Petrol and Heat

[AndreasR]

I was on a trip to the northern parts of the country. It was exremely hot. About 44 degrees.

After traveling for a few hours I pulled into my campsite and started setting up. After a few minutes I heard a gurgling, bubbling sound coming from the car. At first I thought it was maybe the water in the radiator cooking but on inspection I found that it was the petrol in the tank. I had filled up about 80km earlier.

I have a second tank that is gravity fed to the main tank with a return pipe back to near the top of the filler neck. When I removed the petrol cap I could see the petrol pouring out of the return pipe back into the tank. This continued for about 3 minutes and then everything settled down. I can only assume that the lower tank got so hot that the heated petrol expanded to such an extent that it was pushing up the return pipe back into the extended tank.

Is this normal or should I be concerned?

[ThysleRoux]

Petrol will expand when heated, but it should not be excessive. The set-up sounds a bit weird, though. it the second tank gravity feeds into the main tank, the fuel level in the second tank should drop together with that in the main tank or, if it is higher than the main tank, it should empty first, unless there is a valve controlled by the driver between them. The "return pipe" you refer too is probably a vent/expansion pipe, fitted for exactly this occurrence.

Is your second tank situated close to the exhaust or gearbox? While driving, the wind flow under the car will deflect heat radiated by these components, but when you stop heat generate will remain in the vicinity for longer. Maybe a heat shield between any heat generating part of the car and the second tank will solve the problem.

[AndreasR]

I think it was the heat causing expansion of fuel in the original tank thus pushing the fuel up the breather into the second tank which is higher. Will have to look at diffusing some heat away from the tank.

[ThysleRoux]

Your second tank probably does not have enough expansion room, if the combination was still full enough for the petrol to push out of the breather after 80 km. All fuel tanks should have an expansion section that allows for petrol to expand. If you trickle fill the tank, you fill this area with fuel (usually 5 to 10 extra litres) and it pushes out of the breather pipe when it expands