Mercury Repair Questions? Ask a Mechanic for Answers ASAP
Ok, one possible cause is the gas cap is over tightened which prevent vacuum relief, so your neighbor's suggestion is good place to start.
If the code returns, it will need to be smoke tested to test for restrictions/obstructions in the evap system at the vapor canister, vacuum tubes, purge valve and purge solenoid, cv solenoid filter, and fuel filler vapor tube that runs parallel to the fuel filler tube.
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Smoke testing requires a special machine that hooks up to the evap system and uses a special dyed smoke. Only a shop will have this, not a parts store. It is 1 hour labor for a shop to do this