Loss of power.

Started 19th February 2017 at 09:06 · Andrew J Pacey

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My XJS starts & runs well for about 6ish miles then it loses power & eventually stops.
This is a new one on me, if I wait 30 mins or so the Jag will start & run perfectly for a few miles then slowly the power decreases until the engine cuts out again.
I have a good fuel supply.
The best way to describe this problem to loss of power is as if the plug leads are being removed one by one from the spark plugs.
Any advise to this problem would be greatly appreciated.

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Replies
- 19th February 2017 at 11:56 #3047
  
  Robin
  Participant
  
  Always difficult to diagnose remotely but firstly what year (’88?), engine v12 or which 6cyl.
  Do you get an error code on the computer trip readout?
  
  Robin
- 19th February 2017 at 13:16 #3051
  
  Andrew J Pacey
  Participant
  
  Good afternoon Robin.
  Thank you for your reply.
  Its a 1989 xjs v12 5.3 sports convertible.
  This morning I Started the car no problem, it ran well for about 5mins after which time the engine began to falter & cutout altogether.
  It seems when the engine is running up to temperature it starts to lose power.
  The engine cooling system if fine, no leaks, temp gauge reads normal.
  Kind Regards.
- 19th February 2017 at 17:24 #3060
  
  Robin
  Participant
  
  I’m more familiar with my 3.6l than the v12’s, hopefully be someone with better expertise than I on V12 problems.
  
  Has it been running ok until recently or is it the first time you have run for a while – i.e. been put away for winter months?
  
  Good news is it starts! If it is relates to engine temp there are a few clues maybe. First is the engine temp sensor (different to temp gauge sensor) which inputs to ECU, giving cold start enrichment. They can go faulty and give incorrect resistance reading – or even the wiring connector gets corroded.
  When it dies will it restart immediately or do you have to wait?
- 21st February 2017 at 18:12 #3108
  
  Stephen Heywood
  Participant
  
  Hello Andrew,
  
  Laurence asked me for what my suggestions might be to your issue, which is difficult not having seen the car at all but I’ll give it a go.
  
  From my experience I would suggest you took a look the Ignition Amplifier or the Coil Outputs – faults on these can cause exactly the symptoms you are explaining which occur as the engine warms up, and then disappear again as the engine cools while being stood for 30 minutes.
  
  It shouldn’t be too much bother to swap these for a working ones.
  
  Hope this helps
  
  Steve
- 11th March 2017 at 17:46 #3597
  
  Robin
  Participant
  
  Andrew, Have you found the cause yet? Always interested to understand what cures faults for future reference!
  
  Robin
- 12th March 2017 at 20:31 #3612
  
  Andrew J Pacey
  Participant
  
  Good evening to all members.
  Please accept my sincere apologies for my late replies.
  
  I now have an update to my XJS cutting out problem.
  This same fault occurred about a year ago to which the front crankshaft speed sensor was @ fault & duly replaced.
  At the time this work was being carried out it was noted the water pump had a slight leak.
  This water pump leak wasn’t very noticeable & over the year never caused any problems !!!.
  Would you believe it, when my XJS broke down with this same fault as last year due to the faulty speed sensor, Lester the Jag Tec man found that over the year the slight leak from the water pump was finding its way down the front of the engine & causing damage to the electrical sensor sited lower down
  Having the sensor cleaned etc, my XJS is one again running well & a pleasure to drive.
  
  A new water pump has been ordered !!!.
- 15th March 2017 at 20:21 #3631
  
  Robin
  Participant
  
  Thanks for the update, most interesting. Always good to know the solution.
  
  I had a failure with my own crank position sensor after long term storage on my 3.6L and it took me ages to find source of problem. At that time I hadn’t realised it even had a sensor..
  Costly item on the 3.6 too.
  I was never quite sure whether it was the sensor or just the plug to wiring connector corroded but replacement cured the ignition problem.
  
  Robin
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