I had this happen to my Nissan Sani with aux tank AND the Pastor's Mazda B bakkie up in Savuti ..
The culprit was the exhaust running too close to the fuel tank .. on the nylon Nissan tanks this never was a problem, but the steel fuel tanks gets too hot with teh exhozt & gearbox heat inderneath the vehicle when drivin gin he3avy sand on a hot day and you never get above 30 KPH ..
I scavanged aluminium heat shelad off a BMW in the scrap yard, placed it between exhozt and fuel tank .. end of sheizz.
Valves & that have nada to do with this -- just, as Clem states, heatsheild between exhoz and the tank ..
[LOBO now has that problem .. I need some BMW alum plaatjies again
