Fuel Tank Pressure

[MrBrooks]

Hello Fellow Nomads,  We currently have a 2005 Coachmen on an E450 chassis. There is quite a bit of pressure in the fuel tank when I loosen the cap. Seems to be more when temps are higher or when I stop to fill the tank. I went to top off for weighing the other day and removed cap too fast and a small amount (maybe a cup full) came gushing out. Tank was about 7/8 full. Anyhow, I was wondering if anyone else had this experience with the E450. Our last motorhome,  also an E450 chassis had a similar issue. 

[Chalkie]

I have not had an E450 but have had a number of vehicles that let off pressure when removing the filler cap. Generally it seems to happen on hot days or after changes in altitude. The filler caps on vehicles with emissions controls seal pretty tight. In fact I had one truck that would show the check engine light if the cap was not tight. Filling stations and other places do NOT recommend "topping off" the tank instead quit filling as soon as the automatic fill triggers off. I have also learned that just like opening the radiator one should open the filler cap slowly to allow the pressure to equalize.

Personally I would not worry to much about it especially since I have never seen a totally accurate fuel gauge on vehicle I have owned over the last 50 or so years (i.e., your tank may have been completely full) and with a full tank an air bubble may have caused the blow back.

[Sehc]

The fuel cap prevents pressure from escaping, allowing air into the tank when needed. The vapor canister is where the pressure "escapes". When the engine is off, the canister is also closed. The vapors vent into the engine when running.  If your situation seems extreme, you could get the vapor canister checked.