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Old 04-17-2017, 09:37 AM   #1
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Need Evap Vent Control Debug Help!

Hi All,

My 99 Boxster 2.5L is unhappy and throws DTC P0446 ("Evaporative Emission System Vent Control").


I researched the issue for a while and decide to take a look at the Evap system by the tank filler neck.
Here is what I found out so far:
The valve attached to the carbon canister seems to be working, I removed the valve and applied 12V and Ground to the two leads. The valve actuates and closes the plunger/diaphragm. Are there any other things I should check?


I also removed the other valve which was attached to the filler neck (see below).
When I blow in the red part (green arrow), the air flow is blocked. Once I push in the black plastic piece (yellow arrow) air can flow through the valve from green to green arrow. I assume that is correct behavior.


While I was on it I also removed the carbon canister. I can blow air through the hose (from green to green arrow) so it doesn't seem like there is blockage in there either...


How can I debug the actual carbon canister?

All of the other hoses are attached to the vehicle and lead to and from the tank I presume...?

What else can I do to debug the fault? I'm kind of running out of ideas....
Any help is appreciated at this point!

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Old 04-17-2017, 10:24 AM   #2
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Do you have other related symptoms - like difficulty filling with gasoline ?
Are all the hoses that remain on the car correctly connected and free of splits/kinks etc.Very tedious to check !
Did you try deleting the code and driving for a while- maybe an transient fault?
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Old 04-17-2017, 11:01 AM   #3
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Try to blow air though the other hose that connects to the bottom of the canister. There is a valve on the other end of that tube ( in the engine compartment) that gets stuck.
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Old 04-17-2017, 11:02 AM   #4
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Quote:
Originally Posted by PBoxstar View Post
Hi All,

My 99 Boxster 2.5L is unhappy and throws DTC P0446 ("Evaporative Emission System Vent Control").


I researched the issue for a while and decide to take a look at the Evap system by the tank filler neck.
Here is what I found out so far:
The valve attached to the carbon canister seems to be working, I removed the valve and applied 12V and Ground to the two leads. The valve actuates and closes the plunger/diaphragm. Are there any other things I should check?


I also removed the other valve which was attached to the filler neck (see below).
When I blow in the red part (green arrow), the air flow is blocked. Once I push in the black plastic piece (yellow arrow) air can flow through the valve from green to green arrow. I assume that is correct behavior.


While I was on it I also removed the carbon canister. I can blow air through the hose (from green to green arrow) so it doesn't seem like there is blockage in there either...


How can I debug the actual carbon canister?

All of the other hoses are attached to the vehicle and lead to and from the tank I presume...?

What else can I do to debug the fault? I'm kind of running out of ideas....
Any help is appreciated at this point!
You are at the wrong end of the system, the purge valve is under the intake manifold:

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Old 04-17-2017, 11:49 AM   #5
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Quote:
Do you have other related symptoms - like difficulty filling with gasoline ?
Are all the hoses that remain on the car correctly connected and free of splits/kinks etc.Very tedious to check !
Did you try deleting the code and driving for a while- maybe an transient fault?
The error code keeps coming back even after I clear the fault.
Maybe to help debugging, I'm also getting:
P1124 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold (Confirmed)
P1126 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold (pending)
P0130 Oxygen Sensor bank 1 (pending)
P0150 Oxygen Sensor bank 2 (pending)

Quote:
Try to blow air though the other hose that connects to the bottom of the canister. There is a valve on the other end of that tube ( in the engine compartment) that gets stuck.
What is the expected behavior when pushing air through this hose? Should I expect resistance or not (with ignition off)? I guess the evap valve is at the other end as you pointed out.

Quote:
You are at the wrong end of the system, the purge valve is under the intake manifold:
Thanks, is there an easy way to get to this valve and check functionality?
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Old 04-18-2017, 08:29 AM   #6
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[QUOTE=PBoxstar;534146]The error code keeps coming back even after I clear the fault.
Maybe to help debugging, I'm also getting:
P1124 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold (Confirmed)
P1126 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold (pending)
P0130 Oxygen Sensor bank 1 (pending)
P0150 Oxygen Sensor bank 2 (pending) QUOTE

I am happy to help but I don't understand why we have to coax you into giving us the whole story from the beginning. Doing this diagnosis thoughtfully from keyboard is time consuming even with all the info.But with only bits and pieces coming in stages - it leads to misleading suggestions and a needlessly lengthy Thread that potentially helpful people abandon.
So before we go further, what else is there that may be relevant ? For example -what work has been done on the car immediately before the problem occurred ? Under the Inlet Manifold for example.

Last edited by Gelbster; 04-18-2017 at 08:32 AM.
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Old 04-18-2017, 08:33 AM   #7
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Mine just did the same thing. Po 446 -$5. Talked with my indie. Replaced the gas cap was first and the cheapest. Cleared the code new cap no light so far.
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Old 04-18-2017, 08:37 AM   #8
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Look for a crack in the gasket at the base of the cap - that is the usual cause. May be able to just get a new gasket ? There is a hack with a plastic bag/Nitrile glove to temporarily seal it(Youtube probably) .This allows confirmation that the gasket is at least one aspect of the CEL problems.

Last edited by Gelbster; 04-18-2017 at 08:50 AM.
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Old 04-18-2017, 09:37 AM   #9
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The gas cap was the first thing I replaced. The error still persists...
In terms of recent work done on the vehicle, the only thing that is even remotely related to the engine is the drive serpentine belt which was replaced about two months ago. The error showed up even before this was done.

How can I troubleshoot the purge valve by the intake manifold as mentioned by JFP? I'm not even sure where the valve is located...

Thanks for your help!
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Old 04-18-2017, 10:18 AM   #10
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It is directly beneath the intake, you need small hands to get to it.
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Old 04-18-2017, 10:41 AM   #11
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Originally Posted by JFP in PA View Post
It is directly beneath the intake, you need small hands to get to it.
Thanks. What is a good way to troubleshoot if this valve is working correctly? Same for the hose. Should I try to blow compressed air trough the hose and should I expect blocked or free air flow?
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Old 04-28-2017, 01:21 PM   #12
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Ok, so I disassembled pretty much the entire valve, hose and carbon canister assembly in the passenger wheel well. Blew compressed air (from my tire inflator :-) )through all of the hoses and put it back together. The car hasn't complained in two days so I'm pretty hopeful that is was a blocked hose or loose connection somewhere. Before that the CEL would come on shortly after clearing it. I will keep you posted.
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Old 04-28-2017, 01:29 PM   #13
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Congrats ! Your persistence and help from other Forum Members paid off with a reward - no CEL.
If it was a blockage or loose connection - have you any idea which one specifically ?
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Old 04-28-2017, 01:39 PM   #14
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Congrats ! Your persistence and help from other Forum Members paid off with a reward - no CEL.
If it was a blockage or loose connection - have you any idea which one specifically ?
I don't know which one of the hoses was the culprit but since I didn't see any debris or liquid coming out of the hoses directly located in the wheel well I assume it must have been one of the hose that lead to the tank.

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