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Old 06-21-2020, 06:37 PM   #1
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Could y'all walk me through the evap system?

Hey y'all!
In my quest to get this car through emissions inspection, I've now solved the o2 sensor issue I was having.

The only persisting issue now is an evap issue. Code po446, which i know can mean several things, but accompanying symptoms include strong gas odor and difficulty starting the first time after filling the tank.

Last weekend I popped-off the inner fender liner (front right) and unhooked the lines from all the various crap in there. I blew compressed air through the various hoses and didn't encounter any blockages that I could detect.

So the problem I have right now, is that I'm vague on what each item is, and what it is supposed to do, and when.

I searched this forum and Googled, and most discussions center around the purge valve under the intake plenum, left side. So with a test light, I've confirmed voltage at the valve..... but i don't know WHEN it's supposed to have voltage, or whether that voltage opens or closes the valve, so i can't really test the valve.

I could use a basic flowchart-type narrative of the entire evap system. Can y'all help me? Or steer me somewhere where it's already laid-out for me?

Thank you!


Last edited by maytag; 06-21-2020 at 06:48 PM.
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Old 06-22-2020, 05:40 AM   #2
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Old 06-22-2020, 08:51 AM   #3
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Thank you, JFP. you're always good for the details, man!
I saw this diagram previously, and it's somewhat useful (though I'll confess that the stuff under the fender liner doesn't look a LOT like that, haha).

But what I could really use to accompany this diagram, is a flow-chart narrative:

"when xyz happens, valve A opens and releases gasses to CDE"
and "under these conditions, the purge valve should get voltage, which opens/closes the valve....."

etc etc.
Y'know? I can't tell which parts aren't working the way they should, because I cant ell which part is supposed to do what, or when it's supposed to do it.

I'm afraid it's all too very-much like plumbing. And I really SUCK at plumbing.
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Old 06-22-2020, 09:44 AM   #4
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OK, P0446 indicates that the EVAP canister shutoff valve is operating below flow limits. Normally this is caused by the purge air line (#3 in diagram) being blocked. Normally, this can be fixed by disconnecting the line at both ends and blowing compressed air thru it to clear it. If this line is clear, the only other possibility is that the flow thru the EVAP canister carbon bed is blocked, which requires replacing the canister.
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Old 06-22-2020, 04:17 PM   #5
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OK, P0446 indicates that the EVAP canister shutoff valve is operating below flow limits. Normally this is caused by the purge air line (#3 in diagram) being blocked. Normally, this can be fixed by disconnecting the line at both ends and blowing compressed air thru it to clear it. If this line is clear, the only other possibility is that the flow thru the EVAP canister carbon bed is blocked, which requires replacing the canister.
Thank you, JFP!
Before I hit "buy" on a replacement canister: no chance I've got a problem at the purge valve? It's definitely the canister? (I've blown the lines already)

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Old 06-22-2020, 08:58 PM   #6
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I would check the valve first, it can also get plugged or become inoperative.
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Old 06-23-2020, 04:46 AM   #7
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Meanwhile, I'm replacing the fuel line vent valve on a car that spits out gas when you try to fill it. I hope you don't mind if I add this here for future reference to the evap system for others who end up here.

This tech article is on the 996 section of the PelicanParts DIYs, but it is identical to the system on my '01 986:

https://www.pelicanparts.com/techarticles/Porsche-996-997-Carrera/106-FUEL-Replacing_Your_Fuel_Line_Vent_Valve/106-FUEL-Replacing_Your_Fuel_Line_Vent_Valve.htm

Video included on that page:


Maytag - I'm going to be pulling the evap system out of a parts car (low flood damage). Let me know if you want any cheap 'probably good' parts from an '01.
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Old 06-23-2020, 09:56 AM   #8
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I would check the valve first, it can also get plugged or become inoperative.
That's what I'd like to do.... but I'm too fuzzy still. I assume the valve is holding vacuum from the intake, right? Or is it holding pressure from the tank / evap?
And should it be "open" or "Closed" (flowing either vacuum or pressure) when voltage is applied?
And under what circumstances is that voltage applied / not applied?


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Maytag - I'm going to be pulling the evap system out of a parts car (low flood damage). Let me know if you want any cheap 'probably good' parts from an '01.
That sounds very interesting to me. ABSOLUTELY. I'd say let's think about the canister & the purge valve and whatever else makes sense right there.
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Old 06-23-2020, 10:20 AM   #9
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I really is not important if the valve faces pressure or vacuum, only that it works when power is applied, and that when it is open, there is flow through it.

The EVAP system is supposed to absorb (actually adsorb, but that is another science lesson) hydrocarbon vapor accumulating in the fuel tank onto the activated charcoal in the cannister; under the correct operating conditions (warm engine, high intake system vacuum), the purge line opens, sweeping air thru the cannister media and pulling the hydrocarbons into the intake to be burned rather than released to the atmosphere as they were with the vents on older fuel systems. The the lines need to be clear, and the valves have to open on command from the DME to get rid of the vapors.
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Old 06-23-2020, 11:00 AM   #10
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I really is not important if the valve faces pressure or vacuum, only that it works when power is applied, and that when it is open, there is flow through it.

The EVAP system is supposed to absorb (actually adsorb, but that is another science lesson) hydrocarbon vapor accumulating in the fuel tank onto the activated charcoal in the cannister; under the correct operating conditions (warm engine, high intake system vacuum), the purge line opens, sweeping air thru the cannister media and pulling the hydrocarbons into the intake to be burned rather than released to the atmosphere as they were with the vents on older fuel systems. The the lines need to be clear, and the valves have to open on command from the DME to get rid of the vapors.

aaaahhhhhh this is good stuff, JFP! I understood, of course, the 100k foot view of what it is supposed to do, but not the mechanics of it.

So here's where I'm at then: Cold engine, I have voltage at the purge valve. didn't bother to pull it out yet to check it, because it's a PITA to get to, and I didn't have any way of knowing if voltage should be opening or closing it.

So SHOULD I have voltage when the engine is cold? And if the answer is yes, then I can assume the valve is normally-open, and the voltage holds it closed. Yes?

OR - do I have a problem in the control someplace, indicated by the presence of voltage when cold?
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Old 06-23-2020, 12:25 PM   #11
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When you say you have voltage at the purge valve, is that key on or off? And where is the valve you are testing, in the wheel well or under the intake manifold?
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Old 06-23-2020, 03:43 PM   #12
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When you say you have voltage at the purge valve, is that key on or off? And where is the valve you are testing, in the wheel well or under the intake manifold?
Details! that is with the key on, and it's the valve under the intake. Engine off and cold.

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Old 06-23-2020, 05:41 PM   #13
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Details! that is with the key on, and it's the valve under the intake. Engine off and cold.

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That valve is under intake level vacuum and connected to the intake near the throttle body (item#20):



It should show 12V with the key on and be open under electrical power.
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Old 06-24-2020, 08:18 AM   #14
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It should show 12V with the key on and be open under electrical power.
there's the nugget I was looking for!

But your question makes me wonder: is there ANOTHER purge valve somewhere that you thought I might've been referring to? Should I be checking a DIFFERENT valve?
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Old 06-24-2020, 08:24 AM   #15
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there's the nugget I was looking for!

But your question makes me wonder: is there ANOTHER purge valve somewhere that you thought I might've been referring to? Should I be checking a DIFFERENT valve?
The operating purge valve at the cannister (item #7 in the top drawing).
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Old 06-24-2020, 12:13 PM   #16
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The operating purge valve at the cannister (item #7 in the top drawing).
Thank You!
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Old 06-25-2020, 04:20 PM   #17
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Maytag, we can keep our price discussion private (I know that Ferrari guys have lots of money), but it's easiest for me to just put pics here.
You asked about 3 things, but how about the other one that sits forward in the fender? It has two electrical valves and one hose to the charcoal box and another that goes to behind the fuel filler lid.


The fuel line vent valve mount cracked while I was taking it off. I plastic welded it "Stronger than new".



Purge valve under the intake: closed with no power, open with +12v. (test equipment not included)

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Old 06-25-2020, 05:05 PM   #18
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Maytag, we can keep our price discussion private (I know that Ferrari guys have lots of money), but it's easiest for me to just put pics here.

You asked about 3 things, but how about the other one that sits forward in the fender? It has two electrical valves and one hose to the charcoal box and another that goes to behind the fuel filler lid.





The fuel line vent valve mount cracked while I was taking it off. I plastic welded it "Stronger than new".







Purge valve under the intake: closed with no power, open with +12v. (test equipment not included)



Perfect.
And you know we're all broke from the cars, right? If we have money, it's spent in more cars. ;-)

I sent you a reply to your pm.

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