P0442 Buick -

Question

I get "gas cap loose" message when gas gauge gets down to approximately 7/8 full. Then, until it is reset, I get the message each time I start the car. If I don't reset the fault code for a length of time (30-50 miles of stop start with frequent starting and stopping the engine), I then get a "service engine soon" message. I have had my Buick dealer check it out 3 times after changing the gas cap. They said the fill pipe had a bit of rust/corrosion and they sanded it smooth. The next time they did a pressure check on the system and said there was no loss of pressure. Then they said they wanted to do a "smoke test" and I opted not to have it done at that time. They also said they might have to drop the tank and check for a leak around the fuel pump access plate. Incidentally , I checked in the floor of the trunk and found an access plate in the floor, when removed gave me access to the fuel, pump plate. Are they just trying to spend my money, or is this problem exceedingly difficult to analyze. I have had this problem for 2 years and frankly, it is a nuisance. Also, I can avoid this message by keeping the gas tank full.