Gen I Systems

Components
How the System Works
How the Self-Test Works
Bad Graphs
Diagnostic Tips

Components:
Gen I is absurdly complex and unreliable, and the fact that Toyota got rid of it eventually is a testament to common sense. It consists of several components:

A charcoal canister, which would trap vapor from the fuel tank.

Canisters were typically found under the vehicle, towards the back, between the fuel tank and the underside of the trunk pan. On very old vehicles, you may even see them up front, under the hood near the brake master cylinder.

A purge valve, which would allow the trapped vapor into a the intake to be burned with the rest of the gas. When the valve is open, it is sometimes known as a purge event or purge.

The valve is sometimes just known as the evap VSV (vacuum switching valve).

On the V6s, these would typically be under a cover attached to the plenum. On the V8s, they'd typically be on the passenger side (right) of the plenum or on top of an engine compartment-mounted canister. On the 4 cylinders, they could be in either spot.

A vent valve, or fresh air valve, which would allow air from the outside to replace the vapor lost during a purge event.

Typically on or near the canister. On the V6s, they're attached to the plenum under the plastic cover, with a set of hoses that link it from the airbox to the canister.

Also known as the canister closed valve or VSV for CCV.

A tank bypass valve, which separate the tank and canister during testing, to essentially allow each component to be tested individually.

It's typically on the canister, or mounted between the tank and the canister.

It's also known as a pressure switching valve.

A vapor pressure sensor, which was built onto the tank.

About a million little rubber vapor hoses which were all prone to cracking and creating tiny leaks everywhere that are impossible to trace.

A gas cap, which sealed the fuel tank and prevented fuel vapor from escaping into the atmosphere.

If you don't know where to find this, there's not much help I can give you, bud. Maybe think about a different line of work. Something you can ride the bus to get to.

How it works
This system is also known as a 'key on' evap inspection system, because the test can only run when the engine is running? Why, you may ask? Well, they hadn't figured out how to put a simple little vacuum pump on the canister for testing yet, so it required the full vacuum of the engine to test for leaks. By introducing engine vacuum into the evap system and sealing it, the vapor pressure sensor could measure how quickly the vacuum was lost. Too quickly, and it would indicate a leak. However, for us mere mortals, it was incredibly difficult to tell the threshhold for 'too quickly', which is what made these things such a pain to diagnose. The very sensitive components, combined with the million little hoses that would crack with age, combined with the very slow refresh rate of the computers made diagnosing these things a royal pain in the ass.

At any rate, what would happen is that the engine would be running at operating temperature for a predetermined period of time. At some point, the evap system would open the vent valve and the purge valve at the same time. If everything was working correctly, the system would read a drop of about 6mm/hg. It couldn't pull down more than that because even though the engine vacuum was introduced to the whole system, the canister vent was wide open, allowing air to come in and replace the lost vapor. Once the system sensed that the correct amount of pressure drop was accomplished, it would shut the purge valve and allow the tank and canister pressure to equalize.

Next, it would close the vent valve and open the purge again. Now full engine vacuum was introduced into the system, but with the vent closed, the pressure would drop deeper than -20mm/hg. Because of the very deep vacuum, the purge valve would shut as soon as it hit this level, because constantly applying that level of vacuum would damage components. At this point, both ends of the evap system (purge and vent) would be closed, and system vacuum would be around -25mm/hg if everything was working correctly.

If there were no leaks, the thought was that although there would be some loss of vacuum, the loss would occur very slowly. After some period of time (which has never really been determined), the system would see 1) how slowly the vacuum was lost, and 2) that it never quite made it back to atmospheric pressure, which would read "0" on a gauge. Once a determination had been made, it would open the vent and purge to drop it about -6mm/hg again, just to make sure all those components were still good.

While it was doing this, it would shut the tank bypass valve. I have no idea why on earth this mattered, but for whatever reason, the system will test that the canister and tank can be effectively separated. With the tank bypass valve shut, it would open the purge valve. If the tank bypass valve was working correctly, none of the vacuum being applied to the canister would leak into the tank, so the pressure on the vapor pressure sensor would continue to slowly rise. Nobody has explained to me at any point why anyone should give a shit about this. Toyota obviously didn't think it was important enough to keep on the Gen II evap systems, but whatever.

How the self-test works

Note: the system only works when the engine is at operating temperature and the tank is between 1/4 and 3/4 full. If these conditions are not met, the test will not run.

In step 1, you can see the VSV for Evap is closed, meaning no engine vacuum is being applied to any of the evap components. The tank should read a slight positive pressure.

In step 2, the system opens the VSV for Evap (purge), but leaves the CCV (vent valve) open and the pressure switching valve (tank bypass valve) closed. On some cars, it will keep the tank bypass valve open to apply vacuum through the entire system.

In step 3, the CCV will shut. This will prevent any fresh air from entering. Because the purge valve is open, the evap system draws into a deep vacuum. Usually this will happen very suddenly and the system will automatically close the purge valve to prevent damage. You have to manually advance to the next step when it does this.

In step 4, all the valves are closed and the system is under vacuum. The vacuum should very slowly release on its own, but never make it up to atmospheric pressure (in other words, it should slowly rise but always be negative).

Other common (but bad) graphs:

Under Construction

Diagnostic tips

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