Caution: duel fuel tanks gas bomb

I posted recently about cleaning out my mk10 fuel tanks and did get the tanks back in. I R&Red the left fuel pump but was waiting for new mounts before installing it. The right pump was working fine. What I learned is that there is no check valve in the fuel lines on these cars - the system relies on an intact line and competent valves within the fuel pumps to stop back-feed from the opposite side. I should have checked that before running the car on one tank/fuel pump, but didn’t. The net effect was 2 gallons of gasoline in the boot, leaking onto the shop floor. I was lucky, as the rear of the car was at the open end of the shop, and I caught it right away (car quickly ran out of fuel on the lift). I’m presuming that the earlier cars with dual tanks have a similar setup. Did they add check valves for the XJs? I thought about adding check valves, but with the giant a/c unit in the forward end of the boot, it would be challenging. Check your fuel lines ladies and gents.

Thanks for the warning. Our Series 1 XJ6 has no check valve. It’s a good thought, 10 imperial gallons each side, all of it above the pumps. All my kit is newish but valves would be nice. Paul.

I have the same setup in my S-type with 2 fuel pumps, but SU fuel pumps are one way only,
Only a defect fuel pump, where the membrane is defect will cause a issue.

Not the membrane would cause an issue, but the reed valves? There are small aluminum check valves from china that one could put in the fuel line, with a short run of extra fuel hose. Or simply plug a line. Thanks for the warning, will be useful soon!

When I pulled the tanks and fuel pumps from my series 1 XJ6, I part filled the tanks with diesel fuel until I was absolutely sure that the whole fuel system all the way to the carbs was totally leak proof. I then drained one tank and filled it with petrol before firing up the engine. Once this worked well, I did the same for the other. I cannot understand why anyone would do otherwise.

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I have filled one (empty) with water and put it in the sun. The fuel sender leaked. Once installed I put it in the sun and with a bit of fuel inside I didn’t smell any leaks. Same procedure with the hoses (fuel injected). I can’t smell diesel. One tank at a time is better. My car is in a shared garage and fuel will have to wait until on the road.
With one tank full and one disconnected this could have become a mess even though I always plug disconnected lines.

The S-type and also the 420 do not have any valve just a simple T-piece with pipes/Hoses going directly to the Pumps

John, it’s often plain in retrospect what one should have done - I’m just trying to help someone avoid my mistake. I do like the idea of testing a tank with diesel.

If you don’t rely on the internal pumps and wire some purolator/facet pumps for each tank you will find they have one way valves built in to avoid back flow or cross flow to the other tank.


Perhaps some late cars had internal pumps, but my 65-66 pumps are external SUs, so replacing them with a modern pump would not change anything. My experience with some of those style pumps is that they are also quite loud. I haven’t mentioned it before, but what a pita it is getting those pumps out from the nether forward reaches of the boot cavity. Anyways, I’m sticking with my SUs, which are now functioning fine. So, if I can get the cooling system and brakes back together, I can actually drive the thing.