XJ12 S3 Engine Woes

[gpf1973]

Hello,

I drove my '82 Series 3 yesterday for this first time in 3 weeks. The car starts and cruises around town fine, albeit on idle it's a shade lumpy; however, when I try to accelerate relatively hard either from stationary or when kicking down the RPM seems to 'hunt' or hesitate around 2000 rpm and the car makes an intermittent banging noise. Not having experienced this on this or any other car I'm assuming this could be a misfire. When I backed it into the garage I gave it a few revs and again it ran roughly with a strong emissions smell.

I put in fresh fuel at the start of the journey before this happened and the fuel already in there wasn't that old. Temperature and transmission appear fine. I've had the car a year so not sure of its history - i.e. the last time plugs and leads were replaced - but I've had nothing done to it since or has anything suggested things are untoward.

Any ideas as to what might be the trouble and fixes? Looked at the forum and can't see an identical issue to this, although there are some that share symptoms.

Grateful for any thoughts,

Glyn

[Brakebuster]

this could be stuck bob weights for the advance in the dizzy

BB

[gpf1973]

Thanks BB,

I've looked online at bob weights and it looks a fairly straightforward job to check they are free (think positively!).

When I get some time I'll take a look and post an update. However any advice you or others can give re things to be careful of when doing this or other possible causes gratefully received.

Cheers,

Glyn

[mike020150]

As BB says, stuck bob weights are a relatively common problem and they often result in poor performance and fuel consumption. From your description, I'd guess that the misfire is due to too much fuel ('emissions smell') rather than a lack of petrol, if it's not an ignition issue. The simplest failure on this side of things is the temperature sender for the fuel injection system. These can and do fail without prior warning. If they fail 'rich' there is a smell of petrol and black smoke under acceleration. The senders are typical NTC (negative temperature coefficient) units so their resistance falls with increasing engine temperature. They can be tested with a standard automotive test meter and the resistances measured at different engine temperatures (eg 15 degs ambient & just hot engine, say 85/90 degs) and the values measured checked against those in a Haynes or Jaguar workshop manual. I'm afraid I haven't a manual for an HE V12 to quote but I'm sure someone reading this thread will be able to list them for you. Mike.

[oldtimer]

Might be worth starting at the easy end.

Plugs or distributor cap could be the culprit.

Careful comparison of the colour and deposits on 12 plugs can tell a story.
Cut 12 holes in the bottom of an upturned cardboard box and place the removed plugs in the correct order and then examine the variations.

Check the inside of the distributor cap for tracking (burning marks)

And the rotor arm and bob weights while you are there

Of coarse, a good run can sometimes clear these faults, low vehicle utilisations can have negative effects

Oldtimer

[gpf1973]

Thanks everyone for your suggestions. I've done some further research today on the world of V12 distributors and bob weights so I've an improving appreciation of what may be involved and to look out for.

I'll keep my eyes out at this weekend's Manchester Classic Car Show for any dizzy-related items. Last year there were plenty of items of interest.

Update when I know more.

Thanks again and grateful for your ideas

Glyn

[alec2824]

Off topic! Is the Manchester show good?

[gpf1973]

Yes. I went last year which I think was the first time it had been held and was very impressed. Plenty of cars covering all eras (including a good few Jaguars) and enough stalls and clubs etc. to keep me occupied for a few hours. Well worth a visit for a tenner (used XJS club discount) but even at £15 on the door good value. Busy but not too busy (if you know what I mean) and free parking. Recommended.

[alec2824]

Many thanks. The wife says she does not want to go - but I can. But, she wants to go out for dinner on Saturday! Gets tricky!!