I have a 1986 xjsc, just wanted to know when i start the car the car idles kind of rough, when im driving its fine but when i come to a stop it idles rough, does anyone know what it may be? spark plugs vacuum leak, misfire? thanks its also 12 cylinders.

 

Moved your question from General to XJS section where it should get a better response from other owners of the same model.

Graham

 

V12 rough idle. LONG list, but the main are:

The dreaded "black goo" around the throttle discs.

The spark plugs are OLD, or gapped wrong, should be 0.025".

ECU fuel "pot" required trimming.

Being a US car, O2 sensors out of whack, no idea, we dont have them on that year.

Vac leak, HELL YES. start with one hose, and finish with the last one, it will take time, and a very good imagination, and also ONE hose at a time. Not like someone else did, ALL off, now where did that sucker go, duh.

AAV not shutting 100%.

Cruddy air filter elements.

When the spark plugs were last changed, did they do all 12, and I say that coz I see regularly V12's with 10 new sparkers, and the 2 front ones are the old ones, and the story goes "too hard to remove so left it", and rough idle is guaranteed.

 

I'll add clogged fuel injetors to Grant's list.

I've had the fuel injectors professionally cleaned on several cars (including 3 Jags) and doing so has improved the idle quality on all of them.

Don't be surprised if your rough idle is a *combination* of various faults rather than a single fault. Such is life with older cars. It can take some effort and perserverence to methodically check, correct, and eliminate all the possibilities.

Cheers
DD

 

Doug is correct. It is usually a bunch of little things that add up to a big problem on older vehicles. Spray pattern makes a huge difference in idle quality as does volume of fuel injected at idle. Worn or partially fouled spark plug makes a difference. Throttle plates being cleaned and adjusted properly. Throttle linkage adjustments also needs to be checked. Of course timing matters. Any small vacuum leaks, corroded connections, etc. Most likely it is not an O2 sensor issue because they are usually in open loop at idle. However it can't hurt to check and replace them if need be.

 

I also have an XJSC and have the same problem. My mechanic looked over the car when I got it and suggested that a full tune up would make for a smoother ride. Also, he suggested a possible valve job, as I do have a slight tapping noise which might just be that.

 

Is it missing or just lumpy is the vac advance working what is it idling at 500 I can fix it or just diag it steve