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Discussion Starter #1 (Edited)
My '84 Hilux with 22R California / 5-speed failed smog check yesterday for the evaporation emissions system. This is the check where they apply pressure at the filler and see that the system holds pressure (with the tank vent line capped at the charcoal canister).

I imagine the most likely cause is a cracked hose. I plan to replace them all and be done with it. That shouldn't be too difficult to deal with, but I do have a few questions.

(Q1) I tried blowing compressed air into the tank vent - the 1/4" hose from the top of the charcoal canister. I had hoped to hear the air escaping at the leak. That's not what happened. Instead, the hose appeared to be blocked. No air flow. After repeatedly blowing air alternately in the tank vent and gas return lines, a little air began to flow in the vent line. No where near a freely as the gas return, though. Is restricted air flow normal on the tank vent?

(Q2) I visually traced the two lines (tank vent and gas return) to the back of the undercarriage, where they seemed to disappear near the front/top of the fuel tank. I couldn't get a hand or a line of sight on them. Do I need to drop the tank to access these two lines?

There doesn't appear to be a massive leak. The tank holds pressure for the 30 seconds or so I hold the air gun on the hose. But just a slow leak down over several minutes is enough to fail.

Any advice on how to get the system in shape to pass smog test would be welcome. Thanks!
 

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Wekadog

Don't know if this will help or not.

I have a 1986 sr5 4wd. Had a smog test in CA a few years back. It failed first time. I was pissed to find out that smog station screwed up. Here is what happened to me.
After they told me I failed, I had to go and have what they call a smoke test to see where the issue was. Well, come to find out the second place I went to for a a test told me there
was nothing wrong and that they did not have to do a smoke test. I went back to the first tester (TEST ONLY, by the way) and come to find out these guys forgot to fill some cannister
that they use for the evap test. Forgot what he cannister was called and what they pit in it.. They retested and it passed. Only thing they gave me was a free 6.00 certificate.
Never went back to these guys. CA runs a racket on these smog tests. What a scam.

Jim
 

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Discussion Starter #3 (Edited)
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.
 
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