clutches. I'd much rather tweak a bolt than rebuild a transmission.