My brother's XJ8 just started showing the fuel level above F all the time after the car is started. When the key is off, it shows about 1/3 level (between 1/4 & 1/2). Anyone have a cause and fix for this?

When we select miles to empty on the trip computer it seems to be accurate.

 

I would suspect that the fuel gauge sender is malfunctioning. This is submerged within the fuel tank, so is difficult to get at.

 

Even though the range function works?

 

I believe those are separate functionalities. Anyway, the way almost all gas gauges work in almost all cars is that there is a float inside the gas tank, that rides up and down with the quantity of fuel. This in turn moves an electrical wiper across the surface of a variable resistor, so that the gas gauge sees a varying resistance as the fuel level changes. The highest resistance corresponds to the highest fuel level. However, over time crud and oxidation inside the tank, on the surface of the variable resistor, add extra resistance, causing an inaccurate reading. The remedy is either to replace the sending unit inside the tank with a new one, or take the sending unit out and (carefully) remove the crud from the surface of the variable resistor. (I have this same problem on another of my cars, though not a Jaguar. "Half full" is actually only one quarter)

I'm not sure how the range function works. I suppose it monitors the fuel flow to the injectors and does a calculation. But how it knows the quantity of fuel available, I am not sure.

But I am about 99% sure that your problem cannot be fixed without getting into the gas tank. Any differing views?

 

No

We know that corrosion on the sender contacts can cause problems.

New senders are not expensive and just clip on the side of the pump housing.

 

Because it shows ~1/3rd when the ignition is off, and doesn't go down to zero, I'm inclined to think it's the gauge that's at fault, and not the senders.

 

"Because it shows ~1/3rd when the ignition is off, and doesn't go down to zero, I'm inclined to think it's the gauge that's at fault, and not the senders."

If there is extra resistance in series with the sender (from, for example, a dirty contact) then that could still mean that the problem is in the tank. Or maybe a poor connection somewhere in a wire harness.

 

I'm with PatricktheCat if the gauge is not going to empty when the car is shutdown there is something wrong at that end. That should be fixed before doing anything with the tank.

 

Well first off there are two senders, one for each side of the saddle fuel tank. Secondly they don't control the gauge directly, they input to (I think) the rear electronic module which works out what the fuel amount is and sends that, in amongst all the other data it needs to communicate, via one of the data buses (CANBUS sort of thing) to the instrument cluster.

The needles of both fuel and temperature gauges are driven by stepper motors. If you notice, when you switch the ignition on they both initially dip slightly below the zero mark; then the temperature gauge (if the engine's cold) will come back up to the zero mark, and the fuel gauge should rise what ever level the REM dictates. The dip action is the way the gauges re-calibrate their zero points.

[And even if were an 'old fashioned' rheostat and milli-ammeter type system, the gauge should still go to zero when the ignition is turned off irrespective of the sender rheostat position because the circuit will go dead. I've never heard of a car with an electrical fuel gauge that stays live when the ignition is off; certain receipe for a flat battery].

When you switch off both gauges should go down to their zero points. If the fuel gauge only goes down to 1/3rd, and doesn't dip down to below zero when you switch on again, then something is sticking inside the gauge or its stepper motor and preventing it going below the 1/3rd mark.

If it goes right to the top even when the tank isn't full that's because the stepper motor, after the abortive 'down' steps, is going up by the number of steps it thinks it should go by from the zero position, but because it's starting from 1/3rd up it goes right to the top.

Edit ... Pragmatic: sorry, I started my reply hours ago, but got called away to give someone a lift. Didn't see your reply 'til I finished just now pressed the 'send' button.

 

Thanks for the replies guys, that is what these forums are all about. We all want the quick fix and answer, of course that's what I wanted but hey, I don't mind the grind. Was hoping there was some common issue that many have suffered and there was a quick fix.

My 1990 Volvo 740 GL wagon is suffering from a bad fuel indication system. I'm 90% sure it's the sending unit, below zero when off, erratic levels while driving, etc.

Right now it is mysteriously working properly again. I usually tell people, don't worry, it will get worse. As Arnold said, I'll be back.

 

You could do a diagnostic on the gauge if you wished to; http://www.jagrepair.com/images/TSB/...%20Cluster.pdf

 

RSpi--Similar (but different) problem a week ago. Full tank showed 3/4. Drove it down to 1/8 (figuring "empty.") Lucky for me I didn't go further!! Took 20 gals !! As I hoped, gauge must have developed a "full empty" signal, which gave "Full" gauge reading immediately. BTW, the "Range" function comes off gauge reading, so you really have no "reliable" way to use Trip Odo. I "estimated" around 270-280 miles, because of "city" driving, and got VERY LUCKY!! (Settled for "no name" 93 octane, tho.)

 

Well, I'm back. Just to recap, I believe the initial problem was with the mega plug in the top of the trunk. Several things went wacky and putting pressure on that plug corrected everything. Probably messed it up pulling the tank for fuel pump replacement.

NOW: Yesterday I helped my brother get the car started, battery was dead. Tried to jump it with 2 different jump boxes, just got a click, then with cables and his other car, just clicks. Took the battery to get charged and was told it was bad (2016 battery). Installed a new battery and it fired right up.

AGAIN the fuel gauge is messed up. With the car off it reads way pass full. Start the car and it doesn't move at all, just stays pegged above the full mark. Noticed a few other things when the car started: 1. Check engine light immediately came on. 2. Code reader will not connect to the system to retrieve codes. 3. I can NOT hear the blinders flash when they are on (they seem to work).

I do appreciate any fresh tips. I'm headed back to the car to check fuses and that massive plug in the trunk. TIA

 

Made it back to the car today and was able to connect my Scan Gauge scanner to it. It showed a code P1638, instrument cluster problem. With the gas / fuel gauge pegged out past the full mark, I understand the reason for the code. When I insert the key and start the car the fuel and temp gauges do NOT swing test. The fuel gauge stays pegged out and the temp gauge raises just a little to show cold.

Question: Assuming that the instrument cluster is bad, can I swap a used one in and the car / replacement cluster work (if it's good) or will the vehicle reject it because it will need programming?

 

this should probably be moved to the 308 forum.


a used instrument pack with the same vcats number will work. has to be from a 2000+. lower mileage ones are usually priced higher for obvious reasons

 

Is it possible to test them to see the mileage with the cluster out?

 

some of the ebay guys record the mileage before pulling and can possibly correct it for you. you can make a trip to the junkyard and use a jump pack on it or just go by oil change stickers etc. the part number for the instrument cluster "INSTPK" is here, will probably be LJE4300AB/002