XJ6 & XJ12 Series I, II & III 1968-1992

79 XJ6 gas is changing tanks

 
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Old 02-23-2012, 03:47 PM
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Default 79 XJ6 gas is changing tanks

Hi All, its been a while since i posted. Hope everyone is doing well.

I filled up my left tank today and saw it deplete quickly into the right gas tank. Then the car started showing signs of fuel starvation, lurching while driving like its going to die. Never did die and idles fine at the lights. What's my issue?
Thx.
 
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Old 02-23-2012, 07:27 PM
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Assuming you have a late USA Ser II with fuel injection or a very early Ser III.

My guess is that gas is being drawn from the left tank but being return to the right tank. Most likely a faulty return valve.

This might help
Fuel Tank Switchover Checklist, Series III XJ6

Cheers
DD
 
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Old 02-23-2012, 10:15 PM
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Thanks Doug, but why the intermittent loss of thrust while driving?
 
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Old 02-24-2012, 03:39 AM
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Then the car started showing signs of fuel starvation, lurching while driving like its going to die
Because the system has dumped the fuel in the other tank, so you're running on empty.

Of course you may have a blocked tank pickup filter as well. Many happy hours or even days can be spent on the fuelling system on these cars.

Did I say happy ?
 
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Old 02-24-2012, 07:25 AM
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Yeah, running out of gas and/or clogged fuel filter, perhaps. Emptying the fuel filter into a clean container can be very revealing here.

Or......

An entirely unrelated problem that came along coincidentally with the fuel changeover problem. Weird things can happen :-)

Cheers
DD
 
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Old 02-24-2012, 10:01 AM
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So if the fuel is running from the left tank to the right tank, the left side return valve is faulty, not the right side correct? What if I strictly run on the right gas tank, should the driving be normal?
 
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Old 02-24-2012, 10:30 AM
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In theory, it should be normal. However, it seems like these fuel tank changeover valves don't make a perfect seal, so over time, the tank levels will equalize (very slowly), so essentially it is possible, I suppose, to have your left tank overflowing with gas eventually, even when you are only using the right tank.

I solved this problem by replacing my changeover valve with a bunch of brass fittings. Now the car runs great and I never have to worry about the dual tank system! I just need to remember not to park on heavily crowned road...
 
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Old 02-24-2012, 04:08 PM
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Aplogies if you know all this already.

There are three valves and one switch controlling the fuel flow.

1. Changeover valve - found near the fuel pump, and swaps feed to pump from the left or right hand tank. This can sometimes stick on changeover when you least want it to. Quick fix is to give the valve a light tap with a spanner or small hammer to make it move. As this valve is buried behind a cover in the spare wheel compartment, doing this tapping is a real nuisance when the trunk is full !! As happened to me around 1992 coming back off holiday.

2. RH and LH return valves - found at the back of each rear wheel arch behind a cover, and are handed left or right. Each one has a default setting based on what side they are fitted. On one, power opens the valve, and spring closes it, and on the other, its the other way around, thus diverting returning fuel to one or other tank.

3. Changeover switch - on the central dash panel. These can fail and if they do the valves will be set incorrectly and cause chaos in the fuel circuit. Petrol can end up being sent to the wrong tank, and overfill it. I once had fuel flowing at quite a rate onto the road via the overflow pipe. Pretty scary !

Poor electrical connections can have the same effect as well.
 
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Old 02-24-2012, 04:30 PM
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Not to hijack the thread, but is there any real reason that a functioning dual tank system is better than just having a brass T fitting replace the fuel tank changeover valve? I guess drawing from each tank individually would probably keep the fuel fresher, but other than that, I can't really think of why the system exists in the first place.
 
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Old 02-25-2012, 04:09 AM
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I can't really think of why the system exists in the first place.
Nor me !!

If you look at the X350 like mine, it has a "saddle" tank with effectively two compartments, but has a system for keeping the fuel level in both sides using a "suction jet-pump". Of course, Jaguar started off with two tanks when they used carburettors, and there was no need for fuel return, it was put in when the XJ went over to fuel injection. Things move on, and now my X350 also has no fuel return, the pump pressure relief valve and overflow is back at the pump end.

As you will see here, somebody has converted to make it effectively one tank and hence avoid the problems.
 
 
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