Foreign Service

Motor,  Jan 2006  by Marinucci, Dan

• E-mail
• Print
• Link

Evaporative emissions system repairs are something your shop can't afford to avoid. Dan begins a series on Toyota's major evap systems to remove the intimidation factor.

Many technicians have told me that Toyota's evaporative emissions systems are too confusing. Consequently, they avoid working on them. It's a shame because there are so many Toyota products on the road and someone has to fix these vehicles.

At the same time, I sympathize with these techs. Toyota evap systems can look very intimidating to anyone who's accustomed to working on domestic vehicles. Therefore, this month begins a multipart series on operation and diagnosis of the three most popular Toyota evap systems. For many MOTOR readers, the easiest way to learn the topic is to compare and contrast Toyota setups with, for instance, common Ford and General Motors evap systems. I'll do just that whenever it's appropriate.

Whereas this Toyota evap series is not a complete substitute for a pile of shop manuals and/or in-depth factory training, I trust it will dispel those initial fears of working on something different. Make sure you keep all your MOTOR issues handy because space limitations require me to break up the topic into bite-size chunks. That also means that sometimes I'll have to refer you from a current column back to a topic in a previous one.

The first of these evap setups I'll cover is one that's commonly called the early or nonintrusive system. It first appeared in 1997. A typical domestic enhanced evap system has a green service port. One of the confusing things we've learned firsthand about this early Toyota evap system is that some vehicles have the familiar green service port but many do not. So don't get concerned if you notice the service port is missing from many popular Toyotas.

A common domestic enhanced evap system has a purge solenoid valve under the hood. As I explained in a MOTOR article in July 2003, the purge solenoid functions as the front door to the entire evap system. Electrically, this solenoid valve is normally off (deenergized) and mechanically the valve is normally closed. The ECM or PCM pulses this solenoid on and off-as opposed to turning it full on-whenever it decides to purge the system. One side of the purge solenoid valve is plumbed to manifold vacuum; the other side goes to the charcoal canister.

All of this applies to Toyota's nonintrusive system. However, Toyota likes to call these components vacuum switching valves (VSVs). So the front door of this entire evap system is labeled vapor purge VSV, or purge VSV. (The original equipment scan tool identifies it as an EVAP VSV.) The purge VSV is in the foreground of photo 1 on page 16.

The canister may be mounted inside the engine compartment, below the master cylinder. But on other nonintrusive evap systems, the canister is under the rear of the vehicle, usually right above the main cross-member. Photo 2 shows the valves and related plumbing on the end of a typical undercar canister on a nonintrusive system.

The typical Ford or General Motors enhanced evap system you service has a fuel tank pressure (FTP) sensor monitoring the positive or negative pressure inside the tank. Toyota's version of the FTP sensor is called a vapor pressure sensor and it operates like a 1980s GM vacuum sensor. That is, it's always vented to the atmosphere because it's constantly comparing atmospheric pressure to evap system pressure.

On some nonintrusive systems, the vapor pressure sensor is up on the firewall, a few inches from the power brake booster. Look closely at the top of a firewall-mounted Toyota vapor pressure sensor and you'll see a little dimple or indentation. There's a tiny vent hole in that dimple; make sure it's clean.

But very often you'll find this pressure sensor hanging on a canister bracket back under the vehicle. The vapor pressure sensor, which has two hoses connected to it, can be seen in the upper left of photo 2. Its pressure-sensing hose connects to one of the ports on the VSV in the upper right; its other hose is teed into a hose leading to the atmosphere.

Here's a major, major difference between Toyota's nonintrusive evap and a common Ford or GM setup: The FTP sensor on a common domestic system always senses the pressure of the entire evap system. But the Toyota vapor pressure sensor senses either the canister side or tank side of the system; it can't sense both at once! That's because a VSV is spliced into the pressure-sensing hose between the sensor and the rest of the system (see the illustration above). Normally, the vapor pressure VSV is electrically off, or deenergized. When this VSV is off, it connects the vapor pressure sensor to the canister. When the ECM decides to leak-check the fuel tank side of the system, it turns on the vapor pressure VSV. Now the sensor senses fuel tank pressure.

So the computer is monitoring canister pressure when the vapor pressure VSV is deenergized and fuel tank pressure when it's energized. The vapor pressure VSV, which has a blue/green electrical connector on it, is visible near the firewall in photo 1 and in the upper right of photo 2.