Lately when I fill up I've noticed that the gauge does not read full. At first it was showing around 7/8 full after a fill up, but now it seems to be only a bit over 3/4. I have always stopped filling when the pump switches off and not force fill it any more. I BELIEVE this is a gauge problem and not that the pump is turning off too soon, based on how much fuel I can put in when nearly empty. Not 100% sure about that though.

Old school arm chair diagnosis would conclude this is a sender unit that is getting stuck and no longer floats to the top when full. But, this being an XK, I would be surprised if it's that easy.

Any insight into this would be appreciated.

 

It COULD be a problem with the Evap Canister. Maybe. Have you tried to first fill up to the stop and then wait about a minute and try to top it off? It's not a procedure that is recommended, but if you get a goodly amount of fuel on the second attempt it may be a clogged up Evap canister.
If your second attempt only allows a few seconds or so of flow, then your tank is probably full and your gauge IS bad.
This is just a troubleshooting idea, not a fix.

 

Okay, I ran it down to "Range 4 Miles" and filled up. It took 17 gallons before the gas pump automatically clicked off. So we can rule out the Evap Canister. But I appreciate the suggestion of this possibility.

After putting in 17 gallons the gauge only read 3/4 full. After driving around for 5 to 10 miles it went up to 7/8, but no higher.

 

The X150 Fuel tank is the "saddle bag" style. One electric fuel pump is located in the right hand lobe. A jet pump is located in the left hand lobe.
There are numerous reasons that fuel may be imbalanced but a 3/4 full reading on a full tank is likely a fuel sender issue.
See the attached service bulletin.
If you decide to access the fuel tank, search this forum for tank removal discussions.
Best regards.

 

Thanks for that bulletin. This sounds exactly like what the service advisor at the Jaguar dealership was describing. 6 hrs labor... no wonder it was going to be so expensive. My hesitancy is that it doesn't sound like this is an absolute fix. That it may or may not solve the problem.

 

The repair identified in the service bulletin was originally (I believe) part of a recall. You might ask the dealer if JLR assistance is available.
The JLR service procedure includes dropping the rear suspension, drive shaft and exhaust. Six hours is not bad.

Diagnosis by the dealer should have been based on a test using JLR SDD.

The JLR SDD Measurement feature provides a real-time view of the sender outputs individually. If the tank is more than half full and there is a substantial difference in the readings, a sensor problem is a pretty good diagnosis.
Sensor 1 is the left side tank. Sensor 2 is the right side tank.
So, when observing both senders while filling an empty tank, sensor 2 will raise first followed by sensor 1. Then, both sensors will raise approximately equal amounts until full.

For those experiencing fuel stranded on the left side lobe, you may "run out of gas" before the gauge indicates "empty" or a low fuel warning light.
Fuel stranding in the LH lobe may be due to a malfunctioning jet pump or associated components OR marginal electric pump pressure due to low voltage (failing alternator) or a failing/partially clogged electric fuel pump.

 

Boy to I feel stupid now. I've been trying to hang the gas cap by the plastic cord. I guess I missed that part of the owner's manual.

 

Correction to Post #6:
" The JLR SDD Measurement feature provides a real-time view of the sender outputs individually. If the tank is more than half full and there is a substantial difference in the readings, a sensor problem is a pretty good diagnosis.
Sensor 1 is the left side tank. Sensor 2 is the right side tank.
So, when observing both senders while filling an empty tank, sensor 2 will raise first followed by sensor 1. Then, both sensors will raise approximately equal amounts until full."
I should have said less than half full. When less than half full the fuel is two separate volumes, left and right.
Whenever the electric fuel pump (RHS) is running, it provides pressurized fuel to the jet pump in the LHS which returns a greater volume to the RHS to balance the volume.
If there is a problem with the jet pump, the fuel remains in the LHS.