Erratic idle

[cathaypacificblue]

Hi new to the forum,
Recently purchased an 89 xjs convertible and on the odd occaision after starting her up the engine idle revs become erratic a kind of hunting.
Can anybody shed any light on this and how to cure the problem.

Regards

Andy

[kelsham]

Hi this is a sign that the automatic air valve is sticking. this causes the engine revs to rise and reach the cut off point when the ecu realises the throttle is closed and cuts the fuel.It then switches back on and repeats the cycle. The AAV is situated at the rear of the engine passenger side. I have removed it on my car and freed it off. You need torx screw bit to remove it and a new gasket to replace it. If you let the car go stone cold and leave the radiator caps in place coolant loss is minimal. If you remove it and require further advice contact me regards Kels

[davidr]

It could be that the micro-switch is not being engaged fully? This should be switched on when the throttle is released. Does this only happen when the car is cold? Perhaps the heat expands the throttle rods enough to engage the micro-switch when the engine is hot? Just guessing but worth a look!!

[Gordon@1962]

Strange enough mine has started to do the same thing but in reverse.

When its hot it ticks over steady and then the revs will fall to the point of the ignition light coming on and then it will pick back up again.
I did think it was a fuel problem but mine is more complicated in the fact that my car is dual fuel.
It appears to be worse when running on LPG but I can still sense it when its running on Petrol as the idle is rough and feels as though the car is missing a beat. The only difference is on petrol it doesn't drop of as much to the point of the ignition light coming on.

Just as a matter of course I replaced the air filter which was filthy and noticed the distributor cap and rotor arm were well worn, so replaced them as well.
The cars performance has improved no end ( ) but unfortunately still runs rough/erratic at idle

If it is problematic on both LPG and petrol do you think it may be some kind of electric/electronic fault?
Any ideas appreciated.

[irisphoto]

Hello, just a stab in the dark, bit it might need a service. Look to see if the air filter needs cleaning. If you do you own servicing give the carb a clean with Cabr cleaner. It really might be that simple. Phil

[masterb]

carb?
on a xjs?

[NeilMFox]

Hi new to the forum,
Recently purchased an 89 xjs convertible and on the odd occaision after starting her up the engine idle revs become erratic a kind of hunting.
Can anybody shed any light on this and how to cure the problem.

Regards

Andy


Same problem myself, Took the AAV off gave it a good clean and then put it in the kettle to watch it close, After full closing cleaned it again WD40 then under the cold tap to open did this a few times then remembered the ultra sonic cleaner, Seems a lot better carry a 13mm spanner to adjust tick over when hot that is the bolt on the side of it.
Have a spare they are pressed together and the allum is crimped I think so will have a bash at pulling apart. Jaguars sell at 144BPS and its well known that they sell a few.
Neil.

[irisphoto]

Mrb39, ok inlet manifold, but its know as Carb cleaner. It get's rid of all the rubbish!!