[motto]

Fuel cap venting has been a topic of some discussion over the years and is often misunderstood. Basically, a fuel tank cap must have venting unless the venting is provided elsewhere. Having said that, the fact of the matter is that on a hot day no venting is needed as the fuel vapour will pressurise the tank anyway. After all, a LPG cylinder does not require a vent to expel it's contents, vapour pressure does that. The difference is that LPG vaporises at a much lower temperature than petrol.

I once put a jerry can in the sun and it went up to 3 psi which is fairly close to fuel pump pressure. The standard WW11 4" cap used on GMC, Jeep, etc. had a relief pressure of around 3 psi and this was not without reason. On hot days, this pressure in the tank helps prevent vapour lock which, as we know, can immobilise a vehicle.

Cold conditions are a completely different ball game. We've all seen jerry cans with the sides sucked in that had been filled in warm conditions and then subjected to cold. Your fuel tank will do the same if not vented and this is made worse if fuel is being removed from the tank. For this reason, those 4" caps that have the 3 psi outwards relief also have (if I remember the figure correctly) 1-1/2 psi inwards relief.

In summing up, for cold climes, a simple vent hole in a cap is sufficient. For hot climes, a pressurised tank through use of a relief valve is a definite advantage.

David