blzn90

I had a friend call me last night he has a 04 Grand Am he told me that his second to last fill he forgot to put his gas cap back on and got about 100 more miles out of that tank. So he left it off on his next fill up and got about 125 more miles per tank than usual which came out to be about 500 miles on a tank unreal I know on his way to go fill up the check engine light finally came on after two weeks with the gas cap completly off. Any one have any ideas why with the gas cap off he got better gas mileage. I know all OBII systems usally kick on the check engine light with it off but it didn't for about two weeks and around 900 miles driven. Any ideas or help would be great thanks feel free to email me.
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hillbilleous

dealing with a lot of OBDII cars in the dealership myself, there is no reason he should be getting better gas mileage. I believe it is coincidence