I have a project XJS 5.3 v12 HE pre facelift without Cats that i am working on doing up. At the moment the rear subframe is out and she is sat on axle stands. Also all inside is out but not the dash.

Suddenly the other day when running her up to temp (as i do each weekend) she started revving up and down from 800 rpm to about 1800 rpm. I have read about the idle control valve and have shut the screw for the idle all the way in and it made no difference. I also blocked the air inlet to the idle valve with a wooden plug and that made no difference. As the reving happens from cold (I left her stood for a few days) i am not sure that the valve is the problem but when i took it off i couldnt get the piston inside to move. not by putting the bulb in hot water or even trying to move it against the spring by getting a screw into the holes at the bottom and pulling on it. I am guessing it should move against the pressure of the spring without to much pulling but nothing is moving it. Guessing it been seized for some time as the car was sat for about 7 years before i got it but she was driven on and off the loader and has started and run to temp each weekend for about 4 or 5 months without this problem. I will get one later but at about £260 i was going to wait untill the car was near finished.

I have also disconected the throttle possition sensor during running and she stopped revving up and down instead she just sat at about 1800 rpm steady as you like lol. Connecting the sensor back up she started reving up and down again. I have just ordered a fuel pressure gauge to test pressure this weekend in case its fuel problems.

I have read something about the fuel cuts off when the rpm reaches a set lvl (not sure what that is) and the throttle is off so as not to burn fuel when coasting down hill. I am wondering could it be that once the revs hit the set rpm's and the on board computer knows the throttle is closed it shuts off the fuel bringing the revs back down, once the revs drop below the rpm lvl the fuel is switched on and she revs back up to the limit then the fuel is shut off and so the cycle keeps going causing the up and down. If this is the case i also wonder what is causing the revs to be so high in the first place. When i open the throttle just a little bit she stops going up and down and just revs at 1800 rpm again. Also when i open the throttle the revs climb as they should. I have a video on my phone that i will try to upload and put a link here but in the meanwhile any ideas ???

 

Assuming no mechanical fault with throttle linkages, and both throttles gapped to .002" and synchronized.......

You have extra air coming from *somehwere*.

Are the air filter housings off? If so, the vacant bolt holes are a big air leak.

Cheers
DD

 

Damn, I had taken one of the air filter boxes off to look at some surface rust and all four bolts are out. I will be very happy if thats what caused it. I wont get a chance i dont think to do anything till this weekend, i cant remember what came first me taking the airbox off or the revving but i am getting excited just at the thought it might be the bolt holes lol. Thanks for the advice i will update as soon as i can

 

just happened to someone the other day. that is pretty much gonna be your answer

 

I think it has happened to *everybody* at one time or another .

Cheers
DD

 

got home and it was dark but i managed to fumble around under the car cover and fit the air box back on, it was to late to test her as the exhaust back boxes are off along with the subframe and she roars a bit at the moment. Dont think the street would have been to happy lol

Should get home in time today and if weather is ok will try starting her and see what happens. must remember to open the idle control valve screw as well as i had done it all the way in and i guess she wont start and run like that.

Thanks for all your replys i feel this is 99% the cause. Will let you know how it goes once she does :-)

 

So last night got home turned back the car cover and turned the key !!

She immediatly fired into life and sat purring at a lovely steady rpm. I am sure she actualy runs slightly lower possible due to pulling heating and general knocking around of the idle control valve trying to get the damn piston inside to move. This weekend i will let her get to running temp and see if i have done more harm than good playing around with it.

As for the cheapest fix i have ever done on a jag i am over the moon. I cant believe that it was diagnosed so quickly on this forum. i am truly indebted to Doug for the first reply and the rest for backing it up.