I failed a smog check here in CA. There is a new test they do where they put 1psi of vacuum at the fuel inlet and the system has to hold that for a certain amount of time. So I've been going through and re-doing all fittings, cutting back hose and re-clamping. Still not keeping the vacuum. To test, I've been putting a vacuum hand pump/gauge on the "to tank" line that I pulled from that canister against the firewall on the passenger side.

Where do the other lines go on that canister? The other nipple says "purge", then there is a hose hanging from the bottom that is open and not connected. I tried clamping it off and still no luck. Where else can I check for leakage??

 

The bottom line is "open to the air," and the other connected line goes through a VSV to engine vacuum. So if you are unable to hold vacuum in the tank direction, then your problem is "that-a-way."

Do make sure that your fuel cap is good; it has to hold vacuum there (and they are cheap and easy to replace.)

Unfortunately, you may need to drop the tank to get a good look at the top. One possibility is that you have a lot of rust on the top-of-tank fittings, or in the fuel filler hoses. This is more common in the salt belt, but I doubt you know where your truck has been its entire life.

Ultimately, you may need to carefully cap off the various items on the fuel tank to see which one is the one leaking.

Alas, this problem is exactly the problem Calif's new Evap test is designed to catch.

 

1 PSI vacuum? didn't know vacuum exerted pressure...

most likely, the VSV for the purge line to the manifold is failing.

 

Well, I tried clamping off the purge line and same problem. I don't think there is a rust problem on the top-of-tank fittings, my dad bought this truck new and it has been in CA Bay Area its whole life so has zero rust. I guess I will try the gas cap, but I'm pretty sure one of the smog tests is on the gas cap, and it passed that one. I'm near done with fittings, and they all looked ok to begin with, so I'm kinda stumped.

 

the test facility should have given you an indication of the probable causes of failure even if it's a test-only station.

if you're spending money tracking the issue down without reaching a conclusion, call the bureau of automotive repair and talk to them.
http://www.bar.ca.gov/

 

Well, they told me how to test it myself after going through the system and checking for obvious stuff.

I've now gone and changed all of the spring clamps to screw-tightened clamps, but still no luck.

The thing that is wierd is that it holds NO vacuum at all. I can put my mouth on the "to tank" tube and draw air from it. Its not super easy, but if I suck on it, it equalizes a second later.

 

This may help some:
http://personal.utulsa.edu/~nathan-b...74layoutan.pdf

 

Can you have them do a "smoke test" on the system to see where it's at? I think the guys who did my test, said something about that if it ever failed.

 

Tell them they will see 1 psi of vacuum in their #ss after you pull your foot out........