Mitsubishi Montero (1991+). Manual - part 104

converter with the exception of the catalyst. See  Fig. 5. The TWC

converter uses rhodium, with or without platinum, as its catalyst.

Rhodium helps reduce NOx emissions, as well as HC and CO.

         Visually check for presence of catalytic converter(s). Also

check for presence of any required air supply system for the oxidizing

section of the converter. Check for external damage such as severe

dents, removed or damaged heat shields, etc. Check for pellets or

pieces of converter in the tailpipe.

         Three-Way Catalyst + Oxidation Catalyst (TWC + OC)

         This system contains a TWC converter and an OC converter in a

common housing, separated by a small air space. See  Fig. 6. The 2

catalysts are referred to as catalyst beds. Exhaust gases pass through

the TWC first. The TWC bed performs the same function as it would as a

separate device, reducing all 3 emissions. As exhaust gases leave the

bed, they pass through the air space and into the second (OC)

converter catalyst bed.

         Visually check for presence of catalytic converter(s). Check

for external damage such as severe dents, removed or damaged heat

shields, etc. Check for pellets or pieces of converter in the

tailpipe.

Fig. 6:  Typical Three-Way + Oxidation Catalytic Converter

Courtesy of General Motors Corp.

         FILL PIPE RESTRICTOR (FR)

         A fuel tank fill pipe restrictor is used to prohibit the

introduction of leaded fuel into the fuel tank. Unleaded gasoline pump

dispensers have a smaller diameter nozzle to fit fuel tank of vehicle

requiring the use of unleaded fuel (vehicles equipped with catalytic

converter).

         Visually inspect fill pipe restrictor(s) for tampering, i.e.,

restrictor is oversize or the flapper is non-functional. If vehicle is

equipped with an auxiliary fuel tank, ensure auxiliary fuel tank is

also equipped with a fill pipe restrictor.

         EXHAUST GAS RECIRCULATION (EGR) SYSTEM

         Single Diaphragm EGR Valve

         This type uses a single diaphragm connected to the valve by a

shaft. Diaphragm is spring-loaded to keep valve closed in the absence

of vacuum. As throttle valves open and engine speed increases, vacuum

is applied to the EGR vacuum diaphragm, opening the EGR valve. This

vacuum signal comes from a ported vacuum source. Variations in the

vacuum signal control the amount of exhaust gas that is recirculated.

See Fig. 7.

         Verify EGR valve is present and not modified or purposely

damaged. Ensure thermal vacuum switches, pressure transducers, speed

switches, etc., (if applicable) are not by-passed or modified. Ensure

vacuum hose(s) to EGR valve is not plugged.

Fig. 7:  Typical Single Diaphragm EGR Valve

Courtesy of General Motors Corp.

         Dual Diaphragm EGR Valve

         This type uses 2 diaphragms with different effective areas

and 2 vacuum sources. Although similar to the single diaphragm type,

the second diaphragm is added below the upper diaphragm and is rigidly

attached to the valve seat. See Fig. 8. These diaphragms form a vacuum

chamber which is connected to manifold vacuum.

         During highway cruising when manifold vacuum is high in the

center chamber, manifold vacuum tends to pull the valve closed.

However, the vacuum signal applied to the top side of the upper

diaphragm overcomes the downward spring force and the manifold vacuum

pull, due to the diaphragm’s larger piston. This regulates the amount

of EGR.

         When manifold vacuum is low during acceleration, the higher

vacuum signal opens the valve, permitting more EGR. When manifold

vacuum is high during highway cruising, the valve is only partially

opened, reducing the amount of EGR.

         Verify EGR valve is present and not modified or purposely

damaged. Ensure thermal vacuum switches, pressure transducers, speed

switches, etc., (if applicable) are not by-passed or modified. Ensure

vacuum hose(s) to EGR valve is not plugged.

Fig. 8:  Typical Dual Diaphragm EGR Valve

Courtesy of General Motors Corp.

         Positive Backpressure EGR (BP/EGR) Valve

         This type uses both engine vacuum and exhaust backpressure to

control the amount of EGR. It provides more recirculation during heavy

engine loads than the single diaphragm EGR valve.

         A small diaphragm-controlled valve inside EGR valve acts as a

pressure regulator. The control valve gets an exhaust backpressure

signal through the hollow valve shaft. This exhaust backpressure

exerts a force on bottom of control valve diaphragm. The diaphragm

plate contains 6 bleed holes to bleed air into the vacuum chamber when

backpressure valve is in open position. See  Fig. 9.

         Verify EGR valve is present and not modified or purposely

damaged. Ensure thermal vacuum switches, pressure transducers, speed

switches, etc., (if applicable) are not by-passed or modified. Ensure

vacuum hose(s) to EGR valve is not plugged.

Fig. 9:  Typical Positive Backpressure EGR Valve

Courtesy of General Motors Corp.

         Negative Backpressure EGR (BP/EGR) Valve

         This type has the same function as the positive BP/EGR valve

except valve is designed to open with a negative exhaust backpressure.

The control valve spring in the transducer is placed on the bottom

side of the diaphragm. See Fig. 10.

         When ported vacuum is applied to the main vacuum chamber,

partially opening the valve, the vacuum signal from the manifold side

(reduced by exhaust backpressure) is transmitted to the hollow stem of

the valve. See Fig. 10. This enables the signal to act on the

diaphragm, providing a specific flow. Thus, the EGR flow is a constant

percentage of engine airflow.

         Verify EGR valve is present and not modified or purposely

damaged. Ensure thermal vacuum switches, pressure transducers, speed

switches, etc., (if applicable) are not by-passed or modified. Ensure

vacuum hose(s) to EGR valve is not plugged.