Mercedes-Benz 420SEL Gas cap won't come off Inspection at your home or office.

Common reasons for this to happen:

There are three common reasons why your gas cap might get stuck on:

• Vacuum sealed: Under certain conditions, your gas tank can create a vacuum-like seal around the gas cap, which seals the cap tight. This vacuum situation occurs when the pressure inside the gas tank changes, which can happen due to extreme changes in weather or altitude, among other things. This high pressure situation creates a vacuum seal that makes it nearly impossible to unscrew your gas cap.

• Poorly threaded cap: Be it out of haste or simply not noticing, sometimes people screw on their gas caps incorrectly. If the thread does not match up, you can sometimes force the cap on, just like you can force a glass jar lid on even when you’re not screwing it on correctly. If this happens with your gas cap, it can be extremely difficult to get off.

• Debris in the threading: If dirt, leaves, or any other form of debris contaminates the gas cap and gas tank threading, the cap can easily get stuck. This debris can get trapped in the threading, creating extra pressure and friction, making the cap resist that much more when you try to unscrew it.