i have just bought the dreaded p38 4.6hse on gas. now i filled it up when i got it and it took about 70ltr in a 90 litre tank which i allowed for only filling to 80%, this was when it was on the gas reserve red light. after filling i ran it to empty to see how far i got to a tank and when i went to refill it would only take 34 litres any ideas why as it still shows full on the gauge which has always been quite accurate