My 1998 gmc envoy code #P0453 has been a problem as well for me forgot to mention earlier on last question. any help, thanks

Are you the guy who had P0302 code.P0453 is the fuel tank pressure sensor is reading to much pressure to high that would explane why it would pass emissions Iwould check a part called the canaster vent soleniodif its stuck or cloged you would have a high pressure level in the tank it sends fuel vaper to the charcoal canaster from the tank where it is stored until your at highway speed then the canaster purge sloeniod opens and the vaper is burned in the engine. Ihave a 96 blazer and had a simaler problem but i was getting P0304 # 4 cylinder misfire although i had to replace the fuel pump to but that was part of the problem.You need to remove the spare tire and its on the top of the frame cross member above the tire mount check the filter thats in it and you can check to see if it works by jumping 12 volts across it it should click you will feel it move if its good. Ican't say for sure but i can't see the vehicle and this makes a lot of sence to check first and likely will clear up the problem it cost about 30 dollars for a new one