Disconnected Evap Purge Solenoid = Very Happy LNF
I got that P0496 CEL today, car had erratic cold start since i done repair of the broken lash adjusters a couple weeks ago, i had some unstable idle at coldstart before the repair, and some erratic coldstart once in a while, may be 2-3 times per year...
But since about 2 weeks the engine run erratic at each cold start for a second or 2 then the idle will vary a little for few seconds and then its alright when catwarmup is almost done.... i also noticed sometimes it feel rich, exhaust tone ''sometimes'' isnt as ''clean'' as it was before... i got retuned a week ago thinking it was my intake, it helped since fuel trims were slightly off , it runs better since retune, but now that i got that code that was probably the problem...
Any idea on wich solenoid valve i should change?
the evap solenoid on the canister near fuel tank or the evap purge solenoid part 12592015 that is on the intake manifold?
But since about 2 weeks the engine run erratic at each cold start for a second or 2 then the idle will vary a little for few seconds and then its alright when catwarmup is almost done.... i also noticed sometimes it feel rich, exhaust tone ''sometimes'' isnt as ''clean'' as it was before... i got retuned a week ago thinking it was my intake, it helped since fuel trims were slightly off , it runs better since retune, but now that i got that code that was probably the problem...
Any idea on wich solenoid valve i should change?
the evap solenoid on the canister near fuel tank or the evap purge solenoid part 12592015 that is on the intake manifold?
if you only wanted to try one then try the one on the intake manifold.
Damn, I will have to try this because my check engine light came on while i was driving last night, and this morning pulled it and it was a p0496 code
I'll try pulling the plug on the solinoid to see if it clears the check engine light
I'll try pulling the plug on the solinoid to see if it clears the check engine light
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Joined: 03-30-11
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From: Long Island NY
I'm so excited about this that I couldn't wait until tomorrow to report it... 
Summary:
Had random erratic AFRs at idle and cruise, causing engine to not idle very well and to stumble while cruising. Unplugged electrical connection to my Evap Purge Solenoid and all AFR and driveability issues went away. Car runs like new compared to what it was. I am very excited.
The background:
I had been having some random erratic AFRs through light to moderate throttle (<800 PSI or so fuel pressure). The car would hold good AFRs and then suddenly swing rich, followed by swinging lean. When it swung lean, you could feel the car lose some power, and in some cases would sometimes stumble. I couldn't tell any rhime or reason to it. At moderate throttle and above (>800PSI fuel pressure), the car would stick around stoich like glue.
I took it in to the dealer and they said that it wasn't anything they hadn't seen before. I wasn't convinced, so I had them write up a report on it to have on file anyway.
I had been in touch with Gettinausernamesucks and More_Torque_More_HP, exchanging ideas on possibilities. I had changed the wideband O2 sensor in the exhaust housing of the turbo and still had the issue.
Finally More_Torque_More_HP mentioned the possibility of the Evap Purge Solenoid having an issue, as they had some field problems with them going bad. Well after troubleshooting other things first and not finding any solutions, I flipped back and found that PM and tried disconnecting the electrical connection to the solenoid tonight.
The results:
The car now runs like it did when it was new. Driveability is much improved. Gobs of power and response are back. AFRs are spot-on in idle, cruise, and WOT (found out that the tune I've got is actually a ~12.2 tune... sorry BYT/T2...guess I better get to asking for a revision).
I can't believe the problem is gone. I have been stressing over it for so long now.
So now with that said, I'm faced with where to go from here. I'm guessing my purge solenoid went bad at some point, but it never tripped a CEL. Speaking of which, since disconnecting it, I still haven't gotten a CEL like I thought I would. Perhaps it'll take more key cycles or me filling up the tank again.
Summary:
Had random erratic AFRs at idle and cruise, causing engine to not idle very well and to stumble while cruising. Unplugged electrical connection to my Evap Purge Solenoid and all AFR and driveability issues went away. Car runs like new compared to what it was. I am very excited.
The background:
I had been having some random erratic AFRs through light to moderate throttle (<800 PSI or so fuel pressure). The car would hold good AFRs and then suddenly swing rich, followed by swinging lean. When it swung lean, you could feel the car lose some power, and in some cases would sometimes stumble. I couldn't tell any rhime or reason to it. At moderate throttle and above (>800PSI fuel pressure), the car would stick around stoich like glue.
I took it in to the dealer and they said that it wasn't anything they hadn't seen before. I wasn't convinced, so I had them write up a report on it to have on file anyway.
I had been in touch with Gettinausernamesucks and More_Torque_More_HP, exchanging ideas on possibilities. I had changed the wideband O2 sensor in the exhaust housing of the turbo and still had the issue.
Finally More_Torque_More_HP mentioned the possibility of the Evap Purge Solenoid having an issue, as they had some field problems with them going bad. Well after troubleshooting other things first and not finding any solutions, I flipped back and found that PM and tried disconnecting the electrical connection to the solenoid tonight.
The results:
The car now runs like it did when it was new. Driveability is much improved. Gobs of power and response are back. AFRs are spot-on in idle, cruise, and WOT (found out that the tune I've got is actually a ~12.2 tune... sorry BYT/T2...guess I better get to asking for a revision).
I can't believe the problem is gone. I have been stressing over it for so long now.
So now with that said, I'm faced with where to go from here. I'm guessing my purge solenoid went bad at some point, but it never tripped a CEL. Speaking of which, since disconnecting it, I still haven't gotten a CEL like I thought I would. Perhaps it'll take more key cycles or me filling up the tank again.
there should really be a recall on these things haha
I know its what my problem is... Ive had crazy swings in fuel trims for a awhile and now this code will reappear every friggen driving cycle.
It doesnt take smoke and mirrors to determine the stupid solonoid has given out based on the huge upheaval here on CSS.net over this thing.
It doesnt take smoke and mirrors to determine the stupid solonoid has given out based on the huge upheaval here on CSS.net over this thing.
disconnected the solonoid, took it off and attempted to blow into it to see if it was closed or open. it was closed and I felt like I was blowing a breathalizer lol. bolted it back on and hooked it up. started the car and it seems to be running fine. I will really know tomorrow on my way to work./
P0455
P0496
and other are likely one of the two main Evap parts so throwing $50 at the car and having new parts to get her running again is worth it.
It's not like we are stabbing in the dark.
okay, I drove to work today. I got a CEL and a new DTC P0443 along with the P0455 code still.
Driveability felt like it improved, I must be crazy but it felt like power that had disappeared in the lower RPMs has now returned. I floored it to redline and no extra power was made. peak power was the same. LTFT = 0.0%, STFTs bounce more in the negative percentages... -8.0% to -5.0% for split seconds during partial throttle blips. At idle sitting at a redlight, I see +3.2% to +1.5% STFTs.
Over all, I like the outcome. I will buy a new part soon and replace it. Hopefully its not the vent valve on the gas tank cause I have no Idea where to find that.
Driveability felt like it improved, I must be crazy but it felt like power that had disappeared in the lower RPMs has now returned. I floored it to redline and no extra power was made. peak power was the same. LTFT = 0.0%, STFTs bounce more in the negative percentages... -8.0% to -5.0% for split seconds during partial throttle blips. At idle sitting at a redlight, I see +3.2% to +1.5% STFTs.
Over all, I like the outcome. I will buy a new part soon and replace it. Hopefully its not the vent valve on the gas tank cause I have no Idea where to find that.
welp. car almost didnt start today, scared the **** out of me. The RPM went all heywire and then climbed to 2500rpms during cold start... seemed alot higher than usual. Alot of choking and bogging then it evened out after the minute ordeal. I plugged the sensor back in.
My goal was to leave it unplugged and see if the p0455 code would come back, it didnt which tells me that when plugged in and the car is on, my EVAP purge leaks. The only code I had was the p0443 which = out of voltage range for the EVAP purge (because its unplugged).
This also tells me that my car choking this morning has nothing to do with that sensor unplugged, but you never know... Some one else said they experienced the same phenomena during start up on here..
My goal was to leave it unplugged and see if the p0455 code would come back, it didnt which tells me that when plugged in and the car is on, my EVAP purge leaks. The only code I had was the p0443 which = out of voltage range for the EVAP purge (because its unplugged).
This also tells me that my car choking this morning has nothing to do with that sensor unplugged, but you never know... Some one else said they experienced the same phenomena during start up on here..
okay, I drove to get lunch and I had plugged the sensor back in. The car ran fine and dandy. Felt like it did when I first experienced not having it plugged in, I cleared teh DTC p0443 during cold start and man was that a mistake, LOL. car sputtered like crazy and I got ~ 10 misfires per cylinder. Then it ran fine. No DTCs and no check engine lights now. I wonder if taking the sensor out and putting in my mouth fixed it?
Thread Starter
Senior Member
Joined: 02-09-09
Posts: 4,374
Likes: 5
From: Tejas
welp. car almost didnt start today, scared the **** out of me. The RPM went all heywire and then climbed to 2500rpms during cold start... seemed alot higher than usual. Alot of choking and bogging then it evened out after the minute ordeal. I plugged the sensor back in.
My goal was to leave it unplugged and see if the p0455 code would come back, it didnt which tells me that when plugged in and the car is on, my EVAP purge leaks. The only code I had was the p0443 which = out of voltage range for the EVAP purge (because its unplugged).
This also tells me that my car choking this morning has nothing to do with that sensor unplugged, but you never know... Some one else said they experienced the same phenomena during start up on here..
My goal was to leave it unplugged and see if the p0455 code would come back, it didnt which tells me that when plugged in and the car is on, my EVAP purge leaks. The only code I had was the p0443 which = out of voltage range for the EVAP purge (because its unplugged).
This also tells me that my car choking this morning has nothing to do with that sensor unplugged, but you never know... Some one else said they experienced the same phenomena during start up on here..
Basically with that fuel tank vent stuck, air can't fill the tank as you use the fuel out of it, there becomes a low pressure/suction within the fuel tank, and then the car has trouble getting fuel to the engine. This problem becomes two-fold when the engine is trying to suck the air with the gasoline vapors out of the tank at the same time.
Sometimes people notice this problem right after topping off on gas, because that's when there's the most gasoline in the tank and the least air.
Here's a link to that post within this thread: https://www.cobaltss.net/forums/5495711-post178.html
Last edited by Stamina; Sep 17, 2011 at 01:35 PM.
Service Information
2008 Chevrolet Cobalt | Cobalt, G5 VIN A Service Manual | Engine | Engine Controls and Fuel - 2.0L | Description and Operation | Document ID: 1236733
--------------------------------------------------------------------------------
Evaporative Emission Control System Description
EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
Large Leak Test
This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.
Small Leak Test
The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative emission (EVAP) system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.
The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a sealed system, to determine if an EVAP system leak is present.
The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.
Canister Vent Restriction Test
If the evaporative emission (EVAP) vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP purge solenoid valve ON, commanding the EVAP vent solenoid valve OFF, and monitoring the fuel tank pressure (FTP) sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.
Purge Solenoid Valve Leak Test
If the evaporative emission (EVAP) purge solenoid valve does not seal properly fuel vapors could enter the engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the EVAP purge solenoid valve OFF and the vent solenoid valve ON, sealing the system, and monitors the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that the EVAP system vacuum increases above a calibrated value, DTC P0496 will set.
Check Gas Cap Message
The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.
EVAP System Components
The evaporative emission (EVAP) system consists of the following components:
EVAP Canister
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.
EVAP Purge Solenoid Valve
The EVAP purge solenoid valve controls the flow of vapors from the EVAP system to the intake manifold. The purge solenoid valve opens when commanded ON by the control module. This normally closed valve is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.
EVAP Vent Solenoid Valve
The EVAP vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The control module commands the valve ON, closing the valve during some EVAP tests, allowing the system to be tested for leaks.
Fuel Tank Pressure Sensor
The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.14.9 volts. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.
Š 2012 General Motors. All rights reserved.
2008 Chevrolet Cobalt | Cobalt, G5 VIN A Service Manual | Engine | Engine Controls and Fuel - 2.0L | Description and Operation | Document ID: 1236733
--------------------------------------------------------------------------------
Evaporative Emission Control System Description
EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.
Large Leak Test
This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.
Small Leak Test
The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative emission (EVAP) system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.
The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a sealed system, to determine if an EVAP system leak is present.
The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, DTC P0442 will set.
Canister Vent Restriction Test
If the evaporative emission (EVAP) vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP purge solenoid valve ON, commanding the EVAP vent solenoid valve OFF, and monitoring the fuel tank pressure (FTP) sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.
Purge Solenoid Valve Leak Test
If the evaporative emission (EVAP) purge solenoid valve does not seal properly fuel vapors could enter the engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the EVAP purge solenoid valve OFF and the vent solenoid valve ON, sealing the system, and monitors the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that the EVAP system vacuum increases above a calibrated value, DTC P0496 will set.
Check Gas Cap Message
The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.
EVAP System Components
The evaporative emission (EVAP) system consists of the following components:
EVAP Canister
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.
EVAP Purge Solenoid Valve
The EVAP purge solenoid valve controls the flow of vapors from the EVAP system to the intake manifold. The purge solenoid valve opens when commanded ON by the control module. This normally closed valve is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.
EVAP Vent Solenoid Valve
The EVAP vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The control module commands the valve ON, closing the valve during some EVAP tests, allowing the system to be tested for leaks.
Fuel Tank Pressure Sensor
The fuel tank pressure (FTP) sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.14.9 volts. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.
Š 2012 General Motors. All rights reserved.
Thread Starter
Senior Member
Joined: 02-09-09
Posts: 4,374
Likes: 5
From: Tejas
Hey man, I saw you PM'd me. Sorry about the full PM box. I've been getting a lot of questions about various things recently and I haven't been able to field them fast enough recently due to being away a lot recently. Was your message about this?
Thread Starter
Senior Member
Joined: 02-09-09
Posts: 4,374
Likes: 5
From: Tejas
Okay, so I heard back from Vince. He verified it's on his list of things to look into.
In the meantime, he recommended looking into using the capping method and then using a resistor of similar resistance to hook to the electrical connection to keep the ECU happy.
Are there any more updates or findings on this?
I'd use a 22 Ohm resistor to fool the ECU if I was certain that your solenoid was correct. I'll see if I can't do some research with the OEM and back out what the design resistance is. Hopefully it'll be the same as what you found.
Update: I found that the Evap Solenoid (GM Part#: 12608378) comes from ACDelco and offered under the part number 214-2125. I verified that it's a 12V solenoid, but other than that so far I can't find any other technical info about it.
In the meantime, he recommended looking into using the capping method and then using a resistor of similar resistance to hook to the electrical connection to keep the ECU happy.
Are there any more updates or findings on this?
I'd use a 22 Ohm resistor to fool the ECU if I was certain that your solenoid was correct. I'll see if I can't do some research with the OEM and back out what the design resistance is. Hopefully it'll be the same as what you found.
Update: I found that the Evap Solenoid (GM Part#: 12608378) comes from ACDelco and offered under the part number 214-2125. I verified that it's a 12V solenoid, but other than that so far I can't find any other technical info about it.
Update:
The part number for the Evap Purge Solenoid has recently changed. I haven't seen the new one, so I don't know if it's physically the same or if it's been changed/revised.
Old part number (printed on driver side of the solenoid): 12608378
New part number: 12592015
Just an FYI for those still having trouble.
The part number for the Evap Purge Solenoid has recently changed. I haven't seen the new one, so I don't know if it's physically the same or if it's been changed/revised.
Old part number (printed on driver side of the solenoid): 12608378
New part number: 12592015
Just an FYI for those still having trouble.
So I just wanted to give everybody an update on the original problem.
In a nutshell: The Evap Vent Solenoid was bad despite it never throwing any codes. That particular solenoid is the one that vents the fuel tank/evap system and is located on the evap canister under the rear of the car.
So my troubleshooting continued now that I've had some freetime over the Break and such. I basically disassembled the whole system, checked all the piping for plugs or leaks, checked the check valves, re-checked the Evap Purge Solenoid, disassembled and cleaned the canister (as much as I could anyway.. it's a sealed container for the most part), and never seemed to fix the problem.
Finally, when coming back home from the Break, I had a 5 hour drive to just think... and naturally the Evap issue came back to mind. I hadn't seen the Evap Vent Solenoid show anything but open on the Dashdaq for a long time. Could it be stuck open? I got to thinking about the symptoms of what something like that stuck open would be, and it seemed to fit what I was having happen.
The purge solenoid would open, and since the vent solenoid was stuck, and the outside air pressure is higher than the intake manifold pressure, it would force evap through the tube and then force fresh air through the tube before the ECU could compensate and/or shut the purge solenoid. Usually the gas fumes are sucked through the evap system/tubes like a straw, with the solenoid opening while in vacuum and the manifold vacuum pulling it, but now it was being force fed.
This was reflected by my random very rich and then very lean swings. Too much evap that threw it rich and then a bunch of fresh air that would then throw it lean. The car would sometimes stumble, sometimes even register knock, and finally forced me to disable the system until I could find out what it was.
I recently received and installed a replacement Evap Vent Solenoid, and it seems to have finally fixed the problem! I'm planning on testing it to see what the point of failure was. Right now, I'm leaning towards a mechanical failure, since the plunger that cuts off the outside airflow seems to get stuck along the edge of the "cage" that is rides up and down within, and the fact that if it was electrical, it may have thrown a code. As you can see, the solenoid itself changed over time too. It looks like it was redesigned.
The new solenoid (left) vs. the old solenoid (right)
The GM part number for the Evap Vent Solenoid is: 25932573
Thanks to all of y'all for your help/support!
In a nutshell: The Evap Vent Solenoid was bad despite it never throwing any codes. That particular solenoid is the one that vents the fuel tank/evap system and is located on the evap canister under the rear of the car.
So my troubleshooting continued now that I've had some freetime over the Break and such. I basically disassembled the whole system, checked all the piping for plugs or leaks, checked the check valves, re-checked the Evap Purge Solenoid, disassembled and cleaned the canister (as much as I could anyway.. it's a sealed container for the most part), and never seemed to fix the problem.
Finally, when coming back home from the Break, I had a 5 hour drive to just think... and naturally the Evap issue came back to mind. I hadn't seen the Evap Vent Solenoid show anything but open on the Dashdaq for a long time. Could it be stuck open? I got to thinking about the symptoms of what something like that stuck open would be, and it seemed to fit what I was having happen.
The purge solenoid would open, and since the vent solenoid was stuck, and the outside air pressure is higher than the intake manifold pressure, it would force evap through the tube and then force fresh air through the tube before the ECU could compensate and/or shut the purge solenoid. Usually the gas fumes are sucked through the evap system/tubes like a straw, with the solenoid opening while in vacuum and the manifold vacuum pulling it, but now it was being force fed.
This was reflected by my random very rich and then very lean swings. Too much evap that threw it rich and then a bunch of fresh air that would then throw it lean. The car would sometimes stumble, sometimes even register knock, and finally forced me to disable the system until I could find out what it was.
I recently received and installed a replacement Evap Vent Solenoid, and it seems to have finally fixed the problem! I'm planning on testing it to see what the point of failure was. Right now, I'm leaning towards a mechanical failure, since the plunger that cuts off the outside airflow seems to get stuck along the edge of the "cage" that is rides up and down within, and the fact that if it was electrical, it may have thrown a code. As you can see, the solenoid itself changed over time too. It looks like it was redesigned.
The new solenoid (left) vs. the old solenoid (right)
The GM part number for the Evap Vent Solenoid is: 25932573
Thanks to all of y'all for your help/support!