V12 AAV issue identified

[Brakebuster]

since getting my restored XJS back on the road, the idle revs have been hovering around the 1100 mark , which is way to high , and was actually pulling on the auto box to engage drive

a little bit of fettling with the TB's brought the revs down to around 900 , which was better , but still an issue , and after driving the car around this could rise to 1000 rpm again at traffic lights etc,

I know the AAV is working , as the revs start out at 1500 , and gradually drop as the engine warms , but it never fully closed.

so i bit the bullet today, and after warming up the engine before doing any work , i then got to blanking off the AAV inlet pipe , there is a 25mm internal diameter hose from the left air box tot he AAV , this also has a takeoff pipe from the top going off into the engine bay , my adapter had broken due to age, but this is not an issue at the moment,


i found an appropriate blind grommet to blank the inlet to the AAV , used some high temp tape over the top after it was snugly pushed into the AAV and then secured around with a tie- wrap ,

i did the same to the back of the air box as i didnt want to introduce debris to the engine, as this is after the air filter in the box

i then turned over the ignition , as she was warm she fired right up and instantly settled down to 750 rpm , this is the desired tick over with no extra air required

i have read reports that this type of modification can stay in permanently , as the weather is rarely that cold that the engine needs it , and if it does its a simple keep your foot on the accelerator pedal for a few seconds while things get going

so for the time being i will leave it like this and see how she does ,

i recovered a spare AAV from my local breakers, but it seems different as its from an earlier model ,
new are available from Jaguar but are £200 , i will buy if i need to , but will try and resolve mine from not fully closing first

its been left in the engine bay for now, as its sealing the water rail that its fitted to

and i set the TB's to 2 thou on each side, and new capstan throttle links were obtained form Jaguar for £4 each , and are still available if you need them , that and they are new and shiny.....lol

BB

[almcl]

BB

I can't imagine that you haven't already seen it, but just in case, Kirby's book has got a discourse on rejuvenating AAVs.

[Brakebuster]

yes, have seen the AAV rebuild, but thanks for the pointer

more issues though ,

definitely not the AAV ,as its been bypassed as above

driving down the road this morning, only a quarter of a mile from setting off, my revs suddenly dropped down to 300 rpm or so , and it felt like the gearbox disengaged its self, then just as quickly , it got back up to driving revs of around 1500 , and it all came back to life, but just as quickly dropped again... and so on , so i nured her back home.

so , lumpy running with the revs dropping right down , not stalling the engine, and then recovering its self ,

i have checked for all type of leaks etc on the vacuum , but nothing found

could be the anti-stall relay , or the TPS , or a multitude of other issues

i also tried to see if it was fuel starvation , and though not confirmed , there wasnt excessive vacuum in the tank ,


it wasnt doing it at tick over on the drive at a constant 750 rpm ,

but then suddenly dropped down to 300 , and stayed there.........

restarted the car , and everything seemed fine for a while but then did it again

frustrated, as i was gona go for a drive in the sun

now i got to cut the grass, lol

BB

[almcl]

Can you rig a fuel pressure gauge that you can see while driving?

Might help to chase down if it's fuel or ignition?

[Brakebuster]

got nothing that can do it right now

but i'm leaning more towards fuel ,

i have done about 200 miles so far, and though i fitted a new fuel filter, i think the pickup in the swirl pot might be the cause , as now, even at tick-over at 750 , it just drops to 200-300 revs and flufs around before going back up to 750 etc

could be a faulty non return valve in the pump , or the pump its self...

will get myself some clamps and give it a clean , even if its just to eliminate it


BB

[AlastairCM]

Almost sounds like its an overrun cutoff kicking in and cutting off the fuel to the injectors until the revs drop. I think they deleted the overrun valves from post 1988 cars and it was done by ECU trickery and a throttle switch rather than a throttle closed vacuum signal with a rev count above a set point (above 1500 I think?).

The test on the older cars was to block both overrun valve openings in the airboxes and see if that cures the idle problem. I've not got any manuals dealing with the test process later cars.