Yes. The reason for the vent is to let you fill up the gas tank easier and to allow air to fill the gas tank as you use the gas inside the tank.

There's no pressure in the fuel tank. That's part of why the purge valve has to be connected to the manifold, because it uses the engine's vacuum to suck the fumes from the vapor canister like a straw. That's also why your gas cap doesn't come shooting off like a cap off of a shaken 3L bottle of soda when you get gas. (Sometimes there will actually be a small vacuum in the tank) There is a pressure sensor in the tank to tell the car if there's a problem.

I haven't gotten an Evap Related CEL yet either (before I disconnected the solenoid), but More_Torque_More_HP is allowing me to take his E-Mails with me to show my dealer to assist me. I'll report back how it goes and what they find.

Here's an Evap System diagram that I thought might be helpful for people to understand how it works:

 

Well, at least this'll make stft tuning easier.

IamBroke's disabling the code on his car, if that unfreezes LTFT's then that'l be solid. I haven't had time to **** with my tune yet.

 

Drove it home with the CEL on, LTFT's locked at zero, beat the hell out of it and STFT's all in line with what I had before including 15 mins of stop & go traffic on the expressway. Trimmed a few more MAF correction cells I had no data on before.

Edit: Disabled the P0443 code for now but it looks like the LTFT's are still locked at zero. Time will tell.

 

So this disconnection will help with LTFT's? Mine are sitting at 12 right now on stock tune, and I am noticing it.

 

No, your STFT's will just hunt around trying to compensate for it. If your fuel trims are off that much you need a tune for the intake.

The LTFT's just trend an average skew to help with the compensation.

Having the LTFT's at zero once the MAF sensor tune is close will help fine tune the STFT's since the LTFT's aren't a moving target. We currently use the sum of the two to help dial it in.

 

Very interesting info in this thread.

My car threw PO443 about a month ago, so I brought it to the dealer to have it checked out. They wouldn't look into it unless I paid for 1 hour of diagnostic time (since they said were't sure if the part would be in warranty, which is BS). I said no way I'm paying $100, so they reset the code and I left.

Its stayed off since then, but I have had some noticeable driveability issues. When cold, its very rough, and sometimes it really feels sluggish at lower RPM's. Perhaps the Evap purge solenoid is te root cause of these issues.

 

sorry to be a newbie - but can some one please spell out these acronym's for me?

I am pretty sure I'm following the discussion, but just want to be sure.

Thanks in advance!

 

AFR: Air-Fuel Ratio. This is the ratio of molecules of air to molecules of fuel. At about 14.7, the mixture is said to be stoichiometric ("stoich"), and is the point where all the air molecules will react with all the fuel molecules. If there is more fuel than air (<14.7), then the mixture is said to be "rich". If there is more air than fuel (>14.7), then the mixture is said to be "lean". There are advantages and disadvantages to being rich, stoich, or lean. Typical car engines usually run stoich for normal operation, rich for warm-up and full throttle, and lean for long cruising to save gas. *Note: 14.7 is stoich for gasoline. Other fuels, such as E85, require a different ratio to be stoich due to their different chemical makeup (about 9.76 in the case of E85).

Lambda: This is the Air-Fuel Ratio expressed as a quotient, when stoich is the divisor. For example, if you're running at 13.0 AFR, then that would be about 0.88 (13.0/14.7). Some applications, such as tuning for the LNF, are programmed instead in Lambda.

STFT: Short Term Fuel Trim. This is the amount of fuel that the engine computer adds or subtracts to maintain the desired AFR. It is usually expressed as a positive or negative number representing a percentage. The STFT is updated and tweaked very quickly by the engine computer. Intakes and other factors can cause the Mass AirFlow sensor (MAF) to mis-read the amount of air going past it into the engine. The engine computer also uses STFT to help compensate for this.

LTFT: Long Term Fuel Trim. This is the amount of fuel that the engine computer adds or subtracts over time to maintain the desired AFR.

The engine computer looks at both STFT and LTFT when deciding how much fuel to mix with the air it takes in. It then sees how close it got to what it was wanting and re-adjusts the STFT and later LTFT again. This adjust-feedback-adjust approach the engine computer uses when the engine is in normal operation is called "closed loop".

 

Stamina;

thank you very much for going above and beyond on that post!!

 

Going to yank the line outta the intake tomorrow but keep the solenoid hooked up still w/a breatherline at the end, see if it fixes my rough idle and random boggyness. Evap commanded vs actual is off by a decent bit for me.

 

Just take the line off of the solenoid, leave the sol in the manifold, cap off the solenoid end and vent the line to the canister if you think you need to. I was going to try that next week to get the LTFT's back. The ECU will still see the solenoid connected, but it won't vent the canister sucking in air since the connection is capped. Only drawback may be if the ECU richens the commanded lambda during the purge cycle. Something else to watch.

 

Regarding EVAP what is the "Target" Evap commanded and actual pressure. When I log the EVAP it runs around -8200 coinsistently with an occassional +800 hiccup. The number does not really change at all, maybe a 100 less from time to time, whether the ECU commands 100% or 10%.

I am new to the logging on cars considering I never really drove many OBD cars, so can someone explain to me what these numbers actually mean? Would these numbers be of any concern regarding the functionality of the purge solenoid? And would these numbers have any effect on why I keep popping a P0496 code?

Once I originally put on the intake for 2-3 days the car ran pefect. I could feel the low end and top end pull harder. Then I popped a "high surge EVAP" code, cleared it and now the LTFT are at 12.5. The STFT only jump on average about 3-4 in either direction, except in third gear at WOT, then I can see a jump of 17 for a split second, and then it settles back to 0, and it stays there for the most part.

I have been noticing what sounds like a mis fire between 1-2 gear when I shift into 2, it almost sounds like a NLS pop, but more aggressive. And today I heard the same exhaust note in the high RPM's at around 5500. The car seems to fall on its face alot more now.

I imagine the intake is causing all of these problems, and I wonder would a laminar help at all? Even a piece of screen inserted in the intake, held firmly in place by a coupler of some sort. I am typically a function over form type of guy, but in the case of the intake, I would prefer to keep it. I imagine my plugs probably look like **** right now as well.

Not meaning to threadjack, regarding the intake, but I just wonder if the intake could of caused the EVAP code in some way. Maybe make the car run richer than desired, and created more fumes for the EVAP system to "get rid of". I know I am not completely making myself clear on my statements and I may be talking out of my ass, because I lack some of the knowledge needed and terminology needed to completely say what I am trying to say, so feel free to decipher my inability to commmunicate properly, lol.

 

The GMofHouston "Skunkworks" is looking into air straighteners this weekend to see if it's promising.

 

I've also got Vararam (Also located in Houston) looking at making an intake for the LNF so maybe we can finally have choices when it comes to a trouble free solution for intakes.

 

Great explanation thx.

 

I took a 400 mile road trip this weekend with the solenoid disconnected, and the car continues to run very smoothly, and my fuel economy was excellent. It really does feel like a new car again after running just a big rough for months now.

Any reason not to leave this permanently disconnected, other than having an annoying check engine light?

 

From what I've researched in other cases, it can be left unplugged without adverse effects. That being said...

The main concern right now about leaving it unplugged on our specific cars is that it possibly leaves the LTFTs at zero. Although this wouldn't be a big deal for properly tuned cars, it could cause issues over time (running consistantly too rich or lean over time), with the ECU having no way of adjusting consistant, overall rich/lean conditions.

Some of the guys with HPT are currently looking into ways of perhaps turning off the code causing the CEL, possibly allowing the LTFT to reenable itself. Then you could unhook the electrical connection or perhaps leave it plugged in and remove/cap-off the evap pipe going to it instead with no AFR or driveability disadvantages and also providing a tuning advantage. At that point there would be no reason at all that you couldn't just leave it disconnected.

 

Hi every one. I wanted to chime in and tell you that the cobalts are not alone. I have an 07 sky redline it has been running bad for about 2 months I read this post and then dissconected the purge solenoid and
WOW!!! car runs like new. If GM wont fix this problem we need to as a community. BTW the sky and solstice forums told me that it was my cat that had gotten clogged. well the car has only 12k miles on it so im thinking thats bogus and seeing as how some of you dont even have a cat how could that be possible.

 

GM does fix it. Just got mine checked out. They replaced the solenoid free under warranty. Mine runs great again. Common knowledge now that I think about it, but if you log the EVAP system the numbers should go negative and positive, back and forth. Mine would only stay positive, and once in a blue moon go negative. This would explain the high purge cel. I imagine if it stayed stayed negative then it would be low purge.

 

can they check it even if there is no cel on? I know its the solenoid but I have no way to prove it other than connecting it driving it to the dealer let them drive it then disconnecting it and letting them drive it again.

 

From what I've been able to see with HPTuners, the answer should be yes. Even on HPTuners you're able to see how "open" the purge valve is. There are two locals guys that had similar issues and the dealer was able to check the evap purge solenoid and vent solenoid for problems. In both cases they were able to diagnose the problem because they weren't getting a reading from the solenoid in question.

If you're on the Sky/Solstice forums, then you may want to let them in on this thread too.

 

Just got back from the dealer. My evap purge solenoid was sticking but not throwing a code. They replaced it under warranty.

Results: Driveability is much improved above what is was and is on par with when I had the solenoid unplugged. Rough idle is gone. Low-mid response is back.

The AFR doesn't flip out anymore, but does wander noticably more than when the evap solenoid is completely unplugged.


Personally: I'd be interested in finding if there's a way to keep LTFT functionality while disabing the evap purge. The AFRs were just more exact and "confidence inspiring". I'd say delete the whole system and save the weight if we're not going to keep it plugged in and using it, but the fuel tank vent is integrated into the canister by the fuel tank and it probably doesn't weigh much anyway.

I will be keeping an eye on things over the next several days to collect more information, as I haven't gotten to drive too long since the fix.

 

Nice. That's good.

 

LTFT's were locked at zero for the 3 days I had the sol unplugged, even with the CEL code disabled. Reconnected it and LTFT's started correcting the -0.8 to +0.8 I see during my commute since dialed in.

Haven't tried plugging the line to the canister yet. Maybe tomorrow.

 

Should be able to delete it and keep the check engine light off by putting a resistor across the two wires going to the solenoid. I can measure the resistance of the purge solenoid itself and find or make a resistor and put it into the plug. But it may be wired differently inside then I am thinking.

I am an electronics technician and most of my panels have line monitoring and it achieves it by a resistor being place in parallel at the end of the line. It can then modulate voltage and maintain a back feed check. For example if it gives the wires 12v it would be looking for 6v back. If I am wrong on the way the ECM monitors the sensors please correct me. I love learning new things.

Then we could make a small delete plate and have one less air leak to worry About