P0446 and the Vapor Cannister

I wanted to share a fix that actually surprised me but am very happy with the result. This is a 98 Chevy Blazer vin "W". The code P0446 has been plaguing me for quite some time now. So I decided to really focus on it one day. Though the code indicated a problem with the vent valve, I was determined not to be a parts changer. I got a new vent valve, but before I installed it I wanted to run through all the checks very carefully. First I smoked the system using my scan tool to close the valve, seal the system, etc. I got no smoke escaping and the flow meter on the smoke machine indicated no flow...that's right 0 flow. That had me completely baffled. So I decided to do both vacuum and pressure tests on the system (since I read that some leaks will only show with vacuum). Again, once the system was pressurized/vacuumed, I got no leaks after 5 minutes...none. I then followed the procedures to test the EVAP canister it passed based off the instructions in the TSM diagnostic chart.

I checked voltages at the vent valve circuit, good voltages. On Blazers you can't really reach the tank pressure sensor without dropping the tank, so using a combination of the smoke machine, a Maxisys scan tool and a Vantage Pro with a pressure transducer, I rigged up a system to determine if the FTP sensor and the Vpro with pressure transducer would correlate. They did. Finally, I had read somewhere that a possible cause for this code is a problem with the vapor canister. But I had tested that and it had passed the tests? It couldn't be the canister. I even put hoses on the canister and lightly blew on them to see if I could detect any restrictions, I could not (note: this was after I had tested it properly using a Mighty Vac). But everything was working, and I was really stumped.

Taking a hard look at every test result I determined the only possibility it could be was the vapor canister. I have to tell you that though I am not a pro, I've worked on a lot of cars I've owned and friend's cars over 30+ years. I had never ever changed a vapor canister. But I decided I had to call that as the problem (full disclosure, not with a 100% confidence). Ordered the AC Delco part, installed it, and it's been three weeks now and the code has not returned (I was getting it every day or so after I would clear the code). But even more pleasant, I even noticed an improvement in the engine performance. Not to mean necessarily an increase in horsepower, but the engine smoothed out noticeably. Wouldn't be noticed by a person that doesn't drive it regularly, but noticeably to my wife and I. Also, this is a 5 speed manual trans, and we have some fairly steep mountains/hills that we have to drive over (speed limit 55). We've always had to downshift to 4th to maintain speed. First day after the fix, I'm driving up one of the hills and all of the sudden I realized, I was in 5th and moving along nicely. Added some right foot and the truck actually picked up speed! (yea baby)

Story continues, I decided to cut the top of the old vapor canister off to see what I could see. As I opened it, it had a really foul smell...not fuel smell...foul smell (which I had noticed a little but when I was testing, but not enough to think anything of it). Yes there was a hint of fuel smell, but that foul smell was overwhelming everything else. It even stuck with me the next day. As I'm typing this my brain is remembering the smell. I carefully removed the top and the sponge filter and stuck my finger in the charcoal and moved it around. The top initial inch or two the charcoal was loose. But little deeper and I could feel clump where the charcoal was stuck together. At that point I stopped put the canister back together and taped the top on. I want to make a video of actually pouring the charcoal out to see what it completely looks like on the inside. I'm expecting to find a large mass of charcoal stuck together. But I want to make the video so it could help other people see what could possibly throw the P0446 on Blazers.

To you pros, I know this is not really surprising because you've probably seen this many, many times. But I am completely tickled pink that I called the right part on a pretty elusive code. That is thanks a lot to Paul and you guys on this forum. Several years ago I would have probably ended up changing the Vent Valve, then the FTP sensor, then the Purge Valve (who knows maybe all the EVAP lines) and still not solved the problem. I would have never considered the Vapor Canister (since I could physically blow through it and not feel any restriction). By the way, I did blow through the new one also before I installed it. There was a slight difference between the new and the old, pretty slight difference, but it's there.

Apparently when the purge valve would open, the restriction was enough to prevent the correct amount of fuel vapor to reach the engine, OR whatever that foul smell fuel is was reducing performance as it was sucked into the engine. I don't know, but it's fixed. So, the moral of the story is with P0446, don't disregard the vapor canister, even if it seems to test correctly. But I only landed on this after, I had done the proper EVAP tests on the complete system and everything tested out in spec and operational. To conclude, there is a brand new, unopened Vent Valve Solenoid going back to the auto parts store to get a refund.

Hope this long winded story can help someone out if they are having the same issue I was.

Damn that code!

Sent from my Moto E (4) Plus using Tapatalk

I may have to look into this on my 2004 Silverado, I've been getting that code and the purge and vent solenoids both looks and test good (both on and off the vehicle) and I replaced the tank pressure sensor a few years ago (probably unnecessarily).

Code still has not returned!

Nice find! The code description can be pretty misleading in this application, IMO, especially when you're unfamiliar with it.

It my area, we see this code frequently on many different makes/models, but especially GM. It always comes down to the customer living on a dirt road, sucking dust into the canister through the vent valve. Canister gets restricted, and the PCM figures it out during the bleed up phase of the EVAP test.

The canisters never feel restricted when you blow on them, just like yours! But they always feel heavier when compared to the new one. :lol: Some TSBs also have you relocate the vent hose elsewhere, to reduce dust dust getting in. If your canister had a foul smell, it'd make me wonder about water intrusion. Maybe have a look at where the vent ends up? Don't be afraid to move it up higher on the vehicle if needed.

Good work! I'll have to keep that in mind. I have had a few that stump me. Honestly if its a convoluted system, I pass on it. Some systems are a simple design and still throw curves. Usually I can figure them out easily enough though but never had one like yours. Any canister I have replaced has always been cracked.

Thanks for sharing! I appreciate everyone who contributes to this forum. Its what makes it so much better than all other parts changer forums out there.

Thank you for sharing.

Good for you.

Shortly after buying my Vic, I was no longer able to lock a fuel nozzle to fill the tank, had to drizzle the gas in (10 min to fill LOL)
Also ended up with the P0446 code.

I also live on dirt roads, so was not a big surprise.
Luckily I was able to easily get to the Canister purge solenoid and clean it out, and put a filter on the fresh air hose they stick up in the frame rail.
I don't like to spend money unless I have to for part, but Panther's are dime a dozen, so I can get most things from pic a part for fairly cleap if needed.
The hose barb portion can be popped off, and then you have access to clean out the valve. Has been working perfectly since!

Hi all - just wanted to add to the P0446 and the Vapor Canister as possible cause to help others. I had a 2003 Chevy Silverado 4.8l with the P0446. After verifying the operation of the Vent Solenoid, Purge Solenoid, and the Fuel Pressure reading during a Purge and Seal and other bi-directional testing I too was left with the flow restriction mystery. I tested and graphed the vacuum pressure results versus % purge opening with my original canister and found that with the Purge Valve set to 100% and the Vent valve venting in a idle condition the vacuum was over 9 inches of water! I also looked at the Vacuum with the vent solenoid and plumbing completely removed from the canister and still saw vacuum over 8 inches of water which rules out that area. I also checked the gas tank vapor line and purge lines to the canister and was convinced that I had no restrictions there. I want to point out that at idle my fuel pressure sensor was reading approximately 1.5V which is the typical pressure voltage; no odd voltage or offset which cleared my fuel pressure sensor as a suspect. I too blew through the canister and was not convinced it was restricted though some mix of dark residue smelling of gas did appear visible at the purge vent port at the canister. Armed with this information I went ahead and ordered a new canister convinced it was the only suspect left. Upon receiving the new canister I did blow through it and it was a bit easier to blow through. My final test was to graph the new canister and compare and the results (drum roll please) showed a vacuum of 3.79 inches of water at 100% purge and venting! This is not the 1 inch of water that I have read here as a rule of thumb but a marked improvement. Currently the P0446 has not returned and I call this a fix.