"check gas cap" message, always...
 

2006 ss 2.0 sc cobalt, 110 K miles, runs great. but every few days the "check gas cap" text in display comes on, then shortly after the check engine light displays, every time I scan it, it says "large leak p0455 evap system". I check the cap its always tight, I also replaced it, but it does the same. I can clear the code but every few days it comes back on...I understand the whole system of evap but...
looking for specific parts that are the most common to not seal or fail in these Cobalts?
what should be checked and or replaced in what order of most likely to fail and trip this code.

thanks

When you are filling the tank (adding gas), does the gas sometimes overflow? Could be a stuck ball check valve in the fill line. Some older Chevy's had this issue and the valve/fill line needs to be replaced.

no, no issue filling

did you check your EVAP purge solenoid on your engine? its right next to the valve cover. has a line running to the supercharger.

Check the purge valve like leemanfor mentioned. If they fail stuck open then the evap system can't seal and the computer reads it as a large leak.

alright I will start with the purge solenoid on the engine, thank you.

I am also looking at replacing the tank filler neck as there is a lot of pitting and rust on this assembly, I suspect possible leaks along this piece, but I find two types of new parts for this, listed one with w/ PZEV (Partial-Zero Emissions Vehicle and one with out, which would be correct?

alright I replaced the (engine mounted) purge solenoid, reset and light came back on about 1 hour after driving, "check fuel cap" message followed by check engine light. I reset it and replaced the tank pressure sender (in the canister back by the tank) I reset it , and drove about 1 hour and sure enough the same "check cap" message followed by check engine light. so which part should I check next?

also how can you tell if the car is one with w/ PZEV (Partial-Zero Emissions Vehicle) or one without

There is possible a broken line.

No idea on the PZEV determination, but I was under the assumption all the cobalts did a fuel cut during coasting

so, when it comes to the tank filler neck assembly, two are available one with w/ PZEV (Partial-Zero Emissions Vehicle) and one without.
how can one tell which version they have/need?

no leaks found in the lines, and all sensors have been replaced,, but still message displays, so replacing the tank filler neck assembly.

finally just received the new tank neck assembly.....so now in the process of replacing this neck assembly, the original was very rusty, thin and able to "crunch" it with my hand. have no doubt it was allowing a leak somewhere..hoping this will eliminate this "leak"
at the same time I'm replacing rad, ac condenser and heat exchanger as they all were pushed back and bent badly. I think some one plowed snow with this car...

Did your car smell like gas too? I'm getting this message and some fuel fumes when driving and when the car is parked.

Keep us posted.

no smell of gas ever., even when I would sniff around under nether, but always that message every few miles, so capped off the tank spout with a solid rubber cap, sealed it with a clamp and reset light, and just a few miles later, check fuel cap message again. so replacing spout assembly, due to other things found I am also replacing rad, intercooler, pump, and ac condenser. well see if the message comes back after that. if it does I guess next would be to replace the entire canister assembly.

I have replaced the EVAP purge solenoid on the engine and the tank filler neck assembly, and sensor in evap canister down by the tank, but message still reappears every few days,

other thoughts?

Have you checked your vent valve yet

yep, that was the first thing I replaced as it was the least invasive to replace.

Have you made sure your getting power and ground at the purge and vent valve connectors?

thanks to all for the suggestions, this weekends plan is to swap all these parts on my cobalt, and hopefully figure out once and for all why the "check fuel cap" message always comes up.

a great weekend as I got a chance finally to swap out a ton of parts including, new intercooler, new ac condenser, new intercooler pump, new radiator, new purge solenoid, new vent valve, new map sensor and new fuel tank fill spout and vent setup. I also checked all lines and connections. reinstalled stock intake.
refilled all fluids and charged ac, all test ran great and no "fuel cap" message or P0455 code so far.
so glad I replaced all front radiator units as they were damaged more then I first believed them to be. pushed back far, bent and all three units leaking.

https://cimg3.ibsrv.net/gimg/www.cob...699f4fe09f.jpg
https://cimg4.ibsrv.net/gimg/www.cob...ab89052f07.jpg
after

https://cimg7.ibsrv.net/gimg/www.cob...272ff3cf12.jpg

alright so after replacing all these things, I reset all engine codes and test ran it, ran perfectly, then yesterday I got to take it for a test drive, it drove and ran perfectly for about 40 minutes, with no "check fuel cap message" and no "check engine light" it sounded great, and everything functioned perfectly.
I then parked it waited about ten minutes, went to move it into the garage and as soon as I started it the idle was very erratic, from 300 rpm's up and down, quickly to around 3K, then back down, and after about 5 seconds, the check engine light came on, (not "check fuel cap message"),it was such a rough erratic idle. all over the place and sounded terrible.
no pedal movement seemed to make any difference either. when I scanned it, it said code PO171, which is a lean run issue.(bank1)
I turned it off and let it sit for about half hour, I started it again and it did the same, but this time when I turned it off it continued to run (diesel) for about 5 seconds.
thoughts on what would be the most likely cause for this, very strange that it all would work great for 40 minutes then suddenly change.
I checked the intake system to make sure there was no leak, also verified that all pumping and wire connectors were installed. but still did the same.

MAF sensor? O2 sensor? TPS? MAP?

I also noticed that the new EVAP purge solenoid ticks like crazy, it should right?

I got lucky, I tried a new MAF first and it took care of it,
I then (just to confirm) put the old one back in and it was instantly "bad" again,
I put the new one back in and it seems fine now. wheeew

my question now is what is this? it is next to the evap purge valve solenoid , it has two lines in the front one is connected the other had foam around it? is this correct?
https://cimg2.ibsrv.net/gimg/www.cob...0088eda68d.jpg

That's correct, it's your boost solenoid that controls the bypass valve on your supercharger