XJ40 Idle Problem :-(

Hi
I have 1991 XJ40 Sovereign (3.2) which was fine a couple of days ago... but now...
when parked up, say at lights, box in drive and foot on the brake, the car
is shuddering quite a bit. I mean, so much that I feel sick believe it or not. It's quite intense but this also happens in Neutral too. I noticed something on the motorway today too. I was just about to hit 100 and there was a bit of knocking and it decided it wasn't too keen on a smooth acceleration for some reason.
Anybody have any ideas?
thanks in advance.
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"tvprotein" wrote: > Hi > > I have 1991 XJ40 Sovereign (3.2) which was fine a couple of > days ago... but > now... > > when parked up, say at lights, box in drive and foot on the > brake, the car > is shuddering quite a bit. I mean, so much that I feel sick > believe it or > not. It's quite intense but this also happens in Neutral too. > I noticed > something on the motorway today too. I was just about to hit > 100 and there > was a bit of knocking and it decided it wasn't too keen on a > smooth > acceleration for some reason. > > Anybody have any ideas? > > thanks in advance.
that sounds like a clutch failing
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It is really hard to diagnose a car over the Internet. You could have any number of problems that can cause this. Basic engine knowledge leads me to believe that you are having a problem with timing/fuel detonation. The engine is shuddering at all speeds and in neutral at an idle. This is usually an indicator that the petrol is being detonated at the wrong time -- thus the rough operation.
My first thought is the Throttle Position Sensor. The TPS or Throttle Potentiometer eventually wears out as it ages and is a common cause of vague and erratic drivability problems, such as hesitation and flat-spots during acceleration. The TPS is located on the end of the throttle shaft on the bottom section of the throttle body. It sends an output voltage to the computer and will alter the amount of petrol delivered to the injectors based upon the position of the valve angle. A bad or broken sensor will send erratic signals to the computer making the engine think that the throttle is moving and thus making the operation of idle totally unstable -- as it sounds in your case.
My second thought is that the CPS (Crankshaft Position Sensor) is on it's way out. This is a common malady and can throw the firing of the engine off terribly. Usually, the engine just cuts out intermittently when the CPS is going, but it has been known to create other symptoms. If the CPS is throwing erratic signal to the ignition computer than it will fire at odd times -- thus the strange operation.
Another possible problem area would be the Mass Air Flow Meter. This is the large box with the electronics on it that is in the air flow stream to the engine. These needed to be cleaned and adjusted periodically. If yours has become dirty than it can give erratic signals and cause the engine to operate poorly.
Webserve

but
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I can see what you mean - thanks for both replies. I have taken the cowards way out and book it into an independant Jag garage tomorrow so I'll post the results - might be handy for others.
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Took it today and one of the valves has burnt out :-( I'm going to get all 6 done (complete strip down) and it's been proced at 600 (UKP) I thought I might as well get it all done as the garage reckon the engine should be good for quite a bit yet once that has been done. He mentioned that one of the cars that still comes in has done approx 327K !
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Not exactly sure what you are having done BUT at 600 pnds taking the head off and replacing all valves seems VERY reasonably priced indeed. Would hate to think of a fellow Jaguar owner getting an unwelcome surprise.
There are some great independent Jaguar garages out there, I can personally recommend two, just depend on where you are.
Andy

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