My 1994 E320 Cabrio started leaking fuel from the gas cap after I filled it and drove a few miles, it spewed all over the rear quarter panel. I've read every post on every forum about gas cap leaks and hear everything from "the tank should be under vacuum" "the tank is under pressure" "replace the cap" "replace the seal" "heat expansion" etc. This is the first time it ever leaked (usually I only put in about 3/4 and don't top it off) and when I opened the cap the pressure spit gas all over my arm so I KNOW it is under pressure. The outside temp was only about 60 degrees so I don't think the heat expansion was that great. I went to the local MB dealer and bought a replacement cap (this one had a black plastic handle and was not a locking cap like my original). When I got home I noticed a slight leak dripping from the cap/seal when I opened the fuel door. I opened the cap slowly and there was a huge release of pressure again. Then I ordered another new cap online from a MB parts house. This one came and it is has a stainless steel handle. I put it on and drove the car for about 15 minutes and checked for leaks, there was a small wet spot of gas just below the cap/seal again. The filler neck surface looks OK, not damaged, I can't see anything that the gas station might have done during the last fill. The two new caps do feel snug when I close them. Also, I recently had the car inspected and the original cap passed the leak test by the mechanic! The car only has 45,000 miles. From what everybody tells me is that the original cap seal just got old and the new cap should have fixed it. Is there anything that can cause the fuel tank to be overpressurized more than normal??? Any ideas?

Thanks everybody, I appreciate any technical advice before I leave it with the local MB dealer. Dan B.