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Hi, I have a 1987 Jaguar XJ12. I bought it a few months ago from an auction as a project car to drive in summer. It runs and drives. The only problem is that the V12 engine dies after about 15 minutes of idling. The first start of the day idles for about 15 - 20 minutes but after that every time I start the car, it keeps dying instantly as the RPM keeps dropping. I took it to my local mechanic and he test drove the car and concluded that the idling was low so he adjusted the throttle a bit to increase the idling, it ran fine at the shop but as I was driving home the car kept dying in the middle of the road every time I came to a stop or pressed the brake. Eventually the battery ran out of charge and ended up causing a huge traffic jam and towed the car home. As long as I press the gas pedal the engine runs fine, its just when I stop the car or when im not pressing the gas, it dies. Please help me with this problem as I really want to drive it in summer. Thanks.
Before the list gets out of control, slide down one section to the XJS area, and the top stickie I wrote will give you a good heads up.
The adjusting of the throttles to mask that issue, is a NO NO.
There is a HOT idle adjuster on the AAV, located on the rear of the B bank (LH) cylinder head.
The throttle bodies get all gummed up with "black goo" and that will eventually kill hot idle, sooooo, a solvent soaked rag, and wipe them out until CLEAN.
Include in all that the CTS (Coolant Temp Sensor), located on the thermostat housing, at the front of the B bank cylinder head. I seriously doubt it is your issue, but at $25 or so, is a cheap item to cross off the list.
Hey Grant, t I read your write ups and looked at my hot idle adjuster on the AAV. To me the adjuster looks like its too much out of its position. In your write up about AAV rebuild, you mentioned about turning the adjuster all the way in and then backing it up 3 turns as a start. My adjuster looks like its way more out than 3 turns. Please look at the picture of my adjuster and advice me if thats what I should start with before tinkering with anything else. Thanks.
Take a 13mm (1.2") spanner and tuirn it IN, then back it out 3 turns, or 18 flats.
Start the beast, wait for warm up to commence and turn it further OUT if the idle gets too low as the AAV piston closes the inner port.
BUT
Unless the throttle discs are CLEAN, and set to 0.002" gap, and the crossrods are set SPOT ON, you are wasting time, fact.
The TPS split voltage at idle position is also a required spec that is 90% overlooked by many.
When I ran AAV's, the HE was about where yours is, but the 2 PeHE were all over the place, one was out heaps, the other not so far.
Its an individual engine thing, one needs more idle bleed air, the other dont, so whatever your engine wants is what is right for your engine, and what others are set at is of no importance at all.
As the weather is getting bettter in Canada, I tinkered with the car. I put my thumb in the AAV inlet when the car was warm and it stopped. I adjusted the AAV quite a bit and now when it is idling the rpm is oscillating, it goes up and down but the car does not stop. As soon as I put transmission in drive or reverse it stops though. Someone told me that they could hear misfiring in the drivers side bank and that it could be ignition coil. Do you think it could be the AAV? If so then there is someone parting out their 1987 XJ12 so I could get the AAV from them.
Understanding the AAV operation is paramount to diagnosis, and they are ALL old now, and have been maintained not so well. When the piston closes the "mouth" 100%, there is still a large amount o fair bled into the system to maintain idle speed, as set b the HOT idle adjusting bolt, and the air bleed by pass passage.
To turn that bolt at will, is going to achieve nothing.
You must ensure the AAV is closing 100%, and if it is, then adjust the hot idle with that bolt to what you want, usually about 750 in P, with the A/C on.
BUT
As I said before, the throttle discs MUST be clean, AND set to spec, or you are wasting your time.
AND
The TPS idle voltage setting of 0.32 to 0.36 MUST be set SPOT ON, and I use 0.34. It is a fiddly thing to set, and sadly must be done right, near enough here will not work.
If it is misfiring, I suggest spark plugs first and foremost. ALL 12 is good, as many are running with 10 new, as tehbfront one on each side needs to a/c compressor moved for access, and most mechanics simply dont bother.
The plugs must be gapped at 0.025", and again near enough does not cut it with the HE engine. I use NGK BPR6EF, and have done so for a very long time, without issue.
The coil is the coil, and supplies spark to all 12 cylinders, so a misfire wlll not be a coil in this case.
The fact it dies on gear selection indicates its probably a 9 cylinder engine. and that needs addressing way before the AAV.
New plugs, leads, cap, rotor, is the very basic first step in sorting this. Followed by the TPS setting, then the AAV.
02-28-2018, 02:10 AM
V12 dies after a few minutes of idling
