Fuel Tank Pressure Off - SOLVED

[Logan5]

Got the P0446 code on a 2006 Tahoe so started the Evap Mystery Game. But since this truck is new to me recently and I haven't done any evap system work on it yet just decided to replace the 3 main parts that go wrong. Purge valve, Vent Valve, and the canister.

Bench tested the solenoids and put them in.

Cleared the code and decided to try this Service Bay Test on the tech2 clone I have. This keeps failing and says "Vent Performance".

I blew some air through from the purge line and air flows freely. I used the tech2 to run a evap seal and it builds vacuum as well.

Started digging deeper and checking the Fuel Tank Pressure, I am showing .9 V / 5 mmHg with the vehicle off. Should read next to 0 mmHg with the vehicle off. So I am leaning towards my FTP sensor is bad.

On the tech2 you can cycle up purge solenoid duty cycle and as I increase it the vacuum does build and the sensor does change. So it looks like the sensor is working properly, but it's just calibrated wrong?

I doubt the sensor can be recalibrated?

Looked for where that sensor is and its part of the fuel pump assy. Now it all sense. I just replaced the fuel pump, and (can you guess?)...I bought a cheap one off Amazon and not the 5x more expensive OEM or equivalent.

Kicking myself now. Guess I'll drop the tank (again) and swap out the sensor. That or eat it and get a whole new pump assy.

Figured I'd throw this out here first before I go drop the tank again.

P.S.Found this link informative (https://aviondemand.com/insider/pulling-codes-story-of-code-p0446/)

 

[swathdiver]

Keep us posted.

 

[Fless]

Mike at Wells talks about the sensor voltages at atmosphere as well as pressure/vacuum if you would want to compare them with yours. He has quite a few good videos on diagnosing evap systems.

 

[Logan5]

Great video thanks.

He gets the FTP sensor off without dropping the tank. Maybe Santa will bring me an auto lift for christmas.

He verifies that the sensor should read 1.5 V ~0 mmHg at atmosphere. I almost wish my sensor was stuck at a voltage so I knew it was def bad, rather than it seemingly working but reading .5 V off.

Guess I'll try replacing the sensor without dropping the tank. Not looking forward to slithering under the truck and worming up between the tank and the drive shaft.

Somehow I foresee dropping the tank again, lol.

 

[afpj]

If you have to drop the tank a second time, put in a "real" fuel pump (ie OEM) and get that Amazon crap out of there...if you drop the tank and don't replace FP, by the third time though I bet you'd be a pro at getting that tank down.

 

[Logan5]

This is the 2nd tahoe I have, and also the 'extra' car that rarely gets driven. Why I risked the cheapo fuel pump. On primary tahoe I have also dropped the tank twice already. Once for fuel pump (a 'real' one), and once for doing brakelines. So that's already 3 tahoe tank drops in recent memory.

 

[Fless]

Great video thanks.

He gets the FTP sensor off without dropping the tank. Maybe Santa will bring me an auto lift for christmas.

He verifies that the sensor should read 1.5 V ~0 mmHg at atmosphere. I almost wish my sensor was stuck at a voltage so I knew it was def bad, rather than it seemingly working but reading .5 V off.

Guess I'll try replacing the sensor without dropping the tank. Not looking forward to slithering under the truck and worming up between the tank and the drive shaft.

Somehow I foresee dropping the tank again, lol.

The driveshaft comes out relatively easily (which is how I got my fat head up there to see), and you can lower the tank slightly for access. No need to drop it all the way for the pressure sensor.

 

[Logan5]

The driveshaft comes out relatively easily (which is how I got my fat head up there to see), and you can lower the tank slightly for access. No need to drop it all the way for the pressure sensor.

Too right. I figure I'll try first just snaking my arm up there, when that fails I'll pull the driveshaft, and if that fails I'll lower the tank partially.

This sounds like a 2-3 beer job.

 

[Logan5]

Doing autowork in sub-freezing temps sucks balls. There was no way a human arm was getting to that FTP sensor without dropping the tank some on this Tahoe, so I did the partial drop and got the sensor out. Dumb ass thing looked new, reads .5V off. Tested the new sensor, reads 1.5V, put that in. Hooked up the tech2, shows 1.5 V ~0mmHg. All looks good. Put it all back together, ran the Service Bay Test, says Passed. Going to call this done and move on to the next problem. (grinding sound, bearings I think).

Gotta say having a little adjustable DC power supply on the work bench has come in real handy. Those sensors want 5V reference and being able to dial in 5V and hook the clips on to the sensor and test it with a standard multimeter was pretty useful.

 

[Fless]

Way to go -- that's a fairly unusual cause but you powered through and figured it out. I added "Solved" to the thread title.