Vacuum from fuel cap

[midnightrumbler]

Ok, I have been having some issues with my recently acquired 1995 Silverado 3500 Dually 4x4.

I seem to have a stumbling hesitation issue that I am wokring on.

I recently changed the fuel tank due to a leak in the old one and after letting the truck idle for maybe 5 minutes, I noticed when I went to put more fuel in the tank when I opened the fuel cap, a ton of vacuum was heard.

Is this normal? I have never heard this on any of my vehicles.

[THEFERMANATOR]

If it is in fact vaccum and not pressure, then most likely you have the wrong fuel cap on it. The gas and diesel caps ARE different and do not interchange with each other. The diesel cap is designed to allow air to freely enter and only hold a slight pressure in the tank whereas the gas cap is designed to only allow high amounts of vaccum or high amounts of pressure to escape. Gas engines have the charcoal canister system for the tank to vent so the fuel cap is designed to be almost completely sealed. A diesel doesn't use a charcoal vent system so the cap must be able to freely flow air in, and allow excessevly high pressures out. Make sure your fuel cap is marked diesel, otherwise go buy a new one and be certain it is for a diesel.

[jerry598]

I also notice either a release of pressure or a bit of vacuum everytime I unscrew the fuel cap on my truck - but, there are no driveability issues.

[JohnC]

The correct cap has a 2 way valve in it. It will hold about 1" of vacuum and about 1 psi of pressure. Any more than that is a problem. 1 psi in a half empty tank can make a pretty impressive woosh when you open the cap!

[anp4god]

I have always wondered about the whoosh on my Suburban as well but haven't noticed any issues. I don't want to spend the money for a new locking cap. I haven't noticed if it says diesel on it. I'll have to check.