P0455 Evap Leak

[Coop8202]

Hi, I have a 2008 GMC Sierra with the 5.3. I got a P0455 Evap code a while back. I can clear it but it comes back within a few days. When filling the truck up, when the pump shuts off gas spews from the canister area underneath the truck. I can usually always smell a raw fuel smell after driving from the bed area. Thanks! 

[sefiroxx]

Check hoses and canister. The system should be sealed. A break will on the "seal" will set the code. In your case, hose from tank to canister is compromised.

[tbarn]

System is normally open. Pcm closes the vent valve to test.

[sefiroxx]

Evap is closed when verticals is off. Vapors from fuel tank are passed to the canister.. If pressure builds, the system has an over pressure release valve at the canister.

When the vehicle is running, a valve opens at a certain rpm to allow the stored vapors to be introduced into the intake.

[riverbanks]

Vent valve is open when vehicle is off,purge valve is closed when off,a guy did a test somewhere on here a ways back,purge valve opens on decel,he hooked up a test light to watch,now he didn't test light the vent valve,so during evap leak test,purge opens,vent closes for small leak test,then opens for large leak test?

[Coop8202]

What’s the easiest way to test all of this without pulling the bed?

[riverbanks]

Take it to a shop

[sefiroxx]

Purge opens at certain rpm. Prior to rpm threshold, the system is sealed and there is some pressure which the computer checks with a pressure sensor. If no pressure, the code is set.

If leaking on fill, then there is a hole somewhere (100million + cars don't leak on fill)

[riverbanks]

What is that certain rpm?

[sefiroxx]

Lots of write ups out there. In general, on earlier cars (20 years +) the valve was opened or closed when engine was warm and in gear and moving.

Later engines, once the engine warms up, the cake is pulse width opened. Small amounts in idle, larger amounts as rpm increases.

The 440 test occurs at one of two points. On some earlier cars, with a pressure sensor and the system purge valve known to be off. In a pulse width system, the system will measure vacuum pressure, shut the valve off and measure pressure again. If the vavuum pressure doesn't change enough, it will set a code.

[Coop8202]

I always shut the truck off when filling it up, but before shutting it off, it’s at idle of course, so around 800-1100 rpm

[Coop8202]

Update: Top of fuel pump rotted out due to design of tank/pump and debris sit down in there.