Chrysler Full-Size Trucks 1997-2000 Repair Guide

Evaporative Emission Controls

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Operation



Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air that escapes contains gasoline vapors.

The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through vent hoses or tubes to a charcoal-filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold.

The PCM determines when these vapors will be passed into the intake manifold.

EVAP Canister

A sealed, maintenance-free evaporative canister is used (some RAM trucks have two). The canister is mounted under the vehicle on the frame rail behind the cab. It may be on either side (or both sides) of the vehicle depending on model and year.

The canister is filled with granules of an activated carbon mixture, which absorb fuel vapors.

Purge Solenoid

All gasoline engines use a duty cycle purge system. The PCM controls vapor flow by operating the duty cycle EVAP purge solenoid. This regulates the rate of vapor flow from the EVAP canister to the intake manifold.

The PCM regulates the solenoid by switching the ground circuit on and off based on engine operating conditions. When energized, the solenoid prevents vacuum from reaching the canister. When not energized the solenoid allows vacuum to flow through to the canister.

During warm up and for a specified time after hot starts, the PCM energizes (grounds) the solenoid preventing vacuum from reaching the canister. When the engine temperature reaches the operating level of about 120ºF (49ºC), the PCM removes the ground from the solenoid, allowing vacuum to flow through the canister and purges vapors through the throttle body. During certain idle conditions, the purge solenoid may be grounded to control fuel mix calibrations.

After the engine reaches operating temperature, the PCM energizes and de-energizes the solenoid 5-10 times per second, depending on operating conditions. The PCM varies the vapor flow rate by changing solenoid pulse width, which is the amount of time the solenoid energizes.

Fuel Tank Cap

The fuel tank is sealed with a pressure-vacuum relief filler cap. Under normal operating conditions, the filler cap operates as a check valve, allowing air to enter the tank to enable fuel flow. At the same time, it prevents fuel vapors from escaping through the cap to the atmosphere. These vapors are therefore forced into the EVAP canister.

The relief valves in the cap are a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.

Leak Detection Pump

Certain emissions packages have a Leak Detection Pump (LDP) is used to monitor the EVAP system for leaks. On most models so equipped, this unit is located beneath the battery tray. A test port for pressurizing the EVAP system is included. The test port is used to pressurize the system with a special gas and serious precautions must be taken to avoid damage to the EVAP system and the fuel tank. This is a procedure best suited to a professional shop, due to the precautions and the equipment needed to test this system. The PCM can store trouble codes for EVAP system performance.

Rollover Valves

Two-door models have one rollover valve located on top of the fuel tank. Four-door models have two valves also located on top of the fuel tank, although configuration may differ by model. Some vehicles have valves mounted at front and rear of the tank, while others may have one of the valves located atop the fuel pump.

Rollover valves prevent fuel flow through the EVAP hoses in the event of an accidental vehicle rollover. The EVAP canister(s) draw vapor from the gas tank through the valve(s).

Tank-mounted rollover valves are not serviceable and can only be replaced with the tank. Pump-mounted valves can be serviced separately.



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Fig. EVAP system incorporating leak detection pump



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Fig. Typical EVAP canister mounting-Dakota, Durango shown



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Fig. EVAP canister mounted on a 97 RAM truck



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Fig. Another EVAP mounting configuration-99 RAM truck



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Fig. Purge solenoid and LDP installation-RAM truck shown



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Fig. Tank-mounted rollover valves



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Fig. Fuel pump-mounted rollover valve



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Fig. A typical fuel tank filler tube cap

COMPONENT TESTING




NOTE
To relieve fuel tank pressure, the filler cap must be removed before disconnecting any fuel system component.

Canister Purge Solenoid
  1. Locations vary depending on model and year. Typical locations include the firewall (left or right side) and the rear of the engine near the firewall.
  2.  
  3. With the ignition off, unplug the connector on the EVAP solenoid.
  4.  
  5. Turn ignition on, measure the voltage at the ignition switch output line, voltage should be 10.0v or more. If the voltage reading is not 10.0 volts or more, repair the circuit from the ignition switch to EVAP solenoid.
  6.  
  7. Disconnect the negative battery cable.
  8.  
  9. Disconnect the PCM harness from the PCM.
  10.  
  11. Check the resistance of the EVAP solenoid control circuit between the PCM harness connector and the EVAP solenoid connector. Resistance should be less than 5.0 ohms; if not, repair the opening in the circuit.
  12.  
  13. Connect the negative battery cable.
  14.  



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Fig. Purge solenoid mounted on V-8 engine



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Fig. Purge solenoid installation on 2.5L engine-equipped vehicles



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Fig. EVAP solenoid circuits and schematic



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Fig. EVAP solenoid electrical connector

REMOVAL & INSTALLATION




NOTE
To relieve fuel tank pressure, the filler cap must be removed before disconnecting any fuel system component.

Evaporative Canister
  1. Disconnect the negative battery cable.
  2.  
  3. Raise and support the vehicle.
  4.  
  5. Label and disconnect the hoses on the top of the canister.
  6.  
  7. Remove the mounting bolts or nut(s).
  8.  

To install:

  1. Install and tighten the mounting hardware to 7 ft. lbs. (9 Nm), if two nuts or bolts are used. Tighten to 18 ft. lbs. (24 Nm), for single fasteners.
  2.  
  3. Install the hoses in their proper locations.
  4.  
  5. Lower the vehicle.
  6.  
  7. Connect the negative battery cable.
  8.  



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Fig. Mark the lines connected to the EVAP canister before removal to ease installation

Canister Purge Solenoid
  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect the electrical wiring from the solenoid.
  4.  
  5. Disconnect the vacuum lines from the solenoid.
  6.  
  7. Remove the solenoid or solenoid with support bracket as an assembly if this is easier.
  8.  

To install:

  1. Install the solenoid or support bracket.
  2.  
  3. Connect the vacuum lines.
  4.  
  5. Connect the wiring harness.
  6.  
  7. Connect the negative battery cable.
  8.  

Leak Detection Pump (LDP)
RAM TRUCKS
  1. The Leak Detection Pump (LDP) and filter are attached to a bracket mounted to the right side inner fender. LDP and filter are serviced as a single assembly.
  2.  
  3. Remove the LDP filter hose.
  4.  
  5. Remove the filter mounting bolt and remove the filter from the vehicle.
  6.  
  7. Disconnect the vacuum lines at the LDP.
  8.  
  9. Disconnect the LDP wiring.
  10.  
  11. Remove the mounting screws and remove the LDP from the vehicle.
  12.  

To install:

  1. Reverse the removal procedure. Be sure all connections are TIGHT.
  2.  

DAKOTA, DURANGO
  1. The Leak Detection Pump (LDP) is located in the engine compartment under the battery tray and PDC. The LDP filter is attached to the outside of the battery tray. LDP and filter are serviced as a single assembly.
  2.  
  3. Disconnect and remove the battery.
  4.  
  5. Disconnect the hose from the bottom of the LDP filter.
  6.  
  7. Disconnect the battery temperature sensor pigtail wiring harness at the bottom of the battery tray.
  8.  
  9. Remove the PDC-to-fender mounting screw at the rear of the PDC. Unsnap the PDC from the battery tray. To prevent damage to the PDC wiring, carefully position the PDC to gain access to the LDP.
  10.  
  11. Remove the battery tray.
  12.  
  13. Disconnect the vacuum lines from the LDP.
  14.  
  15. Disconnect the LDP wiring.
  16.  
  17. Remove the three mounting screws and remove the LDP.
  18.  

To install:

  1. Reverse the removal procedure. Be sure all connections are TIGHT.
  2.  



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Fig. Battery (1) and LDP (2) location-Dakota, Durango



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Fig. LDP filter (1), hose (2), and battery tray (3)-Dakota, Durango

Rollover Valve
  1. Tank-mounted rollover valves are molded into the gas tank and are not replaceable. The following procedure refers to pump-mounted valves only.
  2.  
  3. Disconnect the battery negative cable(s).
  4.  
  5. Drain the fuel tank.
  6.  
  7. Remove the fuel tank from the truck.
  8.  
  9. Disconnect the line at the rollover valve.
  10.  
  11. The valve is kept by a rubber grommet. Pry one side upward and roll it out.
  12.  

To install:

  1. Use a new grommet.
  2.  
  3. Press the valve in using finger pressure only.
  4.  

 
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