Old Jags are a pain


#1

How can anything so good looking be such a pain?

My '74 XJ has two gas tanks, both stink but recently something happened and it will not switch from one tank to the other so I am only running off one tank. At eight gallons to the mile it is a problem. Any ideas on where to start?


#2

Wow… that’s a pretty strange design. Here’s a website that talks about some of the problems:

http://www.davemjohnson.com/hess/fuel_problems.htm

It looks like there’s a second fuel pump in the lower tank that pumps the fuel into the upper tank, which is the one your fuel pump is connected to. It also seems that the venting system for this odd setup is problematic.


#3

Start with the cheapest fix first, the fuse. Then try taking the in dash switch out to see if the contacts are clean (good). Then try checking a manual (Chiltons, Haynes, Mitchell) as to where your pump relay/s are. My guess is you have 2 (just a guess, do not actually know) and they are mounted somewhere next to the firewall on the upper left or right side of the vehicle. Then if you do have 2 relays try switching them and see what happens. If that does not work and you replace the relays to their original position and neither pump works I would suspect some sort of unusual spike (this is highly doubtful but possible - A long shot) If switching relays does not work but then switching them back to original position you are back to square one, try testing the resistance on the wiring running from the switch in the car to your tanks and see if any of those wires have more resistance then the good tanks wires and the wires from the relay to the switch. If you still have not found the problem try (unsure how much room you have to work under there or how it all looks) (after disconnecting the battery) to connect the wiring from the good tank to the bad tank (then reconnect the battery). If there is no pumping in this case scenario then most of the technicians here would more then likely agree that it is your fuel pump. (But that is just a guess)


#4

This is NOTHING…Wait until the so-called carburetors crap out. For the price of rebuilding them, you can buy a new Lexus…


#5

When you say it will not switch tanks, what exactly happens.

The 74 XJ has dual SU or Stromberg carbs, these are fed from of 2 tanks housed in the rear fenders.

Each tank has an SU electric fuel pump, these are located to the rear of the spare wheel well in the trunk. I’d start with the fueld pumps first, to access these remove the housing at the rear of the trunk, you’ll need to remove the spare wheel and a couple of screws to get this out.

You’ll see the pumps on the left and right side of the trunk.

Start the engine with the fuel switched to the side that works and put your hand on the pump you are switched to, at idle you should feel the pump pulse about once every 30 seconds. Now switch to the other pump and feel for the pulse - there is a method to this madness.

If you don’t feel the pump pulse, start the engine again on the pump that works, then switch again and whack the non working side fuel pump with a 3/4" wrench - make sure you hit the steel body, not the plastic cap.

If you’re lucky, you’ll here the pump start up. That means dirty contacts, yes they have a contact set.

If you’re not so lucky, either the points / pump are shot or the switch is bad. First step is circuit test to make sure you are getting 12v to the pump.

If you are getting 12v, you can remove the pumps easy enough, and the plastic cap is simple to remove, very fine wet & dry can be used to clean the points.

Joe Curto in NY can rebuild the pumps or has new point sets if you need them. Burlen fuel systems in UK make contactless fuel pumps, they are show repro and look identical to the originals ~ but with current exchange rates they aren’t cheap.

Almost finally, if the pump is working but the engine still dies, you have water in the tank. The XJ6 tanks are long tall and narrow, water does ingress through the filler neck, particularly when the tank filler seal cracks and dries up and you don’t clean your tank drains. The water just sits in the bottom of the tank and gently rots them out and fills the carb with water when you try to use that tank. I’ve seen 2 gallons of water in these tanks before now. If that is the case, just drain the tank and seperate the water.

Finally, these tanks have a vent that runs up throught the ‘C’ post, if the valve fails on 1 tank, it will run for a very short time before the tank vacuum overomes the pump power, then the engine dies until you open the filler cap.

But this is a simple problem to diagnose.

Good luck, drop me a line if you need anyhting else - and remember this car is 30+ years old, I have a 73 New Yorker with it’s own set of foibles, my 76 Jaguar is ‘sensitive’ but easy to fix.

~ honest.


#6

Wow, nice post.


#7

That sounds like a fair trade.


#8

Rubbish.

The parts are zero since these are variable jet carbs rebuild is limited to a gasket set, jet and needle plus a diapraghm if it’s Strombergs.

Tuning is easier than any fixed jet carb I know of, provided of course you know how, the factory tuning manual is available from Burlen. Problem with the SU / Stromberg carb is they are so reliable they tend to get totally ignored until they start to run very bad.

Find me a fixed jet 70s car of similar mileage with its original carb in good original working order and I’ll recant.


#9

Scudder, the high water mark of British Engineering was the Rolls Royce Merlin V12 aircraft engine which literally saved the world. But it’s been all downhill from there…What are you going to do when the North Sea oil runs out?


#10

I tend to agree…the Merlin was so good they installed it in the lacklustre P51 Mustang and made it the Cadillac of the skies as it’s known.

Strangely enough, the Jaguar 6 pot design is based on the Merlin (not many people know that), the original Jaguar 1934 engine design was 4 cylinder but was extended to 6 cylinders for small aircraft use, the prop would fit at the transmission end…hence the reason for No 1 cylinder being at the rear of the engine.

Honestly though, I believe the apex of british engineering was the bow and arrow - think Agincourt, after that it was all down hill to British Leyland and partner in crime, Lucas.


#11

If there’s water in the tank, a bit of ethanol can be mixed in. If the gas already has ethanol (E10 or E15), then the it probably is not water. If the gas uses MTBE, then ethanol might be a reasonable solution.