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EVAP system troubleshooting

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I got a P0446 code on my 99 Boxster. I've gone through the diagnosis steps for the code. Blew compressed air through the purge air line and it seemed fine. The next step in the OBD diagnosis manual is to replace the EVAP canister; however, while I was in the wheel well, I figured I'd check the other related components. I found that the "Vacuum Control Valve" (Part number 996-201-621-02; however, sold only as part of vent line 996-201-255-08 - Arrow pointing to part in image) allows air to flow in both directions despite appearing to be a check valve (arrow on side.)


​My question for those in the know:




Is this part in fact a check valve? Should it allow air to pass in both directions?


If not, I'm thinking of replacing that first before springing for a new canister.


Thanks all,



99 Boxster

105k miles

Edited by JayUSA123
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Do you know how it does that with only two connections? It does seem odd that the valve allows air flow in both directions. Is there a testing procedure for it? I can't find any testing instructions for it in the service manuals.


Depends on which side of the valve vacuum is being applied.  Normally, when these fail, they are either stuck open, stuck closed, or won't hold vacuum.

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Okay, it definitely seems to be stuck open. I've also noticed that that when I remove and reinstall the gas cap while the car is hot and running, I hear a howling noise for a few seconds that seems to come from the "bleeder valve" (996-201-143-01). I believe this is one of the symptoms of it failing; however, I'm not sure whether I should add a new one to my parts order now, or replace the vacuum control valve first and go from there. Do you think the howling could be related or should I just go for both?


Thanks a bunch!


Edited by JayUSA123
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I'm not a big fan of swapping parts out without knowing why; I prefer to do one at a time, particularly when parts like that bleeder valve retail for nearly $100. But I have the benefit of a shop with spare parts about for most things.  That said, it would depend upon how much the extra parts cost, and how much you value your time (not having to address the same problem twice).

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  • 1 year later...
8 hours ago, Hurdigurdiman said:

JayUSA123.. I have the same problem and no way can I find the answer. I want to know whyI can blow through the vent line both way passed the blue valve. I can. So is my blue valve bad or good? Did you ever solve your problem? Thanks in advance. Mike

I'm just wondering that since it's vacuum actuated, have you tried applying vacuum in each direction?

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