Clementine's Garage
Clementine the Cat
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Yellow R4
Réparateur d'automobiles

Petrol Overflow

Torphichen, Scotland
A week ago I filled up the tank; not over filled, just to the pump’s first cutoff. I drive 28 miles and park in my driveway, pointing up the slope (about 1 in 5, is that 20%?). Some time later the car starts to dribble petrol out of the petrol cap. So I assume the petrol has warmed up and expanded. I syphon some petrol out to drop the level in the tank and notice that the seal on the cap is perished. So I order a new cap after searching without success for a replacement seal. While waiting for the new cap to arrive I fix the damaged paintwork under the tank filler.

New cap arrived yesterday. Nice tight fit. The car has now not moved all week and now, late afternoon, for no apparent reason, it has dribbled petrol again out of the new cap (it is a vented cap). So I’ve syphoned some more and cleaned the paintwork. Why is it doing this? Must I always park facing downhill after a fillup?


I found this happened quite a lot in hot weather, i.e cold morning, warm afternoon. I think the problem is not just the petrol expanding because it happens when the tank is quite a bit under 100% full. Its also trapped air expanding with no vent at the top of the tank on the left hand side. So when you park with the RH side downhill, the LH side has a triangular section of trapped air at the top of the tank, and it pushes petrol up the filler tube. Likewise if you park with the nose uphill and the tail downhill.
If you are full up, try to park with the nose down and the RH side up, so it will vent air before it pushes petrol out. This does seems to help, and I now have less occurrences of emergency response teams putting cones around my car when I drive it to work on hot days!

I have a new unused tank from der fransose and if I ever fit it I will weld in an extra vent neck on the LH side to hopefully reduce this.