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P0446 Code and no vacuum in fuel tank

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On my 98 Boxster, during the first few years I owned it, when I removed the gas cap to fuel, you would hear a lot of air rushing into the filler. Isn't that what should happen? In the last few years there has been no airflow after removing the cap, and I have had the P0446 code which I believe indicated "a large leak in the EVAP system"? I have searched this forum and read all posts that had P0446 code, and they all refer to it as "EVAP canister shut off valve (function) - below lower limit", although my Actron scanner refers to it as EVAP control system vent ckt malfunction. Are these all the same thing or are there subcategories to the P0446 trouble code? The dealer told me if the code came back (which it has and the CEL was on for a few days before it went off) that the carbon canister vent line woud need to be replaced (I think this is the one with the EVAP canister purge valve located under the driver's side intake runner?). I am not sure if he was 100% sure though. I saw one post where someone recommended blowing through the air line from the canister with compressed air to clear any possible blockage. Can anyone tell me how to recognize the correct line to do this to? Can the EVAP canister purge valve be tested by appling 12 volts and verifying it opens? Can I do a vacuum test by applying vacuum just past the purge valve and seeing if the vacuum holds to check for a leak in the line or EVAP system or is this not a proper test for this situation as maybe the vacuum would be vented to atmosphere even on a good system? Does the EVAP system use engine vacuum to scavenge gas vapors from the fuel tank and do the vapors get pulled through the EVAP cannister which filters it before being pulled into the intake manifold? Please correct me if any of the above info is incorrect. I have never had any problem filling the fuel tank, and there has never been any problems starting the car or really any other issues or codes and I had the recalled gas cap replaced with no affect on this problem. Thanks for any help as I would like to make sure I know what the problem is and not waste time and money replacing good parts and not correcting the problem.

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P0446 EVAP Canister Shutoff Valve (Function) - Below Lower Limit

Possible fault causes:

Fault area

- Purge air line blocked.

- Flow resistance of EVAP canister too high.


1 - EVAP canister purge valve

2 - EVAP canister

3 - Purge air

4 - Tank

5 - Pressure sensor

6 - Shutoff valve

7 - Operating purge valve

8 - To intake manifold

9 - Vacuum control valve

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