"Does anyone know if the fuel tank on 440 has positive pressure. There is a hiss when i take the fuel cap off, and i think the air is coming out not in. But i havent heard it before until the hot weather."
There is a breather/ventilation pipe for the tank. Between the tank and the breather, there is a non spill valve in case the car turns over. It stopps fuel leaking out of the breather. It's a black plastic pipe like housing with a ball inside, which drops onto a sealing face if the car is inverted in a smash. In the early 1990 cars, the breather pipe stopped under the rear offside wheel arch. As the cars were being driven, the negative pressure in the tank, as the petrol was being used, sucked in grit from the road surface, the grit being thrown up from the offside rear tyre. In some extreme circumstances, the anti inversion spill valve would get totally clogged, resulting in the fuel tank being compressed by the external air pressure, and the float reading to the fuel gauge would show the tank being half full, but it was actually empty, the underside of said tank being distorted upwards, and the internal design of the tank made to protect the pump filter and fuel level float. (Personal experience) Volvo came out with a modification to extend the breather pipe as far back as the rear bumper, but this still sucks in dirt thrown up from the road surface, especially in long hot dry spells.
Might be wise to check the tank breather and A/IS valve.
Regards
Malky
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