i tried this yesterday on the way home from work... it made part throttle and idle a LOT better but as soon as i went into boost it was a full point lean... then again my LTFT are 10% so i'm guessing unplugging this zeros them out and locks it...

 

Unplugging it freezes LTFT wherever it's at at the time (if LTFT is say -10, then it'll remain static at -10). STFT remains functional.

After a reset I'll typically plug in evap for LTFTs to get where they need to be and then unplug it again. Still haven't found my evap culprit.

 

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!

 

Wow thanks man. I am having very similar problems.

I am even having EVAP codes, long start ups, and i even heard a popping noise from the gas tank area.

Do you have instructions on how to do this?

I really don't know anything about it but it seems like it would be easy on my lift.

Thanks in advance Stamina.

I'm also wondering if I can check the Open/closed state with a DashHawk

 

Yeah, I could do a write-up w/ pics if people need one.

1) Raise up the back of the car.
2) Take off the hoses going to the canister on the passenger side of the canister (push the tab on the end and pull off).
3) Disconnect the wiring harnesses going to the Evap Vent Solenoid and the Fuel Tank Pressure Sensor on the canister.
4) There is one screw hidden up on the driver side of the canister, holding the canister in place on the rail it sits on. Take a 10mm and take it out. ("#23" on the schematic above)
5) Slide canister towards the driver side. It will slide off the rail.
6) Locate the Evap Vent Solenoid on the canister. Twist counterclockwise to unlock and pull to remove.
7) Replace with new Vent Solenoid, twisting clockwise and pushing slightly into the canister until locked (down-facing position on the electrical connector).
8) Reinstall canister on rail, reinstall locking screw, and reinstall hoses and harnesses
9) Enjoy your new well-behaving car.

 

Last question. Any risk of spilling gas? Why can't you turn the vent selenoid without removing the canister?

Thanks for the support.

 

I'm not sure if you can or not on a Dashhawk. I don't own one personally. Maybe somebody that does could answer this.

There are some fumes (obviously I suppose), but no gas leaked at all.

The canister is immediately in front of the spare tire wheel well, and the Vent Solenoid faces the wheel well and is in the "highest" part of the canister when the canister is installed, so you can't get the solenoid out without pulling the canister due to space restrictions.

 

wow!

thanks for the update!

 

Awesome. Points for fellow Aggie.

 

Let this be a lesson...gm truck owners know this well... do not overfill your fuel tank it will destroy the vent sol. It hapens because the system is rapidly fluttering to maintain stable slight vacuum which causes raw fuel to fill up the canister and that's all she wrote.

Another fix is to add a dodge ram evap orifice .020 spliced into the purge sol line that's what I did to prevent this problem at the track v100 isn't really a friendly fuel for the system...nor is e85 it corrodes the plunger.

Let this be a lesson...gm truck owners know this well... do not overfill your fuel tank it will destroy the vent sol. It hapens because the system is rapidly fluttering to maintain stable slight vacuum which causes raw fuel to fill up the canister and that's all she wrote.

Another fix is to add a dodge ram evap orifice .020 spliced into the purge sol line that's what I did to prevent this problem at the track v100 isn't really a friendly fuel for the system...nor is e85 it corrodes the plunger.

 

So for the Evap selenoid on the manifold.

Do you unhook the two connectors on top
Unbolt the bolt on the side
Then replace?

Is there a gasket on the new selenoid?

Anything in particular i need to pay close attention to? Seems straight forward but i can't afford to screw anything up.

I'm going to tackle both the vent valve selenoid and purge selenoid at the same time.

Thanks in advance.

 

For the one on the manifold:

1) Pop off electrical connector
2) Pop off evap hose/line
3) Remove bolt on the left side (has only one)
4) Remove and replace
5) Replace bolt, evap hose, and electrical connector

There's a gasket on the new solenoid. The solenoid replacement is a simple pop old one off and pop new one on operation.

 

Very nice, where did you order your new solenoid from?

 

GMPartsDirect in my case.

 

I just spoke with Tom at CED and he has put the part up on his website:
Ecotec Evap Vent Solenoid 25932573 - Crate Engine Depot

 

my vent sol is bad, just confirmed it, was making my ltft wander day to day between 0 and 9.4 stupid thing...but then I get in there for a good look and it had dirt all over it...mostly thanks to road racing.

 

ya I need to check mine now too, if its the older revision or bad I'll be calling Tom at CED

 

Assuming i did the vent valve right, it didn't fix my issue. Still throwing P0496

Next up Purge Selenoid.

 

Good luck man.

Yeah, there are just two solenoids and a sensor, and none of them are that expensive either luckily.

Here's the rundown on that code:

P0496 Trouble Code Conditions: (Evaporative Emission - System No Flow During Purge)
The ECM detects the EVAP system is not able to achieve or maintain vacuum during the diagnostic test. The condition exists for greater than 4 seconds or for a cumulative of 30 seconds.

Possible Causes:

■Fuel filler cap is very loose, missing or the wrong part

■Fuel tank, fuel filler neck or fuel sending unit 'O' ring is leaking

■Fuel tank pressure sensor is damaged, disconnected or it failed

■Fuel tank vapor line(s) is clogged, damaged or disconnected

■Purge valve vapor line is clogged, damaged, or disconnected

■Purge solenoid is not opening (it may be damaged or sticking)

■Vent solenoid is not closing (it may be damaged or sticking)

■ECM has failed

 

good stuff right here thanks for the reportim having the same problem getting the 0496 cel

 

So, if you are trouble shooting the first thing to check is the Evap purge solenoid? If that does not work then try the Evap Vent solenoid.? I don’t have a CEL so I don’t even know if I have a problem. It is just on some startups my RPM will be at about 1500rmp (I know that’s normal) however if I touch the throttle my RMP drops lower than normal idle (some times I think I am going to stall), however if I touch the throttle after engine is warm, the RMP will not drop below idle. IS THIS NORMAL? I don’t have a RPD or Dashdaq so I don’t know what my AFR is.

 

Wow I over looked something in the posts. The P0496 Trouble Code is associated with a CEL? So, if you don't have a CEL there is not a problem?

 

NO, My car exhibits these idle stuttering issues and only throws a DTC but not a CEL.

Weird thing is if I clear the DTC the problem goes away for a bit.

 

Would something wrong in the evap system cause my car to not want to idle when I first start it up some mornings?

 

mine tends to bounce a lot and throw a few misfires here and there.