P0446, with no symptoms

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Does the purge vent normally stay open? Is this code strictly a diagnostic failure? If yes to both, I won't worry about this. I cleared the fault code to see if it reappears.
Searching other posts on this code, they found unrelated engine issues.
2006 Envoy 5.3

P0446 - GM
Type Powertrain - Auxiliary Emission Controls - ISO/SAE Controlled
Description
EVAP Purge Vent Valve Circuit Conditions

Cause
Engine speed over 40 rpm, system voltage from 7.5-15v, then the PCM detected a short to ground, an open circuit or a short to power in the Purge control circuit during the test period
 
Last edited:

AtlWrk

Member
Dec 6, 2011
674
That information on the P0446 looks incorrect/incomplete (happens a lot with generic/Google DTC references).

The evap system has two solenoid valves: a "purge" valve near the engine and a "vent" valve near the gas tank (there's no such things as a "purge vent" valve). P0446 is not an electrical issue--it indicates a physical blockage in the vent portion of the evap system.

Yes it is normally open and yes it is strictly for diagnosing the evap system. That being said, it should be an easy and probably free fix.

The vent is exactly what it sounds like: it allows air into and out of the gas tank through the charcoal canister. Normally the vent valve is open and the purge valve is closed. However, to make sure there are no leaks the PCM will test by closing the vent and opening the purge valve--this applies engine vacuum to the gas tank and entire evap system, which it can monitor with the fuel tank pressure (FTP) sensor. If the vacuum remains at a set amount then it knows there are no leaks. It then opens the vent valve to release the vacuum and measures how long it takes the pressure to return to normal. This is where your PCM is detecting a problem. It's taking too long for the pressure to return to normal indicating that the vent is at least partially blocked. Good news is this is almost certainly just a clogged filter on the vent valve itself. It's just a plastic/mesh-sponge like filter that can get full of dirt and gunk from being up under the body exposed to the elements and road junk.

TL;DR: Take off the vent valve, open up the filter, clean it out and replace. Worse case scenario the valve itself is stuck closed and you'll have to replace it. Hardest part is figuring out how the vent line comes off (YouTube is your friend).
 

Mike534x

Member
Apr 9, 2012
894
If you can, go under the truck and inspect the vent valve near the gas tank. I had a P0449 and a few other codes a few weeks back. Replacing the one int he engine bay didn't solve it, turns out my vent valve near the gas tank had several cracks that formed around where the connector for the vacuum line (?) runs to it and was enough for it to leak and set off a CEL and other EVAP codes.
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Thanks!
Take a look this evening, weather permitting.
YouTube vids don't mention a filter, is it inside the valve?
 

AtlWrk

Member
Dec 6, 2011
674
I assume most people don't bother trying to fix a $20 part, they just replace the whole thing.

The vent valve looks like a box attached to a cylinder. The cylinder is the solenoid valve and the box part contains the filter. It should have a cap with tabs you can pry open. I'm not guaranteeing it will work and you may have to replace it, but considering trying to clean it wont cost you anything but time I'd say it's worth a shot. Or if you don't want to risk having to get under the truck a second time just pick a new valve now. Up to you...
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Replaced the Vent Valve, near the tank. Mostly because it was so easy to RnR. No joy though. The 0446 code reappears every other day. Today it appeared before reaching 5mph, yet read it was a self test above 40mph. I clear the fault code each time it appears.
That engine dash light really bugs me.

Think I need to somehow attach the air compressor at the vent valve connector, ready with soapy spray bottle. In hopes to find a crack in the charcoal canister. All tubes I can see are clean, no obvious cracks.

Is there an easy way to test the tank pressure gauge without lowering tank? It sits on top of the fuel pump.
 
Last edited:

AtlWrk

Member
Dec 6, 2011
674
P0446 indicates a blockage or restriction, not a leak or crack. The vent valve was by far the most likely cause but since that obviously didn't fix it you're looking at the hose that connects the vent valve to the charcoal canister, the canister itself or the fuel tank pressure (FTP) sensor.

It sounds like you're failing the passive test: at a cold startup the PCM checks the tank pressure. Since the tank vent is normally open, temperature changes shouldn't cause any pressure or vacuum to buildup as the truck sits. If the PCM sees anything other than ambient pressure in the tank it can only assume the vent is blocked and sets a code. This could be because there genuinely is a blockage or the pressure sensor is providing bad data.

To figure out which you can try guaranteeing there's no pressure in the tank before starting it by opening the gas cap. If you do this for each cold startup (tedious, I know) and the CEL doesn't come back as quickly (remember it still runs other diagnostics to check the system) then I would look at the charcoal canister having a blockage. If the behavior doesn't change then I would look at the FTP sensor.

Have you had any trouble with pumps clicking off early when filling the tank or hissing when opening the cap? These are other sure signs of a blocked vent.
 
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CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
No pressure at gas cap, fills fine, even at a high volume pump I sometimes use.
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Idling the motor in driveway to setup and test some of the new TorquePro features added today.
Logged a Pending P0446, did not turn on dash light.
Logged error: evap monitor $3d TID:$c6 max value 15, current reading 15.1
Tank pressure remained at or slightly below zero at varied rpms.
Gas cap has no +/- pressure release before or after starting.
Gas tank at 1/2
 

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CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Tested my old Vent Solenoid. Tested OK and no closer to resolution.
It was only $25, but no need to spend money on a quick fix without testing the old part.
Easy to test: just blow in to the vent tube side, air should freely through device; apply 12v, should hear a click which shuts the valve and air will no longer pass through.
 

AtlWrk

Member
Dec 6, 2011
674
You can get readings from Torque for fuel tank pressure? I tried and wasn't getting a response for that one so just assumed it wasn't functional yet.

How close to zero are you talking? Torque has the PID set up to read PSI but the thresholds for one of the P0446 tests is -2.5 inH2O (.090 psi).
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
GM pid set was a free add-on after adding free TorqueScan to Torque Pro. Not certain the Tank pressure reading is working, the couple times I looked either said 0.0 or -0.01psi
 

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CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Purge Valve Solenoid test good. The tube from this valve to the engine was clean and air passes through freely.
Remove purge solenoid, blow through either side, no air should pass in either direction. Apply 12 volts, bottom pin is ground, air should now flow freely after an audible click.
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Still not fixed but passed emissions. Filled tank, reset fault codes, kept tank topped off for ~150miles until the O2 test showed "ready", Emissions "not ready". NC passes if no more than 1 test "not ready".

Did get a smoke test, no leaks. Tank Pressure Sensor is the only other failure point. DIY tank drop seems impractical.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Evap is the worst system to fix. At east you got it passed.
 

CajunWon

Original poster
Member
Nov 25, 2012
137
Cary, NC
Would really like to cut in from top for fuel pump access-door. Did this with a Jeep, but found cutting measurements prior -made quick access for 2 pump changes.
 

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