Codes that don't light the CEL don't matter. Stop worrying about them. Especially, if the person reading them doesn't know what they mean.

Purge valve is under the hood. Vent valve is by the gas tank. Both have charcoal canister in their full names, easily confused.

 

It was the charcoal canister that came with the rubber seal that goes inside the charcoal canister, that the vent valve goes into. It was a AC Delco (GM) charcoal canister, you know
the one that the dealer wants over 200.00 bucks for. I found it at Summit for 60.00 (clearance) on line in their TX location.

On testing the purge valve I find the use of a hand vacuum pump is a good way to test as you can time and watch the hold time on the gauge. Also to complete the testing we need
to test the electric part of the valve. See this video for general information ...
I'm looking at the purge valve as many have stated it was the problem that fixed it in the end. BUT I did get a one time short notice on the dash to "check the gas cap" about a week back.
So I am uneasy about the filler neck as the cap is new and just tested. I have replaced the gas tank (leaking at the vent lines going into the tank), fuel pump (two times), replaced the fuel tank pressure senior/ sw., the vent valve, and charcoal canister... and have done lots of looking. Still have suspects to recheck. Thanks back later...!

 

All of that changing things on the tank probably left a grommet a little loose.
Whatever, a leaking gas cap will not affect the car one bit. Its sole purpose is to save a tree from hydrocarbon vapor.

 

Update... Still checking and re-checking the EVAP system (large EVAP leak). I have now found that the gas cap is not getting tight??? It will go on and I hear and feel it click but it will move when I touch it, it, does not tighten up. Not just the top part but the bottom / locking part, both will move (loose)???? I put a small amount of silicone grease/ lube on the cap sealing area and inside the filler neck /
locking area and cap seating area/ rim. It's like the top part of the cap is not pushing down on the bottom part so it seals to the filler neck ???? Anyway found this 2-3 days ago, been working on it and today got a one minute read-out to "check the gas cap" If the cap is not sealing this is the (or one of the) large EVAP leak which I got.

Yesterday I did the scanner reading and it showed "NO- CODES" ?? This goes back 3 weeks from the last scanner test.

Question now is... can the filler neck / pipe be some how damage / worn to make the cap leak / not seal??? I have tried 3 NEW gas caps and the one on now is the GM one. I have had them all tested by the state inspection station and ALL the caps tested GOOD???? And yes, I have cleaned and cleaned the filler pipe/ matting area inside and outside. Has anyone had this problem or know the cure for it?

I found this and looking at it maybe the answer ... Has anyone seen this problem ? Your fix ?

 

You should get that silicone grease off, it will kill your catalytic converter.

 

The gas cap is supposed to move after it is closed. That is what the clicking is. You should always do 3 clicks, it will continue to click if you continue trying to turn it.
Usually cleaning the metal mating surface with a scrubby and the rubber with alcohol does the trick. I have extended the life of the gasket by reversing it.

Have you been looking at "historic/permanent" codes? Those are only for information that they once existed, they go away after about 100 starts. The only codes that matter ore "current".

How would a state inspector test your gas cap? I never heard of a test other than the OBD2 test.

 

donbrew- reply... Yes, I do the 3 clicks but some say do only one and some say do 4 clicks. Like most thing the answers are all over. That is a good trick to extend the life of the gasket by reversing it.
May be the codes showing up now are old and just now coming up as the "check gas cap" has been coming up last 2 weeks or so.

Code is by the scanner from the auto store and are not the "historic/permanent" codes.

The state inspector test your gas cap as part of the yearly car inspection. It is part of the computer test equipment that goes to the state office in charge. It goes to a vac. pump that (once removed) the
cap is hooked-up and a vac. applied and it has to hold for a set time to pass. Again it is part of the test / computer equipment which goes/hooks to the state ... so the station can't cheat ... who would think.

PS... the rad. fan is doing lots better and not staying on all the time or lots of the time... I would say it's acting as normal .

 

If you have the original cap 3 clicks is printed on it, or maybe it is on the fuel door. Doesn't really matter, it is "suspenders and belt", the cap does not click unless the seal is engaged.
I never heard of a state that spends all of that time testing gas caps. Most states test by Plugging an OBC2 reader in, it tells you if it is leaking or not.
The kids at the auto parts store don't have a clue what the OBD2 reader is doing. They only know how to use it to make a sale; if a part name is in the description of the test failed then that part must be replaced. They have sold thousands of crankshaft sensors when the code is indicating bad timing chain.

 

Update... Still on the code P0455 EVAP system (large EVAP leak). Got a Smoke Machine and going from the back side of the purge valve line going back to the tank area, the only smoke is from the vent pipe from the charcoal canister... it (vent pipe) goes from the canister and follows the gas filler pipe up and ends along side the top of the filler pipe just to the rear of it. I checked it with the engine cold (vent valve open) and with engine hot (vent valve closed) smoke come out of this vent pipe both ways!

Have replaced the purge valve, charcoal canister, and vent valve. I tried putting the stock vent valve back on to see if it would stop the smoke from coming out the vent pipe, it did not!

I took off the gas cap to see if any smoke had gone through the tank and up to the gas cap... I did see a small amount of smoke coming up the gas filler neck... so the large leak has to be from the vent pipe!

Don't understand how or when the vent pipe closes as if it was open all the time you could not test the system. One would think it would be closed when the vent valve was closed but it is not. You can
hear the vent pipe venting after the engine warms up... it is a popping sound you can hear when opening the gas fill door.

For sure when smoke testing the EVAP system, smoke is coming out of the vent pipe BIG TIME!!! Got to find the problem soon as it drinking gas very fast.

 

The vent closes when the engine starts and opens periodically based on vacuum. It lets air in so gas can go out.