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Join Date: Sep 2012
Location: Ventura Co, California USA
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Thanks for the comments.
Yes, the smog test is less about testing for emissions than simply making old vehicles too expensive, so everyone will buy new ones. One the other hand, I am old enough to remember when the air was brown, so good things are coming out of this bureaucracy run amok.
I was actually able to solve my problem and passed the test this weekend. I was considering writing up a report on the test scheme I cooked up. For general lack of interest, I guess I'll just describe it instead.
The tank you're referring to is dry nitrogen. They use it to pressurize the gas tank for leak-down measurement. They also carefully measure how much nitrogen it takes, since the air volume affects how fast pressure will drop for a given leak rate.
It turns out that the official test is performed by pressurizing the tank to 2 psi (18" to 22" H2O) with the vent line capped at the charcoal canister, and seeing that the leak rate is within spec (35 psi through a 0.050" aperture). That's a damn useless leak-rate spec for a home mechanic, but what I learned is that after repair, my tank held 2 psi for 30 minutes without any noticeable drop in pressure.
On these trucks, there are three lines from the gas tank. The fuel feed line, the fuel pump return line, and the vent line. My test involved lifting the vent hose from the charcoal canister and connecting a vacuum gauge (also reads low pressure). I disconnected the fuel pump return hose and blew in compressed air, then quickly reconnected it. The half second of switch-over from compressor to the fuel pump didn't let too much air leak out. Then I just recorded the pressure every few minutes to determine the leak rate.
I fiddled with so many things as I was figuring out my test, it's impossible to say what was really bad and which action fixed it. Among the things I did were clear an obstruction in the vent line with high pressure air, changed a lightly cracked return hose, oiled the filler cap relief valve, and removed, inspected and refitted as many of tank hoses as I could reach without dropping the tank.
In short, if the tank can hold 2 psi for 30 minutes with vent line capped, it will pass. Maybe this tidbit will help the next guy someday.
Cheers, and thanks for your input.
Last edited by Wekadog; 04-25-2019 at 12:57 AM.