P0446 Evap Diagnosis

[2004XLT]

CEL came on a week after removing my throttle body to clean it, code P0446.

- No vacuum leaks
- Verified evap canister vent valve is functional
- No clogged or damaged hoses to/from evap canister
- Checked voltage at harness to vent valve. Begins at 9.5v and drops to 0v within 10 seconds. I don't know what that means. Should be 12v constant with key on.
- Have not proved the purge valve under the hood is working or if it gets 12v
- If I fill my tank till the pump stops it will spit out a quart or two of fuel. It's done this for at least 50,000 miles. Never had evap code before.
- I now have a very rough idle when I start the exp in the morning. No changes in rpm but it shakes for a minute. I think it's sucking vapor not fuel. Then once the air is purged it runs normal.

Interested in feedback regarding the drop in voltage at the harness and any possible fixes.

 

[Andy J]

Id check the purge valve. Remove it, and see if you can blow thru it when its disconnected. If you can then its likely stuck open internally. Ill have to look in one of my manuals, but if i recall, the voltage drop after 10 seconds is normal, as the engine purges fumes right after startup then stops.

 

[2004XLT]

Thanks for the info about the voltage drop. I just pulled the purge valve and I can blow through it, not freely but it's not holding pressure. Also applied 12v and nothing, no clicks or other behavior. It's definitely stuck.

I'll be interested to see if this also fixes my overflow problem.

 

[Bedrck47]

P0446 - EVAP Control System Canister Vent Solenoid Circuit Malfunction
Monitors the canister vent (CV) solenoid circuit for an electrical failure. The test fails when the signal moves outside the minimum or maximum allowable calibrated parameters for a specified canister vent duty cycle by PCM command.

• VPWR circuit open.
• CV solenoid circuit shorted to PWR GND or CHASSIS GND.
• Damaged CV solenoid.
• CV solenoid circuit open.
• CV solenoid circuit shorted to VPWR.
• Damaged PCM.

To verify normal functioning, monitor the EVAP canister vent solenoid signal PID EVAPCV and the signal voltage (PCM control side). With the valve open, EVAPCV indicates 0% duty cycle and a voltage approximately equal to battery voltage. When the valve is commanded fully closed, EVAPCV indicates 100% duty cycle, and a minimum voltage drop of 4 volts is normal. Output test mode may be used to switch output on/off to verify function.

 

[2004XLT]

I replaced the purge valve on the firewall which didn't make a difference. Hope its not my pcm.

 

[BRIANGEE]

I wouldn't think there's any reason to suspect the PCM at this point. It sounds like you have multiple issues going on that may or may not be related.

 

[2004XLT]

I belive the rough idle/vibration I have upon starting after sitting a few hours is the fuel vapor being introduced when it shouldn't be. According to what bedrck posted the purge valve has nothing to do with P0446 only the vent valve. So to me it looks like my valve is staying open. I don't see the issue since the solenoid is getting power and I've proved it works.

 

[BRIANGEE]

Do you have access to a scanner with mode 6 data? You can check for misfire counts on specific cylinders even when it's not enough to cause a code.

I have a similar issue to yours where I get a misfire for the first 10 seconds or so causing a rough idle and then it clears up until the following day when the engine is cold again. I'm still tracking mine down but I've narrowed it down to a misfire on cylinder 1. It's now progressed to throwing codes (misfire cylinder 1 as well as misfire within 1st 1000 revolutions). Anyway, something to look into and might get you pointed in the right direction.

 

[2004XLT]

I 'll see if I can borrow a scanner to see what's causing the shaking, also go thru the diagnosis bedrck posted above. I have emissions due in a month but no time to fix this.

 

[BRIANGEE]

Personally, I think you have two separate issues with the evap and rough idle. Hopefully Bedrk's [R.I.P.] post is helpful. Please keep us posted either way - I'd love to see you get this resolved.