Hi all,
My beloved '95 4l manual XJS failed me this evening...I was sitting idling at a junction (after a longish drive) when the engine just died. No warning. One moment it was running, the next...not. It turned over, but wouldn't fire up.

After pushing the car to safety, I ran a couple of checks (while waiting for a tow). I confirmed that the fuel pump was running and that there as no spark.

Reading round, I see that the crank position sensor is 'notorious' suddenly failing. Is this a likely culprit in this case?

I should say that until it knocked out, it had been running extremely smoothly, though I had noticed a very slight hesitation at around 3000rpm.

I look forward to reading your thoughts.

Merry Christmas

Paul

 

Sounds like it, but you state its a 4.0L (6cly) ? Its been awhile since I owned one but I'd imagine its no different than the v12 if the signal cuts out the car will just cut off like an electrical panel breaker no warning.

 

Ok...a bit more info...I have also read that an indocator of a failed CPS is that the tacho needle doesn't move when you turn the engine over....

I have just tried that and the needle moves ... hovering between 6-800rpm or so...so maybe it is something else that has died...?

 

Hi YawningAngel

I had the very same problem myself and even fitted a New CPS which didn't make any difference, where after trying almost every trick in the book, the cause of the engine failure turned out to be one of the Two Blue Relays, which are inside the Fuse Box behind the 'Knee Pad Cover' on the Passenger Side in a (UK) Car

They are just above the Fuses and a bit tucked out of sight, where the cause of the actual problem is not the Relays themselves but more to do with the Blades on these not making very good Contact in their Sockets

Where after swapping them around and 'Jiggling' them a bit, the engine fired up on the Button and She's been ok ever since, for which I am indebted to 'Paddy' for that suggestion, without who's help I do not think that I would have ever got her running

Its all here on Page: 96 including photos, where this and the previous pages also gives a guide to Changing the Fuel Pump/Removing the Ignition Coils/Doing a Compression Test/Removing the ECU to clean the pins and checking for dampness/Removing and Replacing the Injectors and loads of other stuff

Trying every trick in the Book to get 'Carmen' Started after sudden Engine Failure including Photos

 

Alas no luck with those relays...they seemed in good condition...no corrosion, but I removed them both swapped them over and reseated them, to no avail.

 

Hi YawningAngel

I wouldn't dismiss those Relays out of hand too quickly because unlike Normal Relays, they are fitted with Diodes which may have possibly blown, which means you cannot swap them with a Normal Relay that doesn't have a diode

And if I remember correctly Jaguar recommended a different make of Relay that has thicker blades

But having said that, there's also something else that you can try

Take another look in that Fuse Box and one of those Fuses is for the ECU and if that's blown then that is your problem right there or else it could be the Fuel Pump or Main Relay, which are located in the Boot/Trunk and are really awkward to get to as they are underneath the inside of the Fender/Wing by the Boot/Trunk Support Strut

Drivers Side on a (UK) Car where the Fuel Pump Relay is the Silver one

 

Thanks...
I should say that I have verified that fuel pump is running...so I guess that relay is OK. However, I assume I should be able to feel the ECU relay under the passenger airbag clicking on when I turn the ignition on...verifying that relay, too?

 

Hi YawningAngel

I'm not too sure about whether you would be able to feel that Relay Clicking or not

Going by what you said about the engine just dying on you, it should be a very simple fix if only we knew which component had gone wrong

So best to check all the Fuses just to see if anythings blown and then you could Test the CPS by disconnecting the plug that its plugged into, while leaving the actual CPS in place, then put a Meter across the 2 Wires and then Watch the Meter while someone else Spins the Engine on the Key and see if you get a reading on the Meter

Which should be set on Volts

Anything to do with the CPS is easier tackled from on top of the Engine and there is no real need to even take off the Top Hose, providing you can take a look and memorise where that Plug is and then do it by feel, as you hand will be blocking your Vision

Its a bit awkward the first time but once you know how to do it, then changing the CPS unit complete is really no more than a 10 Minute Job, that can be done by the side of the road, if you carry a spare CPS like some owners of that Model have been known to do

 

The car is back up and running!

I replaced the CPS and it fired up 1st time. It only took a while as I have been away over the holidays...

Anyway, many thanks for all your comments and suggestions.

Cheers

Paul

 

I have had a few (maybe 4) cases of my engine cutting out suddenly - and unfortunately at highway speeds on three lane highway! But I have been able to manoeuvre to the shoulder without incident though not always easily with vehicles all around me at >100 kph.
All dials looked fine (oil, temperature, voltage) until it stopped.
I then waited maybe 15-20 seconds, restarted the engine and it started immediately and ran fine for the rest of the trip as though nothing had happened.
Every time this happened it was 15 minutes or so into the drive and at speed on the highway.
I was concerned about it being bad gas so did fill up with 94 octane. And for the past three or four trips nothing has happened.
I noted comments about relays that can be not seated properly causing sudden engine cuts. But has anyone experienced anything like this, where almost immediately afterwards the engine appeared to run fine - as I say, as though nothing had happened?
It is a 4litre 6 cylinder 1992 XJS
Bill