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Intermittent P0455 code


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Okay, I've searched, have seen the diagnosis for the P0455 code, and am running out of airspeed and ideas. The car is a 2005 996TTS with 33k+ miles. I've replaced the gas cap, had a smoke test performed with no obviously leaks (none that we could see...), and tested the charcoal canister EVAP valve function with the Durametric. I can hear and watch the carbon canister valve actuate, but not the tank control valve. Is there a way to test that valve? I pulled the fender liner and have two replacement valves that didn't seem to change the function.

The CE typically comes back not long after the tank is refilled, and after resetting the code it won't return until refilling the tank again. Strangely, it won't return for every tank. As far as I know the gas cap is on tight.

I've taken some data logs on the Durametric and would like to know what the readings should be for the tank pressure difference and the carbon canister load.

I'm confounded. I hate to have to drop the tank to see what's on top, but is that where I'm at? Ughhh.


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P0455 Leakage in fuel tank system

Possible fault causes

- Tank cap not closed correctly, leaking or missing

- Purge air line leaking

- Tank vent leaking

- DMTL (Tank Leakage Diagnostics Module) leaking

- Leakage in fuel tank system

Sounds like a purge air line or tank vent leak.

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I was thinking about your fault. If the gas cap can be part of the problem because the tank vacuum for vapor recirculation did not work....The fuel cap being new, how about the main cap on the reservoir itself. May be it is loose. Very easy to found out , four nuts... Are we looking at the impossibility to create negative pressure inside the reservoir? I think so....



Edited by jpflip
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It's a good thought, but it didn't leak any smoke when I put smoke into the gas tank. I figure if there were a leak it would have manifested itself with the smoke.

Regardless, I'll take another look. It doesn't take long to remove the fittings and the ring.



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  • 3 weeks later...

Okay, so I dug back into the fender again today to see if I could kill this one off once and for all. Nope! The P0455 code was present, but the CEL hadn't triggered yet.

The car creates negative pressure when it's running. (0.4 hPa atmospheric to -1.8 to -2.4 while running. It's cyclical.)

The carbon canister loads up after running for a few seconds then the load reduces.

The main cap on the top of the tank is tight.

There is no residual dye anywhere to be seen from the smoke used when pressure testing the tank the first time. I used a black light as the dye shows up under the black light.

The pressure normalizes to atmospheric according to the Durametric shortly after the car is shut off. Telling me there really is a leak somewhere.

Any way to know if the Tank Leakage Diagnostic Module is leaking?

Which hoses should hold a vacuum if I use a vacuum tester?

Loren, thanks for your help so far. Can you offer any further insights?



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