Hi,
I am new to volvo's and love my 99 s40 2.ot although recently it has developed a frustrating problem of when i only have about 6 gallons of petrol left in the car it starts to misfire and lose power, although if i pull up and rev the engine hard and set off again it seems fine for about another 2 miles then same problem until i fill the tank back up and it clears any idea what this could be, i have always used premium unleaded until the first time it happened last friday when i had no choice but to use supermarket petrol I am gutted and hope the car is not ruined as i absolutely love it....
all help would be greatly appreciated

 

When the tank is down to about 6 gallons, if you remove the fill cap does it suck air in? If so, it be creating a vacuum and starving the engine. Could be a tank vent valve or fuel pump.
Stan