I have a '98 Camry, V6 with ~123k miles. Car runs well but has developed an annoying problem where the check engine light trips with code P0446 (EVAP) after re-fueling.
I usually clear the code, go through one tank, and re-fill making sure gas cap is on tightly. The check engine light seems to trips when I am at a stop light, at an incline (facing uphill) with the tank ~75% full.
Strangely, I recently did a long distance 600 mi round trip with no code tripping and I averaged ~ 33 MPG. However, after the trip, sitting at a stop light the damn light came on!
Is it a check valve, or bad gas cap? How hard is it to replace the check valve?
That 97+ generation of carbon canisters had design defects. The air- gas separator or some one-way vent valve doesn't operate properly and in some cases allowing the canister to be flooded or the charcoal disintegrates and sends fine powder all over the evap system. You can google the TSBs on these. It's seldom the gas cap.
However, you may more likely have sticky vacuum switching valves (VSV). One vents the vapor into the intake manifold and the other switches a pressure sensor between tank and canister. It could be possible that the first one isn't closing properly and the second isn't switch correctly. Haynes manual details the testing procedures for these. If they are faulty, try cleaning them by prying open the metal tabs that hold the port cap carefully. Don't crack the plastic or break the tabs or you'll have to buy a new valve.
Since the P0446 test involves operating the VSV next to the canister, that would be a suspect. However, the evap vent VSV (near intake plenum) has a higher duty cycle and is easier to get to so I'd check that first and clean if necessary. Autozone can let you borrow MityVac free (with deposit) so you can tee in and read vacuum. The vent vacuum should alternate between 0 and about 15inHG and not the 0 to3inHG kind. You should hear loud sucking sounds as it operates.
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