What now?
 

1999 boxster 130k miles Po455 (gross leak) CEL

This is what I've done so far:
Replaced purge valve (engine under manifold)
Replaced Vacuum control valve (on side of EVAP tank)
Replaced carbon canister valve (on canister)
Replaced Fuel Cap (OEM)
Replaced vacuum valve (on filler neck)
Smoke checked the entire system. Took off underneath panels to check for leaks (none)

I'm at a loss. No signs of leaks anywhere. followed all lines as best I could. I have read everything I can find about this but I have done everything recommended.

I know there is a pressure differential sensor under the battery at the fuel tank. Could this be not working properly?

Any help would be appreciated. I'm ready to pull what little hair I have left out!

In my reading, it seems this could indeed be the culprit.
However: are we sure the fault code is real? By that I mean, are we sure you have a leak? Or is it possible you have a sensor issue? Do you hear hissing when you remove the gas cap at the fill station? Do you smell gasoline when the car is running?

I assume you've cleared the code since replacing all of those items, and it's returning immediately?

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Thanks Maytag,

Don't know for sure the code is real. No hissing and no gas smell. No trouble filling with gas.

I used a smoke machine and can't find any leaks. I reset the code and it comes back as soon as the system tries to pressurize. CEL then comes on after code is set three times I think.

I checked that purge valve and canister valve work properly.

If there's no hissing when you release the gas cap, that suggests the fault is real. If your system isn't vapor-tight, then there's a leak somewhere. As to how to find that..... you've already done what I would do: smoke it.
Did you smoke it from a single source? ie: one side of a check valve but not the other? I'm just thinking out loud here. If we know the tank isn't pressurizing, maybe build a cap with a fitting that you can hook the smoke machine to. I'm just thinking trying it from a couple of different places.

Otherwise, I'm no help. I'll be watching with interest and hope someone smarter than I chimes in.

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Thanks I'll keep at it. Can't stand to be defeated.

Smoked again from different locations. Still can't find a leak. Replaced pressure differential sensor today. Let's see what happens.

Still get CEL P0455. Can anyone help?

Did you look under the battery tray for leaks?

Yes checked that and the lines under the shields on the bottom of the car.

Did you ever figure out the problem and fix the P0455 code? I'm sure curious if you got it fixed.

This may be obvious, but did you replace the gas cap?


Edit: OOPS, I just read that you did. Sorry!

Bob,
I have the same recurring code PO455 on our newly acquired Boxster. I have replaced the fuel cap (OEM) and filler neck vacuum valve. Now thinking about the purge valve under the manifold. I hate to keep throwing parts at it. Did you ever find the problem with your car?

any help or suggestions would be GREATLY appreciated.
Thanks!

We have the same reoccuring code on our newly acquired 2003 Boxster S. I have changed the fuel cap (OEM) and the vacuum valve on the filler neck. Code comes back.

No pressure when I open the fuel cap.

Hate to keep throwing parts at it.

When under the car, I noticed some oil residue under the fuel tank & on top of the undercarriage cover. If it was a gas leak, I would think that it would evaporate but I could be wrong. Could it be a leak where the filler tube goes into the tank?

Any help or suggestions would be greatly appreciated.

Bob, did you finally find your problem? Please share if you did.

PO455 code
 

Did you find the problem? I am having the same issue. New fuel cap & vacuum valve on the filler neck did not fix the issue.

Any help or advice is greatly appreciated

I had that code not too long ago but unlike him I had fuel vapors coming in the cabin. Turned out to be the fuel sender sitting under the battery, it had a crack where one of the fuel lines was connected, just became brittle with time. Swapped that out, no more fuel leak.

I'm in the same boat (sort of). No code or fuel in the cabin, but I get the howling noise. I've replaced the gas cap, the valve on the neck of the filler, the EVAP return valve (under the intake manifold), the charcoal canister, and the fuel tank vapor control valve (under the fender on the 03 Boxster). I just replaced the vapor control valve, so I haven't tested to see if I get the gas tank howl. We'll soon see.

Tony