I had one of those "funny" experiences today when out in my longchamp. I have been battling a battery and regulator issue, and thought I would venture to the gas station to get some gas. Here in the states, you can not fill both tanks at the same time, so I tend to do one and then the other when I need more gas. I filled the left tank, and yes I am sure it was the left. Drove around and suddenly the right tank had more gas in it. The left was down about a quarter and the right was up a quarter.
I thought I was going nuts. So, I thought I would pull out the parts book and see if there was any way for the gas to run from one to the other. I could not really see a way. Someone told me once that there were check valves connected to the hoses, but not really in the drawings. So, I thought I will try a test. I switched to the left pump. I did not start the car, and I could here the pump pumping away. 5 min later, still going. When I change to the right, it pumps until there is pressure, then it stops. Ah, there must be a check valve! SO test number two, I removed the vent at the tank, placed a plastic bag over the vent, and Low and behold, pressure. SO, I must have a stuck valve in the pump or somewhere along the line.

So, has anyone ever heard of this? I was thinking about installing another check valve at the T where the two pumps meet, but not sure there is something on the market.

We

Wellis,
I had always assumed there was a return line or some sort of crossover between the fuel pumps that routed surplus fuel back to one side. I have often seen a full tank drop and the empty one come up a bit. On occasion with both full one would become overfull and the other drop rapidly. Don't know if that helps your analysis. I do know that the fuel filter (only one shared between the 2 pumps and tanks) was badly blocked so I'm not sure if it has improved since I recently replaced it. It was not easy to access being in between the 2 fuel pumps.
For those unfamiliar the Longchamp has 2 independent fuel tanks, each with a dedicated electric pump, and a switch on the dash from L to R which also changes over the gauge.
Peter

Thanks for the input. I can tell you there is no cross over between the two. I do know with mine, and with my aunt's SIIII the fuel line is in the engine compartment. I looked at the SII that I have, and there is no fuel filter in the engine bay, so it must be like yours where it is in the back.

I have removed the fuel pumps and all the junctions from another spare car, so i will see if I can get some more information that away. My brother suggested that I clamp the hose to the other tank and is if the pump gets up to pressure.

We

I can not add much, except to say that on mine the 2 tanks seem to operate totally separate. I empty one at a time, there's no flow from one to another