Pressure build up in gas tank!

[rickjordan]

I have noticed that when I fill up the SL, there is a strong pressure release when I open the gas cap. Is this normal? I don't remember this happening on our '99 Passat (last gasoline car).

[WANT '71 280SEL]

If it is too much pressure either the in-tank screen is partially clogged or your gas cap vent isn't working correctly. In which case you need to replace the cap. I would experiment with a different cap if it really bothers you. The main effect would be poor performance if either were the case.

Thanks
David

[rickjordan]

The gas cap is an original metal one. I didn't realize these had any venting system to them. Could I try one of the "gas" caps from one of my diesels to see if that makes a difference?

[WANT '71 280SEL]

You could certainly try it. I can't say for sure that's your problem but worth a shot if it's quick and free. All gas tanks/caps have to have some sort of vent to them. To be honest however, if it's not affecting performance, I would leave it alone. Their is suppose to be pressure but I have no idea how much you have over the internet.

Thanks
David

[stevebfl]

Normal tank pressure is determined by the roll over valve. This valve is in the evaporative tank vent system. It is usually near the left, rear subframe mount. It blows at 2psi I think. Vapors blown past are accumulated in the charcoal cannister and under load air is pulled across the charcoal into the intake.