See Figures 1, 2, 3 and 4
The heart of this system is a charcoal canister located in the engine compartment. Fuel vapor that collects in the carburetor float bowl and/or gas tank is stored in the canister instead of being released into the atmosphere.
In order to restore the ability of the charcoal to hold fuel, fresh air is drawn through the canister under certain operating conditions, drawing the fuel vapors back out and introducing them into the combustion chambers.
At idle speed, or when the engine is cold, the addition of any fuel vapor to the correct mixture would cause excessive tailpipe emissions. For this reason, a port in the carburetor or fuel injection system throttle body allows the fuel vapors to be drawn out of the canister only after the throttle has been opened past the normal idle position. If there is no vacuum, the canister purge valve remains closed.
The flow of air and fuel are further restricted by a thermal valve when the engine is cold. This valve prevents the vacuum signal from going to the canister purge valve until the engine reaches a pre-determined temperature.
When the canister purge valve opens, air is drawn under slight vacuum from the air intake. If the engine exhibits operating problems during warm-up and basic fuel system and engine tune-up adjustments are correct, check the thermo valve for proper operation.
Purge Control System
See Figure 5
The purge control system controls the flow of the fuel fumes from the evaporative canister. Check the entire system as a whole first, then, if the system exhibits signs of incorrect functioning, check each of its components to find the exact problem with the system.
- Disconnect the black vacuum hose (2.4L, 3.0L and 3.5L engines' vacuum hose has two red stripes) from the intake manifold and plug the nipple.
- Connect a hand vacuum pump to the disconnected vacuum hose.
- Check the following figures both when the engine is cold (engine coolant temperature is 122°F/50°C or less) and when it is hot (185-205°F/85-95°C).
On 2.0L and 2.6L engines:
- When the engine is cold, the engine is run at 2,500 rpm and the amount of applied vacuum is 15.7 in.Hg. (53 kPa), the vacuum should not leak.
- When the engine is hot, the engine is idling and the vacuum is the same as above, the vacuum should also not leak.
- When the engine is hot, the engine rpm is 2,500 and the same amount of vacuum is applied, the vacuum should leak down. If the system does not do this, check each component to find the faulty part.
On 2.4L, 3.0L and 3.5L engines:
- When the engine is cold, the engine is idling and 15.7 in.Hg. (53 kPa) of vacuum is applied, the vacuum should not leak. Run the engine at 3,000 rpm and check again. The vacuum should still not leak.
- Run the engine until hot and then shut the engine OFF .
- Restart the engine. With the engine hot, idling and the same amount of vacuum applied as above, the vacuum should not leak. Within 3 minutes of starting the engine, run the engine at 3,000 rpm and try to apply vacuum. The vacuum should leak. Then, after three minutes have passed, once again apply the vacuum at 3,000 rpm. The vacuum should be maintained momentarily, after which it should start to leak. The vacuum will leak continuously if the altitude is 2,200 m (7,200 ft.) or higher, or the intake air temperature is 50°C (122°F) or higher.
- Reconnect the vacuum hose.
See Figure 6
The 2.0L and 2.6L engines utilize a vacuum controlled purge control valve. First check all the hoses and connections for proper attachment, cracks, bends and leaks. Many problems relate simply to poor mechanical connections within the system or restricted hoses.
Four cylinder Pick-up and Montero engines require the removal of the valve to test it. Label each hose before removal; correct re-connection is essential. With the valve off the car, connect a hand vacuum pump to the bottom vacuum nipple of the valve. Apply a vacuum of 15.7 in. Hg. (53 kPa) for to check for air tightness. The valve should hold the vacuum. Release the vacuum and blow gently into the canister side hose port. With no vacuum applied, no air should pass through the valve. Now draw a vacuum of at least 8 in. Hg. (27 kPa) and blow into the port again. Air should flow through the valve.Evaporative Emission Purge Solenoid
See Figures 7 and 8
The 2.4L, 3.0L and 3.5L engines control the canister through the Evaporative Emission Purge Solenoid (EEPS) valve. This can be checked in place on the firewall. With the engine off, label and disconnect the two vacuum hoses running to the valve. One hose will have a red stripe on it; take note of which port it was connected to. Remove the electrical connector from the valve. Connect a hand vacuum pump to the port which contained the hose with the red stripe. Draw a light vacuum on the pump; no vacuum should flow (the system holds vacuum).
Using jumper wires, connect battery voltage to the terminals on the solenoid valve. Make absolutely CERTAIN that the polarity of the wiring is correct. The (+) side of the battery must be connected to the terminal which forms the top of the "T" in the pattern of the terminals. Once the solenoid is energized, vacuum should leak (or flow) when the hand pump is used. Disconnect the jumpers and the vacuum pump. Use an ohmmeter to check the resistance across the terminals of the solenoid valve. Correct resistance is 36-44 ohms at 20°C (68°F).
Resistance will change with temperature. Make common sense allowances for temperature variation.
See Figures 9, 10 and 11
On 2.4L, 3.0L and 3.5L engines it is necessary to check the purge port with a hand vacuum pump to check for obstructions in the port itself. Make sure the vehicle is warmed up. Disconnect the vacuum hose from the throttle body purge hose nipple and connect the hand vacuum pump to the nipple. Start the engine. For all vehicles except those equipped with a pre-1993 2.4L engine, check to see that, after raising the engine speed by racing the engine, vacuum rises proportionately with the rise in engine speed. On pre-1993 2.4L engines, check to see that, after raising the engine speed by racing the engine, vacuum remains fairly constant. If there is a problem with the outcome of this test, it is possible that the throttle body port may be clogged and require cleaning.
See Figure 12
The thermo valve found on all pre-1990 4-cylinder Pick-up and Montero engines is located on the intake manifold or cylinder head where its bottom will be immersed in coolant. The 2.4L, 3.0L and 3.5L engines control the purge system through the ECM. The thermo valve blocks or passes vacuum to the purge control valve depending on the coolant temperature. Test after the engine has cooled overnight.
Label and disconnect the two vacuum hoses on the thermo valve. With the engine turned OFF , connect a hand-held vacuum pump to the upper port on the two port valves. Apply vacuum and confirm that the valve leaks or does NOT hold vacuum. Start the engine and allow it to warm up. When the coolant has reached normal operating temperature, disconnect the appropriate hose as before and repeat the test; the valve should hold vacuum and not leak.
Disconnect all of the vacuum hoses from the thermo valve. Connect the hand vacuum pump to nipples (B), (C) and (D) one at a time; plug the nipples not being tested at the time. Apply vacuum to check the valves condition. The first check should be done with the coolant 104°F (40°C) or less. In this case the valve should not hold the vacuum. On the second test of each nipple the engine coolant should be at least 176°F (80°C) and the valve should hold vacuum. If the valve failed either of these tests it needs to be replaced with a new unit.
If it becomes necessary to replace the thermo valve, do so only on a cold motor. Fit the wrench only onto the faceted base of the valve, never on the plastic parts. When installing the new unit, apply sealer such as 3M®No. 4171 or equivalent to the threads and tighten the new unit to 22 ft. lbs. (30 Nm).
2-Way Overfill Limiter
This device, sometimes mistaken for a fuel filter, is found in the vapor line running from the tank to the canister. Usually located at or near the tank, this valve is both a pressure and suction-sensitive unit. Its purpose is to compensate for the pressure changes within the fuel tank. Since the filler cap is tight enough to be considered sealed, the pressure must be equalized somewhere within the system.
When the pressure builds within the tank, such as on a very hot day or after a long period of driving, the valve releases the pressure and vapor into the charcoal canister, thereby venting the tank without raising emissions. Conversely, should the tank develop a vacuum, the valve will bleed some air (and vapor) from the canister into the tank.
The control pressures within the valve are pre-set and not adjustable, but a quick check can be performed as follows:
- Look at the valve and observe which end is toward the tank. Label or diagram the correct position.
- Remove the valve from the vapor line. It may be necessary to remove other obvious components such as a parking brake cable bracket for access.
- Lightly blow through either end of the valve. If air passes after some resistance, the valve is in good condition.
- Install the valve into the line in the correct direction and secure the clamps. Make certain the lines are firmly seated on the ports before installing the clamps.
Usually mounted near the fuel tank, this simple valve is found in the vapor line coming from the tank to the charcoal canister. Normally the line carries vapor which is easily absorbed and held by the charcoal in the canister. If the car rolls over, the line would fill with liquid gasoline and exceed the canister's ability to absorb fuel. Once saturated, the canister would allow the liquid fuel to run out, possibly onto the hot surfaces of the engine. Since an engine fire is the last thing you want when your car is on its head (or any other time), the roll-over valve will block the vapor line and keep the fuel out of the canister.
This valve rarely, if ever, fails. If it must be checked, unbolt it and remove the hoses. Shake it-if it rattles, it's OK. Since even the simplest job can be done incorrectly, make sure the valve is reinstalled right side up. Connect the hoses firmly and install the clamps.Charcoal Canister
Since the canister cannot be tested on the workbench, it should be checked periodically for cracks, obstructions and proper hose connections. Check the maintenance schedule for your car; many models require replacement of the canister after a period of years or miles/kilometers. Additionally, the canister should be considered suspect on a carbureted car after any incident of severe engine flooding. It is possible to deliver enough fuel vapor from the carburetor to overcome the capacity of the charcoal. This can result in the air/fuel mixture becoming too rich and causing driveability problems. If a high mileage carburetor is overhauled or replaced, a new charcoal canister should be installed as well.
When working around the canister, remember to label or diagram every hose before removal. Vacuum and vapor must flow correctly if the system is to work properly.