2008 Impala vent circuit (FIXED)

Thanks for your help, too. I learned a bit on this one and it was my first PCM replacement. It's one of those ones where you really want to be sure. I was pretty confident with the diagnosis but still had a little anxiety when preparing to start it!

Not so fast. Another lesson learned. When I put the computer in, the cel didn't come on and the owner was on a rush, so he took the car before I actually tested the valve functionality.

The car is back in my garage now with the same exact issue.

I ran through all the tests again. The valve won't close with bi-directional control. I'm getting 12v on both sides of the valve, on and off, and 12v at the ecu. Using my test light at the ecu, going to b+ doesn't make it shine when valve is supposed to be closed. Bench tested the valve and it works fine. Being the computer with test light to b- and touching the control wire at the ecu closes the valve. Doesnt that sound like a bad driver again? This is a totally different ecu too.

These tests show that the part is fine, the wiring is fine, transistor isn't doing it's thing, right?

What are the chances that the remanned computer is having the same problem? I unplugged the connectors at the ecu too and the pins looked fine.

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Sorry to hear.

I like where Monde is going. I see that the vent valve functions, but how's the current draw? I don't have a spec, but I'd think anything more than 1A would be an issue. Probably should be closed to 500mA. Maybe try jumpering the vent and seeing if the current draw stays consistent over several minutes? Kinda like when the PCM is doing it's small leak test.

Only other thing I can think of would be an intermittent short to B+.

Monde wrote: Try testing the purge valve and see if the driver opens and closes it. If it does, you can assume that it is a driver issue. Check also for a 5v reference at any sensor to see if the pcm is talking to you.

I can control the purge valve with my scan tool and I have 5v ref.

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What scan tool are you using? And what is the actual code number? P0???

bruce.oliver wrote: What scan tool are you using? And what is the actual code number? P0???

I can't remember what the code was now. I think it was p0446. I'm using a launch scanpad.

Example b. Is what I'm seeing with my tests.



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P0446 Conditions for setting the DTC...
The FTP sensor is more than 8 inches H2O vacuum for 2 seconds during the 13 minute test.
OR
The FTP is less than −2.5 inches H2O or more than +5 inches H2O for 3 seconds after a cold start ignition ON.

I have seen fuel tank pressure sensor set this dtc on gm vehicle before. Otherwise this DTC basically sets when the ECM detects a restriction in the evap vent system - Its a DTC that is set based on "logic" if that makes sense.... kind of like "time to activity" o2 heater circuit logic dtc`s...
Restriction--
With the engine running, the EVAP canister vent solenoid valve open, and the EVAP canister purge solenoid valve commanded to 100 percent, the fuel tank vacuum should not increase to more than 12 inches H2O.
If the fuel tank vacuum is greater than the specified value, isolate the restriction by disconnecting one component at a time while the EVAP canister purge solenoid valve is commanded to 100 percent and the vent valve is open.

maybe just take the gas cap off and see what the fuel tank pressure sensor reads (koeo)...
Multiple ways to troubleshoot this symptom... I would pay close attention to your fuel tank pressure sensor. Try and utilize GM`s evap purge / seal functional test..


FTP Sensor
Ignition OFF, disconnect the harness connector at the FTP sensor.
Ignition OFF, test for less than 5 ohms between the low reference circuit terminal A and ground. If the circuit tests normal, replace the FTP sensor.
Obviously you may want to check signal circuit wire integrity as well as 5 volt ref. not sure why gm didn`t add ohm tests for those wires in their flow chart...

I watched a very good EVAP video that showed a wiring diagram that didn't supply all the info and the guy had to refer to another source (Mitchell) to figure out why he was getting 12 volts on the vent valve circuit with the car in a key off scenario. Turned out the car had a ENOV or California emission set up and there was another secondary circuit that would supply 12v to the vent valve. Don't know if that might help with your 12v reading. The video was made by Wells and the young tech did an excellent job going thru the whole EVAP system.

EDIT: Just found the link to the video

Double check the code number and get back to us. P0446 is not a circuit code. It is a vent control performance code. It's almost always a bad fuel tank pressure sensor, in my experience.

And just because the bi directional control isn't working doesn't mean the PCM is bad, it could be the scantool just won't operate it even though it says it does.

Any updates on this?

Hey, sorry, I didn't realize that I didn't subscribe to this thread so I wasn't getting any updates! I am going to pull the code today and see exactly which one it is and go through all of the new posts. I'm planning on taking another look at the car on Tuesday.

I've been watching some of the solenoid testing videos and will apply some of that, along with all of your suggestions and post back.

One question that may be silly, how do I test the current draw on the solenoid? Set me meter to amps and somehow put it in series with the ground wire like you would when doing a battery draw test? That sounds a little scary. I wish I had a used battery to bench test stuff like this with. Unless of course my method is wrong!

Thanks

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Update on which code it is.



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Tutti57 wrote: Update on which code it is.

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That is not the right description for that code. P0446 is vent control performance, likely Abad fuel tank pressure sensor. Look at data for the fuel tank pressure. Evap vent circuit code is P0449

Good catch. I was using my backup Autel for that pull. The question is, why isn't the computer controlling the vent? The description says that it shuts the vent and opens the purge to create vacuum in the EVAP system, and looks for it to be in spec to set P0446. This would not take place with the valve staying open. I can't find anything that says that with the DTC set, it shuts down the vent valve, but could that be the case here?

Should I ground the valve to the frame to manually close it and do the FPS tests or continue to hunt down the potential vent issue first?

I've got a feeling this is going to be lesson that I'm going to have to pay for!

I would concentrate on the code. The code, P0446, has nothing to do with the vent circuit. It is very easy to check the fuel tank pressure PID on the scantool. It should be roughly 1.5 volts with 0 pressure and 0 vacuum. If it isn't 1.5 remove the fuel cap and it goes to 1.5 then there is your problem. Then educate the car owner on topping off the tank after the pump shuts off. The FTP sensor doesn't like liquid fuel. Eric O on South Main Auto on YouTube has a couple videos on that code with GM.

The owner wasn't at work today but I'm going to bring my scantool to work tomorrow and check out the FPS data. Hopefully he will be there. I'll post back tomorrow at some point with the results.

It is not letting me control the purge valve or vent. The pressure doesn't change no matter what I try, probably because the purge isn't opening. Pending p0446, but the owner is trying to pass inspection by keeping the tank full and leaving this monitor unset, so it won't set the code until he drops below 85% fuel. No cel on right now.

The picture is oem data. Global showed value of -415 and 5.25 with a standard of -0.35 and and -50Pa. Not sure what that all means, but it is showing in red as being out of spec.

You can see in one pic where I have the purge at 40% buy the command is at 0%. What do you make of the EVAP purge sol ckt short volts test status not run? The vent says the same.

Is this a symptom or the problem?

I think I want to bypass the computer and ground the purge and vent to test the tank pressure sensor. Unless anyone sees something obviously wrong with the sensor to begin with. Not sure if that could shut down both EVAP drivers.



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Monde wrote: Try to run the short to voltage test yourself and see. See if you get 12v on both wires, with the sensors disconnected. Best way to do that as you may know is to isolate the circuits and with test light on B ,test light should not light.

Ok. Thanks to everyone for sticking this one out with me.

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Look at the first picture you posted. It shows the vent solenoid as venting but your fuel tank pressure shows an extreme vacuum and 4.84 volts. If the vent solenoid is venting the fuel tank pressure should be 0 pressure which is roughly 1.5 volts. Looks to me like you probably have a bad fuel tank pressure sensor

bruce.oliver wrote: Look at the first picture you posted. It shows the vent solenoid as venting but your fuel tank pressure shows an extreme vacuum and 4.84 volts. If the vent solenoid is venting the fuel tank pressure should be 0 pressure which is roughly 1.5 volts. Looks to me like you probably have a bad fuel tank pressure sensor

Ok. Makes sense.

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