1989 XJS V12 starts fine hits high idle, warms up goes to normal idle this is where the surge happens. any ideas!! Thanks for all that give there time to us that need it you are great!

 

Hi RMF

The Surge you describe or Hunting as we sometimes call it on here, is really not that much of a Big Deal, as many of us have experienced this from time to time on our Cars and is all down to a Sticky AAV Auxiliary Air Control Valve

These can be Horrendously Expensive to replace and for which you could buy a very nice Secondhand Car

So unless you've won the Lottery on Roll Over Week, you will need to fix it yourself

Sometimes the 'Hunting' goes away of its own accord, as it most often tends to occur after a long lay up though if not there are a couple of things that you can try, the First of which is to remove the Rubber Elbow very Carefully, as they get brittle in the Heat and then give the AAV a really good squirt of WD40

If that doesn't work then the Nuclear Option is to remove it and repair it for which 'Grant' has a Fix

Although if it doesn't bother you, then it may be worth hanging in there to see if it puts itself right

 

Ye I have already pulled it off and put in boiling water and watched it close the gap, not fast but it closed then touched to the ice in my ice maker and watched open back up, the travel was about 3/16 of an inch, what do you think is that about normal? And by the way this is one in my collection not a customers car, as I have shipyard I dont have car customers..Just trying to take care of the ones I have. Thanks to all

 

Why is it called the DREADED white shielding wire

 

Break it and the car won't run, feed to the amp

 

Hi RMF

Let's hope you never find out (Lol)

 

Mmm,

1989, is it Lucas Ignition or Marelli??
13 HT leads = Lucas
14 HT leads = Marelli.

You are on a change point with 1989, thats why I ask.

That AAV sounds OK, but the paper is attached.

I would be removing the air filters, take a solvent soaked rag, and CLEAN the throttle bodies internal bores and brass discs. Engine OFF of course.
The "black goo" that builds up in there on rarely used cars is HUGE, and affects hot idle stability.

The Dreaded shielded wire is used as a pulse signal to Pin #18 of the fueling ECU. It also signals the Marelli ECU on those cars. It fails, heat of a V12 engine bay cooks the plastic stuff, and then the :fiddle factor" as owners move the loom for access to spark plugs etc cracks it, and the thing inner wire leaks that pulse to earth, and the engine 'she be dead". Splice in a new section of Coax, and drive on. TV antenna coax is fine.

 

If you are describing a cycle whereby the idle climbs to about 1400 RPM and then cuts, and then starts at about 700 RPM and on like this, then the cause is a stuck open AAV. This is caused as follows: If the throttle is closed, but RPMs are high, the ECU cuts the fuel automatically until the revs drop. This is called over-run fuel cutoff, and it is to prevent fuel usage on a trailing throttle, such as coasting to a halt at traffic lights etc.
The remedy is to fix the AAV.

 

You my Ausie friend are awesome for sharing you knowledge it sir is truly the greatest gift one can give!
My ECU in the trunk on the passenger side say LUCAS 16cu
Next My my ignition amplifier has the red & green wires plugging in to it and 4 w/wires coming out some go to the coil and then there is one of those that go to feed the EFI, now where the EFI wire that comes out of the amp makes connection to the wiring harness there are 2 wires- the EFI wire plugs in to a white wire in the loom and the black wire in the loom does not hook up to any thing but it looks like it may have been soldered at some point, do I leave the black one floating?
Just a bit more info on the AAV when I put it in the boiling water it does close 3mm or so but there is a v (kind of) grove in the piston and when it closes it still has a bit of the top of that notch open that would let air pass thru, is that to be normal? I will do all the solvent rag cleaning you suggested,, this is a cool car and it is worth my time with it to add to my collection and then I have a 86 coupe v12 to round up my Jag pack until I can find MK to finish it off, holy **** those older ones are getting up there a bit in price but are so cool..

You sir I thank! Tod

 

OK,

Lucas Ignition system..

It would be sweet if you added that little snippet to your Sig, as 99% of the people instantly think 1989 = Marelli, the still have a shiiit load to learn.

That Black wire, NO. It will be earthed at the 16CU, and MUST only be earthed at one end. It is the shielding earth attachment, some are at the ECU, some are at the Amp.

That AAV NOT closing 100% IS YOUR HUNTING, and that is fact. 6 beers to sort that out the first time you service one. I tried the cleaning as suggested earlier, Nope, never worked, so major surgery took place. That write up is all you need, apart from the beer.

 

Many beers at the ready!! will start later this afternoon (beers) then repair to follow shortly!! Damn good advise on procedural approach!!

 

Another possibility is a vacuum leak. The leak causes the rpms to go up, but since the throttle is closed, the injectors shut off. When the rpms then drop, the injectors are reactivated and the process starts over.

 

Agreed,

BUT

With an AAV that is NOT closing 100%, there is that Vac leak, and until that is sorted as designed, you are up against the Jaguar Gods, and they be unforgiving at any time.

Followed closely by Throttle Disc Goo in rarely used cars.

 

I had the exact same problem, Problem was solved after I replaced what I believe is called the air temp sensor, located at the front of the air filter box. Discovered that only after replacing the AAV. Expensive lesson learned. ( at least I know the AAV is good for another 30 + yrs ) But she now idles at a smooth 750. Hope this fix solves your problem

 

AHA, more memories awakened, thank you.

Trim the ECU fuel "pot".

BUT

That AAV MUST be sorted first, or you are wasting your time.

The attached explains.