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I've had my 1975 series II XJ 4.2L stored in an enclosed garage next to the beach at my mother's summer home in Galicia, Spain, for fifteen long years (ever since our older son was born).
The car's always been almost perfectly fine all throughout its 'ordeal' of sorts: as I live a good 5.5 hours' drive away from her beach house, I started the engine once a year only but it always did so beautifully and without a hitch (just a few hiccups) every time–this despite the [almost always awful] AED being fitted and in "proper" working order. (Whatever the word 'proper' may mean when talking about AED functioning, that is! )
I NEVER EVEN DID UNPLUG THE BATTERY'S NEGATIVE TERMINAL!!
So in 2021 I decided to bring the cat back to life (or at least to a more dignified kind of life, in any event) and had it towed to my home near Madrid.
It needed extensive work on the IRS due to rust having stuck the parking brake pads to the discs/rotors... and me finding out while driving it on a back road near my mom's: I totally fried the rear of the drivetrain, wrecking the rear-caliper fluid retainers and a few other things in the process . It also needed new tyres/tires, an all-fluids change, some new fuel piping in the engine bay due to rust and all other basic things after such a protracted storage term: marine environments are never ideal for storing away anything metallic–but miraculously, no need for a new battery!
(I need to add that all this despite the fact that I kept the 'cat' in a fully enclosed garage at all times AND under a rain-proof outdoors car cover AND a soft indoors velvety-inner-lining cover as well.)
So I go to a quite renowned classic-Jag specialist that I used to take the car to when it was in full use in the 2000s (when my wife and I were pretty active in car club activities both in Spain and abroad) and he does whatever he has to do (new suspension bushes/-ings included) in order for me to be able to use it again.
After a long EIGHT MONTHS (yes, brexit has made getting spares form the UK that much harder now than it used to) he hands out the car back to me in (supposedly) pristine mechanical condition–after me having to part with some hefty dough, that is!
After a brief "setting-in" period that saw some minor glitches basically gradually disappear on their own, there was this one thing that keeps bugging me (besides the antenna/aerial rising only when it pleases) still today: I get no read from the fuel gauge–it stays at a quarter full no matter what, and it does so when either tank's selected.
But that's not all: some weird things are happening as well along with this; please read on to find out.
Since I get no reliable reading, I always refuel as soon as the tank that I'm running on goes dry: this way I make sure I ALWAYS have the other one full (ain't the quaint–not to say 'weird'–Jaguar twin-tank arrangement a beauty?). Oh, that and the one-Imperial gallon (4.55 litre/liter) petrol/gas can/bin I always carry full of the stuff in my boot/trunk!
But when I'm forced to press that tank-shift electrical switch on the fascia/dashboard on the road and then head for the nearest petrol/gas station, I always end up having to refill BOTH TANKS with pretty much the same amount (around some 6.6 US gal / 5.5 Imperial gal)–well below their capacity!!! (Each tank holds 10 Imp gal / 12 US gal total.)
At first I thought this was just bad 'bookkeeping' on my part (or perhaps that I was going !) but when I decided to start jotting things down carefully every time I visited a refueling / refuelling establishment, things weren't any different: yep, after having burned / burnt only about six to seven US gallons (roughly a bit over half the capacity of each tank) the fuel line still invariably went dry.
Oh, and one other thing (this one's important!): pressing the switch to shift between tanks just allows me to do a few hundred yards at most before I have to either visit the petrol station–or worse yet: extract the gas can from my boot and refill "on-site"! (Yes, I've had to do that on a couple occasions, and now I try to never even let the one tank I'm running on go dry: it may be too dangerous to refill on the road sometimes!)
I have a few (not nice) thoughts about the car's latest visit to the repair professionals, but I'd like to hear some of your own thoughts first.
Cheers / take care... and thanks a lot in advance!
the tank fuel level Sender Floats get stuck when not exercised.
Assuming the changeover Switch is fine, remove each Sender, (empty tanks first), lubricate mechanism with thin oil like 3-in-1, manually exercise them, and reinstall.
In the Series 3 XJ6, (1979-1987), they are removable from the rear of each tank, by first removing each tail lamp assembly. Follow the Service Manual steps to the letter.
I knew all that about the fuel sender units–thank you nonetheless so much for the input. BTW, the right-hand side tank has the Series 3 arrangement you mention (ie, with the sender unit at the rear of the tank behind tail lamp assembly), whereas the left one does not. Go figure!
In my own non-expert opinion, however, there's definitely something else at work here; I wanted somebody else's opinion before jumping to conclusions, but from the way the fuel is behaving it seems the tanks have been physically linked (as per a siphoning pipe, for example).
If this were true (which I'm not saying it is), the pipe would have to be connected to both tanks' sides somewhere a bit below midheight–in which case the tank-shift electrical switch would be useless and the reading on the gauge would be common to both tanks simultaneously.
Besides, either one of the two existing pumps would have to be connected to this imaginary pipe rather than to the tanks themselves; thus the unused near half of the fuel. The other pump would just be "lying there" doing nothing. Of course I'm aware that all this arrangement would need pumps external to the tanks themselves ('dry' pumps as opposed to submerged).
For clarity, below is a schematic graphic representation of what could in the end very well just turn out to be nothing but a figment of my imagination.
I've depicted (or at least tried to depict to the best of my ability with Paint) the two tanks as seen from either the rear of the car or the front:
(Twin fuel tank conspiracy conjecture)
Among other things, it's beyond me why any repair shop would go into all this trouble, even if it were to keep one of the pumps for themselves due to brexit-induced added difficulty to get spares.
Jag twin tanks... Old jags sitting for 20yrs = rust in tanks. Rust goes straight to changeover valve and fuel pumps, then to carbs and anything I'm forgetting that's in between. The tanks are connected by the "imaginary" pipes whenever the valves get stuck partially open due to rust. Later fuel injected cars are even more fun with extra return lines/valves, fpr, csi, filters etc... I assume your 1975 car is still carburetted with in tank pumps. I wouldn't chase this demon piecemeal... New tanks are worth the money.
... and like I said, the cat was always kept well stored at all times under two made-to-fit car covers in a fully enclosed garage, yet rust found its was to almost every place: for instance, chromed parts now have tiny pinprick brown dots all over their surfaces–and although I always took care to inflate the tires/tyres to working pressure (in order to avoid tire flats) AND fill up the tanks to the brim every time I started the car (remember, just once yearly) to avoid water contamination from in-tank air moisture (a precautionary measure I learned from the general aviation world), it's also not less true that every year a lot of gasoline/petrol evaporated between refills.
The Series 2 XJ6 carburetted cars had a pair of SU fuel pumps, one for each tank.
The tank selection switch operated the appropriate fuel pump.
Each fuel pump included a reverse flow valve which stopped fuel being sucked out of one tank being pushed back through the other pump into the other tank.
If these internal reverse flow valves failed then fuel would transfer from one tank to the other.
If the SU pumps have been replaced by aftermarket pumps which may not have reverse flow valves, then fuel will drain from the fullest tank to the lowest tank while sitting as well as fuel transferring from the selected tank to the other tank when running.
Check the fuel pumps and also note that the concept of a pipe transferring fuel as per your "supposed" diagram is probably correct as per my above remarks.
I ran a Series2 XJ6 for 15 years with aftermarket pumps and in line check/reverse flow valves. Series 2 XJ6 SWB
Bill, my '75 had two external pumps in the trunk (carbs) with your aforementioned check valves, and the check valve at the filter iirc etc... but when did they have the in tank pumps? Didn't some of the ser2 cars have those? Maybe early? Just wondering.
Slofut.
The car I put into the photo was a 1974, 4.2, SWB.
I did have a 420G which had in tank pumps, so they do go back a fair way
Re XJ sedans I have only ever seen one example of these" in tank" pumps on a friends S2XJ6 and it was I think a 1978 build.
Cheers
Between Slofut's recommendation ("The tanks are connected by the 'imaginary' pipes whenever the valves get stuck partially open due to rust") and Bill Mac's ("Each fuel pump included a reverse flow valve which stopped fuel being sucked out of one tank being pushed back through the other pump into the other tank"), now I start making sense of some of the twin-tank antics of late in my car!
I have a ‘75 Series 2 Sovereign. 4.2 with twin SU. Manual choke conversion. So exactly the same set up as an equivalent XJ. I am based in the UK. I work on my own car a lot, but decided the fuel tank replacement was too much of a hassle without the benefit of a garage hoist. So I got my local independent Jag specialist to swap the tanks out with new ones (he is a bit of a local expert/hero/celebrity when it comes to Jags, Classic or otherwise). I got him to do the fuel pumps too.
I have almost exactly the same issue. Except my change over switch and fuel gauge appear to work perfectly fine and read correctly. The issue is the fuel seems to be drawing or returning to both OR wrong tanks. It always seems to even out the fuel load between tanks. If I fill them both up and use the vehicle on ONE tank, it usually ends up with both tanks having the same fuel left in them. Not immediately though. Which is odd. I have been through the system and the lines are pretty simple. There are definitely no spurious or non OEM lines installed. No DIY or dodgy hacks or modifications. Interestingly, issue seems more prevalent with more frequent, shorter trips.
I am pretty sure it is a return valve issue. Or a related problem with the type of fuel pumps used. My local expert hasn’t been too helpful. Doesn’t want to waste time on it (unless he is paid for the pleasure). Gave me the advice of setting up some clamps on the fuel lines to test which tank is drawing and also preventing the fuel returning to the wrong tank. Basically, doing fault finding for him.
The Series 2 set up was distinctly different with the use of outbound pumps, while the series 3 used inbound pumps. But there was cross over on the later Series 2 using the newer set up. I prefer the outbound set up. Much better for access when diagnosing, or doing repairs/servicing.
I would love to see what other discover or manage to solve, in the faint hope it might help me solve my own issue. It hasn’t been a huge hassle for me so the job is lower down my list of things to do and solve. 😉
A. So, Slofut: where in this picture would your 'change-over valve' of sorts be exactly?
Fuel lines
... only I can't make any sense of it!!
Yago, no change over valve in that scenario! Dash switch energizes either pump and each pump has a mechanical one way valve. No returns either. But if a non return valve gets stuck partially open you'll get the tanks connected together.
Well-travelled, your problem seems eerily similar to mine. I'm sure somewhat of an at-least-partial answer must lie within the others' replies to my initial post.
By "outbound" pump I suppose you mean external to the fuel tanks–"inbound" then, of course, being 'immersed'.
If you are still running SU pumps most likely the internal valves in the pumps are leaking and the pumps require overhaul which can cost big dollars.
There is a very simple answer to this cross draining problem.
Install a one way valve at the outlet of each fuel pump.
These one way valves are very cheap in OZ costing around $10.
I got rid of the SU pumps and installed Facet pumps and one way valves and drove my S2XJ6 for 10 years with no fuel problems
Well-travelled, your problem seems eerily similar to mine. I'm sure somewhat of an at-least-partial answer must lie within the others' replies to my initial post.
By "outbound" pump I suppose you mean external to the fuel tanks–"inbound" then, of course, being 'immersed'.
No, you interpreted correctly, I was mistaken that your car had the changeover valve. Sorry for the confusion. Btw... You can remove a pump and blow through the outlet port to tell how much leakage it has when closed. The added one way valves Bill Mac describes will be the best solution but you still may have to periodically clean them even with pre-filters installed depending on your tank conditions.
Yago
Just google "one way fuel valves".
That should give you a domestic selection for your part of the world as opposed to what I get in Australia.
cheers
[Rather suspect] Twin-tank functioning
) 
. It also needed new tyres/tires, an all-fluids change, some new fuel piping in the engine bay due to rust and all other basic things after such a protracted storage term: marine environments are never ideal for storing away anything metallic–but miraculously, no need for a new battery!
!) but when I decided to start jotting things down carefully every time I visited a refueling / refuelling establishment, things weren't any different: yep, after having burned / burnt only about six to seven US gallons (roughly a bit over half the capacity of each tank) the fuel line still invariably went dry.
