CEL came on yesterday, 2006 ODB trouble codes

[2006Tahoe2WD]

Test that vent valve by applying 12v to it and try to blow through it -- best done on a bench, probably. Just because the solenoid works doesn't mean the valve is fully sealing.

After disconnecting the hose you can try to blow through it while toggling it on/off with the scanner, or remove it from the truck and apply 12v (polarity doesn't matter for this test) alternately to see if it seals every time. With no voltage applied it should be open, and with 12v applied it should seal. Every time.

Okay thanks. Per the electrical schematic (Alldata) the vent solenoid has 12 volts applied all the time (and is normally open) the "controller" commands the vent valve closed by connecting the ground. I need to get a "12 volt probe" and some alligator clips. I'll try doing it with the solenoid electrically connected to the car as you mention. I also looked at a DIY smoke machine. I'll check the vent solenoid first. Thanks.

 

[Fless]

Be careful with that description of the vent valve circuit. 12v is applied when the key is on; when the ignition is off, no voltage is applied and the valve should be open. Having it commanded on with the ignition off would introduce a full time draw on the battery.

The purge valve is somewhat the opposite; when the ignition is off, the valve is closed. When the ignition is on, the purge valve is closed until the system commands it open to purge the canister of the fumes.

You have a 12v source on the truck; either use the scan tool to command the vent valve open/closed, or remove it from the truck and use small jumpers at the battery. I would not apply an external 12v source to the valve while it's still in-circuit.

 

[2006Tahoe2WD]

Okay, heavy rain here now. I was thinking I could detach it and have it hang down or connect some tubing to blow on it while commanding on/off with ig. on but engine not running. I have to see the tubing connection types - I don't want to mistakenly split tubing trying to pull it off a barb. Thanks.

 

[2006Tahoe2WD]

Break in the rain and I think I made progress. I got the vent valve out and disassembled. It was very dirty inside. I cleaned the filter and housings. You can take the valve off and blow it out. If you blow one way the valve snaps shut and you can feel if it is leaking very much when shut. After the blowing out it seemed to be okay. I assembled it and reinstalled. Time to do another purge/seal test.
I did that and guess what happened? After the seal the negative pressure continued to climb (more vacuum) - which is different than what it was doing (declining quickly to zero). A conclusion is the vent valve is now working and the actual leak is in the purge valve. Does this make sense?

 

[Fless]

Sounds like you kept the engine running after the seal, otherwise the vacuum wouldn't have increased.(?) So you could run the purge and seal test again, and turn off the engine when you seal it to see how fast the vacuum decays.

After that, if it still decays fast, maybe plug the purge valve (connector end) and run again; doing that wouldn't care if the purge valve leaks. IIRC there's an o-ring on the purge valve mount so make sure that's good.

If you had to clean the vent valve a new one might be something to consider.

 

[2006Tahoe2WD]

Okay - good idea, run purge/seal test and turn ignition to "ON" position and watch decay.
The O-ring going into the intake manifold looked okay.
I'm not totally following how to pull the vacuum and then plug something to hold the vacuum while not depending on the purge valve. I'll go take another look.
Thanks much for helping me along on this.
I could just to the dealer and load the parts cannon with a purge and vent valve. They are probably due anyway as you suggest.

 

[2006Tahoe2WD]

I was thinking of getting a vacuum gauge/hand pump. I could take the hose going to the rear of the car off the purge valve and attach the gauge to the valve. If I start the engine if the valve is not leaking (closed) I should not see a vacuum. Right?

 

[2006Tahoe2WD]

or.... with the purge valve unplugged (should be closed) and tube to tank unplugged I could start the car and I should feel/hear no leaking past the valve. Right?

 

[2006Tahoe2WD]

I tried the purge/seal test and then turned the key to OFF to watch the decay. Doesn't appear that this worked. The vent status went to VENTING when I turned the key to off.
I'm still getting rising negative pressure after hitting SEAL.
Maybe one thing to try is pulling the connector on the purge valve half way through the purge part of the test and see if the negative pressure still increases.
The other thing is to measure the ohms on the purge valve and see if the value is within specs. or just go buy a new purge valve and install to see if anything changes.

 

[Fless]

I'd probably start by replacing both the purge and vent valves; together the GM/ACDelco versions are around $50 at RockAuto (there is a thread here on TYF for a discount code). They're not that expensive and are wear items. Either valve can be intermittent, and I'd recommend getting the GM/ACDelco parts.

You could still have a leaky vapor canister or an evap connector that is leaking. A smoke test might be the bees knees to find the leak.

I've forgotten; has the gas cap been changed and the fuel filler sealing surface been cleaned?