Hello, I recently purchased a Crimson Red 2004 Pontiac Bonneville GXP with 56,000 original miles from a man who had stored on the side of his house for five years. We bought the car as a first car for my youngest son. So far, after putting brand new tires, one new fuel injector, fixing a broken air line to the rear shocks and changing the coil packs to the new style the car has been almost perfect. We do have one nagging problem which is related to the check gas cap message. The gas cap is fine based on inspection so I think it is something else. Any suggestions as to what it is?

Lastly, this is one nice car! It handles very similarly to the BMW 540, is loaded with things like satellite radio and on star and looks great. My first car was a 1976 Pontiac Grand Prix that I paid $200 for it and broke it to ball joints within a week.

This should get him through college quite nicely.

The most efficient way to find the cause of the gas cap message is to have a smoke test performed on the EVAP system. If you are in an emissions testing state, it shouldn't be hard to find someone to do this. Likely causes are rust on the metal sealing surface of the filler neck (sand the metal ring), rust between the metal ring and the plastic filler neck (replace filler neck), vent valve, or gas cap. Thing is, it could be a small leak anywhere in the fuel tank venting system.

Nailed it above.

All the EVAP system is doing is monitoring a "decay rate" vs. an expected "decay rate" and if it's faster than that expected, the DTC is triggered and the MIL commanded on. The cap may be fine, but the neck may be bad. Purge and EVAP solenoids are the next culprits.

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Retired Bonneville Owner and former GM Tech:
2004 Pontiac Bonneville GXP: Black/Ebony *SOLD*

Summer Toys: Combined 827 RWHP / 877lb/ft RWTQ
2004 Pontiac GTO: Impulse Blue Metallic/Black/M6: lots 'o mods, 415 RWHP / 405lb/ft RWTQ!
2006 Cadillac STS-V: Light Platinum Metallic/Light Gray/A6 - Spectre CAI, Magnaflow exhaust, Speed Inc. tune, 412 RWHP / 472lb/ft RWTQ

Daily Drivers:
2019 Chrysler Pacifica Limited: Mommy's new RGC
2015 Chrysler Town & Country Limited Platinum: Kids new RGC
2007 Chrysler Town & Country Limited: Sold to neighbor
2009 Pontiac G8 GT: L76, Sport Red Metallic
2003 Chevrolet Suburban 2500: Doeskin Tan - 8.1L Big Block... new plow truck
1999 Chevrolet Suburban: Sunset Gold Metallic - Daddy's winter beater and plow truck

Welcome to the forums!

The EVAP code can be any number of things. Make sure the lip of the filler neck is smooth and free of corrosion.

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-Car Guy Carl
"Penelope" - 2003 SLE - 247k Miles
"Sydney" - 2000 Honda S2000 - 98k miles