I recently purchased a 1994 XJS 4.0 Litre Coupé in excellent condition. Body, paintwork, interior and mechanicals were all in very good condition as the car had always been garaged and had low Km.
It had a few gremlins that I discovered during the test drive but nothing that could not be easily fixed. These included a driver's side mirror that could not be adjusted and a speedo that failed on the way home after purchase.
The speedo failure was due to the signal cable from the sensor on the diff housing becoming disconnected from the cable coming out of the speedo interface unit in the boot. Easily fixed.
I then removed the driver's mirror to check it and as I withdrew the electrical cable from the door it became apparent that the two green connectors at the end of the cable had not been plugged in.
I suspect that the mirror was disconnected during a complete respray some years ago and never reconnected. Once again a fairly easy fix once I figured out how to remove the door card.
Having sorted out the main issues I am now turning my attention to a couple of other issues.

The temperature gauge never reaches the normal mid-point of the gauge. It will sit halfway between cold and normal. I have installed a new radiator cap but it made no difference.

The other issue is with the oil pressure gauge. Once warm it reaches the operating pressure of 4 Bar but the needle constantly flickers around that point.
Could this be an issue with the oil pressure sender unit or the gauge itself. I doubt that the oil pressure would vary that quickly under normal driving conditions.

Looking forward to the response from the body of experience in this community.

 

Sounds like you will need to replace your thermostat. Should be rather straightforward, just make sure you get the thermostat with the flat gasket, not the o-ring.

It can also be the coolant temperature sender, I suppose. A cheaper and even easier fix. Both very common.

The oil pressure sender is failing. It may even be leaking some oil. Very common as well.

 

Paul,

On the 4.0 XJS, if you put your hand on the top hose, you'll feel a sudden noticeable change in its temperature as the thermostat opens. That can help you identify if its already stuck in the open position.

As Vee mentions, the oil sender units can be problematic. Also, to counter criticism from the US market about changing oil pressure gauge readings (isn't that what it's supposed to do?!) Jaguar introduced a revised sender unit that just acts as switch that just sits the gauge at one position unless it was very low, rather than let it vary naturally. If you do need a new sender unit, decide if you want the Jaguar fixed type or a normal variable sender unit.

Good luck

Paul

 

Thanks Vee and ptjs1 for your suggestions. I will take a closer look this weekend and do some more investigating.

 

After a decent run in the XJS I noticed moisture around the coolant header tank seams and also evidence of some leakage along the inside of the mudguard and onto the RH chassis rail.
So it appears I may have a leaky header tank.
I will investigate the possibility of having it repaired at a radiator repair specialist.
If that doesn't work out I will be looking for a good used one as new ones are very expensive.

 

I still think that thermostat is ready to be replaced. Perhaps the coolant temperature sensor as well.

Both parts are easy to find, cheap to buy, and rather straightforward to install. You may want to perform these changes yourself, and leave the more difficult work to the experts. I'm a big fan of reusing, and recoring the OEM radiator. There's absolutely no reason to buy a new one when a recored radiator will function as good, or better than the more expensive products out there!

 

Hi Paul,

Don't know if the header tank arrangement is the same as on my 1990, 3.6L but I have an occasional leak just as you describe on inner mudguard / chassis rail. My leak is coming from the rubber seal that sits around the coolant level indicator on the side of the expansion tank as I can see faint antifreeze staining. Runs over the seam and along bottom of tank then down inner wing. Only a dribble but another job to do.

As said if your tank is different this may be a red herring.

Good luck,
LeeP

 

Lower than normal coolant temp indication tends to happen because of a bad ground connection to the gauge behind the instrument panel.
Normal coolant temp indication operation: The temp gauge readings are supposed to go up as the sensor's resistance decreases with coolant temp increase. But, if something else in the circuit is keeping the circuit's resistance higher than norm, such as corrosion in a connection, the gauge will believe that the higher resistance coming "from the sensor" is due to the coolant temp being still cold, therefore it will keep the pointer still indicating a colder than norm coolant temp. I'm actually having this problem with my '94 XJS 4.0L, which has gone from what you, yourself, are now seeing in your XJS, to the point that the pointer hardly moves from the bottom of the dial anymore. I have replaced the sensor to no avail. This is typically caused by a defective ground connection to the gauge due to corrosion somewhere behind the instrument panel, so the best fix here is to just add a redundant connection with another wire connecting a good ground to the gauge's negative connection. This issue and repair has actually been addressed several times in this forum. I have to get myself busy repairing mine already.