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gto0188

P0446 Plan of Attack

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2001 Sierra 1500 Ext Cab 4.8 4wd 135,000 miles. Throwing a P0446 Evap Code. Cleared code tried just retightening gas cap. No luck light back on.

 

From what I gather there are several causes of this code to include:

 

1. Gas Cap 

2. EVAP Canister Solenoid Vent Valve
3. EVAP Canister Purge Valve

4. Tank Pressure Switch

 

Also any faulty wiring/hoses to these components. 
 

I do not have access to an expensive scanner that will test functionality of these components. I hate blindly throwing parts at something but I'm a bit blind here without the use of a scanner.

 

Is there a pretty common part that causes P0446?

 

My plan of attack is based on easiest-hardest to access parts.

 

1. New Gas Cap

2. New Canister Purge Valve

3. New Canister Vent Valve

4. New pressure switch (looks like bed will need to be removed for this although I did find some Youtube videos where they were able to replace w/o removal).

 

What have you found to be the common faulty part with P0446?

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Must be pretty common because mine occasionally throws the same code. It tends to go away though for weeks at a time before returning.  

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This is a very common problem and GM released a TSB on it.  TSB 04-06-04-055A Dated 03/20/2006.  Here is a link to the TSB:

http://ww2.justanswer.com/uploads/travist80/2009-04-08_014020_Vent+Valve.pdf

 

In short, the air filter for the EVAP vent valve is upgraded and the air filter for it is moved to a place near the top of the transmission.  I replaced mine in 2015 using the Standard Motor Parts kit, part number CP454, and have not had any trouble since.  It was about haf the cost of the GM/ACDelco kit.

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P0446 is burned into my soul.

 

I chased this problem for a very long time and replaced one part at a time, same as you with your strategy. Eventually replaced every component. I have heard stories of dealerships that would completely replace the EVAP system if still under warranty and then total the vehicle if P0446 came back after that. I ended up replacing the entire EVAP system short of the gas tank. I eventually bought a smoke tool and did the pressurized smoke test, only to find no leaks (I should have done this first, there is an inexpensive one on Amazon that uses 12v battery with mineral oil).

You can test both valves by giving them 12v and seeing if you can push air through them, no need to replace them if the valves cycle with 12v.

I can't tell you how many times I took my truck bed off working on this, it was ridiculous. So heavy and hard to do alone. I even wired into the sensors so I could read them on my Fluke in the cab, because the EVAP test takes so long to run and complete. Anyway I went through great lengths to troubleshoot this on my own.

One night I was up late and on my second glass of bourbon and found a guy on Youtube who has a video series on EVAP systems and how they are designed and work. It was much like a college course. Probably 80 hours of videos. I think he was the engineer who designed the evap emissions system on modern vehicles if I recall correctly.

Anyway I benefited from his lectures on YouTube and got a much deeper understanding about how the evap system software works in the truck's computer. In the most basic language, the computer looks for a specific set of parameters from the sensors and plugs it into a formula and expects a certain range of results. One of the variables is a change of fuel level before and after the test. This was one angle I hadn't thought of so I took the truck bed off for the 100+th time and inspected all of the fuel pump/fuel gauge wiring. I noticed one of the fuel pump ground wires getting hot after letting the truck run for 20 or 30 minutes with the bed off and decided I'd replace it. 

New wire, flux, solder, dielectric grease, heat shrink, about ten cents total fixed the problem.

I was an afternoon away from programming an arduino to fake the sensor parameters necessary to trick the computer into thinking there was nothing wrong with the EVAP system.

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FriendlyMan, thanks for the information.  I had problems getting the EVAP system to pass the tests as well.  Drove with less than half a tank of gas; drove early in the morning, late at night, etc.  Finally began looking at the intake air temperature (IAT) and engine coolant temperature (ECT) sensors, and noticed that there was what I considered to be too wide of a difference between the temperatures reported by the two sensors.  I would up changing the ECT sensor and bingo!  Cold engine temps were much closer and finally, all the emission systems readiness tests passed.  Drove to the emissions testing station and finally got a passing test!

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On 10/3/2020 at 8:32 PM, FriendlyMan said:

P0446 is burned into my soul.

I can't tell you how many times I took my truck bed off working on this, it was ridiculous. So heavy and hard to do alone.

Thanks very much for the info FriendlyMan. Looks like I may be headed down a similar path...

 

I was excited since after I put the new fuel cap on the SES light stayed off for awhile. Came back on today.

 

I crawled under the truck expecting to see a round vent solenoid but nope, mine is the rectangular style above the fuel tank between the cab and bed. Only way to access it is bed removal ugh. Mine is this style shown below:

 

3-CE1-E43-F-64-A8-4395-996-C-154-FEC6127

 

Been planning on doing much needed corrosion control anyway and now would be a good time to do it. I will replace the vent solenoid AND the fuel tank pressure switch since that too is on top of the tank.

 

Before all that I will go ahead and put a new purge solenoid in. Easy enough right on top of the engine.

 

Man crap like this makes me miss my 77' K10! 

Edited by gto0188

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It ended up being the fuel tank pressure sensor. I was barely able to replace it by moving the driveshaft out of the way to get my hands up there and work by feel.

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