First, check the condition of your gas cap. If it is of the locking (with turning key) type, air may be getting through the key hole into the gasoline tank.
Gas caps are relatively inexpensive. Might be worth your while to get a new one.
If the 0455 Gross Leak continues after PCM reset and gas cap replacement, follow the vacuum lines path from the gas tank to the engine intake plenum.
Check the condition of the rubber hose which connects the gasoline filler neck to the tank itself. These sometimes develop cracks allowing air leaks.
The P0455 indicates there is a large air leak somewhere. You may hear a hissing sound while the engine is running; try to pinpoint the area and spray water from a low-low pressure garden hose near where you think the hissing is. Do this with a cold engine as not to get yourself burned from the steam created off the exhaust. Spray water sparingly, too. Don't give the engine a full-pressure bath. You're just trying to get water to flow temporarily into the air leak. The water creates a temporary seal and the hiss goes away for a second or two. At least then you can grope around with your fingers to feel for rubber hoses falling apart in the area you heard the water put the hiss to rest for a second or two.
Remember that the EVAP system consists of the gasoline tank as well as that maze of hoses inside the engine compartment. Check for good connections at the gasoline tank (unfortunately, this will require lowering the tank from the chassis). Even a pinhole puncturing the gas tank itself will set EVAP codes.
Finally, EVAP codes are as a result of air leaks on the high-vacuum side of the vacuum lines. Your hood may have a vacuum diagram; if not, try to obtain one. Concentrate on areas where the purge valve is connected, as well as the charcoal canister, and the hoses running to/from the gas tank.
Faulty hoses where the PCV valve is connected, or the hose running from the valve cover to the intake snorkle upstream from the throttle body do NOT NOT have correlation to EVAP codes.
Thursday, December 18th, 2008 AT 10:38 PM