GM Blazer/Jimmy/Typhoon/Bravada 1983-1993 Repair Guide

Evaporative Emission Control System (EECS)

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OPERATION



The Evaporative Emission Control System (EECS) is designed to prevent fuel tank vapors from being emitted into the atmosphere. Gasoline vapors are absorbed and stored by a fuel vapor charcoal canister. The charcoal canister absorbs the gasoline vapors and stores them until certain engine conditions are met, then the vapors are purged and burned in the combustion process.

The charcoal canister purge cycle is normally controlled either by a thermostatic vacuum switch or by a timed vacuum source, though a few later model vehicles may use electronic regulation in the form of a purge control solenoid. The thermostatic vacuum switch is installed in a coolant passage and prevents canister purge when engine operating temperature is below approximately 115°F (46°C). A timed vacuum source uses a manifold vacuum-controlled diaphragm to regulate canister purge. When the engine is running, full manifold vacuum is applied to the top tube of the purge valve which lifts the valve diaphragm and opens the valve. If equipped with a purge solenoid, under proper engine operating conditions the computer control module will signal the solenoid which will then open the vacuum line allowing manifold vacuum to control the purge diaphragm. Most solenoids used on these vehicles are normally closed and will open when the computer control module provides a ground signal energizing the solenoid.

Remember that the fuel tank filler cap is an integral part of the system in that it is designed to seal in fuel vapors. If it is lost or damaged, make sure the replacement is of the correct size and fit so a proper seal can be obtained.

A vent, located in the fuel tank, allows fuel vapors to flow to the charcoal canister. A tank pressure control valve, used on certain high altitude applications, prevents canister purge when the engine is not running. The fuel tank cap does not normally vent to the atmosphere but is designed to provide both vacuum and pressure relief.

Poor engine idle, stalling and poor driveability can be caused by a damaged canister or by split, damaged or improperly connected hoses.

Evidence of fuel loss or fuel vapor odor can be caused by a liquid fuel leak; a cracked or damaged vapor canister; disconnected, misrouted, kinked or damaged vapor pipe or canister hoses; a damaged air cleaner or improperly seated air cleaner gasket.

TESTING



Charcoal Canister and Purge Valve

See Figures 1 and 2

The fuel vapor canister is used to absorb and store fuel vapors from the fuel tank. Vacuum sources are generally ported, either through an internal or remote mounted purge control valve. Engines employing the timed vacuum source purge system usually use a canister purge valve which is integral to the vapor canister. The valve consists of a housing and tube molded into the canister cover, valve assembly, diaphragm and valve spring. The diaphragm cover has a built-in control vacuum signal tube.

The 2.2L diesel engine is not equipped with a charcoal canister.

  1. Remove the vacuum hose from the lower tube of the purge valve and install a short length of tube, then try to blow through it (little or no air should pass, though a small amount may pass if the vehicles is equipped with a constant purge hole).
  2.  
  3. Using a vacuum source such as a hand vacuum pump, apply 15 in. Hg (50 kPa) to the upper tube of the purge valve. The diaphragm should hold the vacuum for at least 20 seconds, if not replace the purge valve (remote mounted) or canister (internal mounted valve), as applicable.
  4.  
  5. While holding the vacuum on the upper tube, blow through the lower tube (an increased volume of air should now pass); if not, replace the valve or canister, as necessary.
  6.  



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Fig. Fig. 1: Common EEC system schematic - 2.5L and 4.3L (VIN Z, except Turbo) engines shown



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Fig. Fig. 2: Common EEC system for the 2.8L engine

When testing valves by blowing air through them, be careful that you are blowing in the proper direction of flow. Many valves are designed to only allow air to flow in one direction and a proper working valve may seem defective if it is tested with air flow only in the wrong direction.

Thermostatic Vacuum Switch (TVS)

The number stamped on the base of the switch (valve) is the calibration temperature.

  1. With engine temperature below 100°F (38°C), apply vacuum to the manifold side of the switch. The switch should hold vacuum.
  2.  
  3. As the engine temperature increases above 122°F (50°C), vacuum should drop off.
  4.  
  5. Replace the switch if it fails either test.
  6.  

A leakage of up to 2 in. Hg (6 kPa) in 2 minutes is allowable and does not mean that the valve is defective.

Canister Purge Control Solenoid

See Figure 3

As stated earlier, most solenoids found on these vehicles use a normally closed solenoid valve. This means that when the solenoid is de-energized it is closed or, when it is energized it will open allowing vacuum to pass. On most vehicles equipped with this solenoid, fused ignition voltage is applied to the solenoid through one of its terminals. When the ECM recognizes proper engine operating conditions, it will provide a ground through the other solenoid terminal in order to energize the solenoid.



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Fig. Fig. 3: Normally closed canister purge solenoid schematic - commonly found on California emissions vehicles

To test a normally closed solenoid valve, try blowing air through the valve fittings when the engine is OFF , air should not flow. When the engine is running the solenoid should de-energize during engine warm-up and energize once it has reached normal operating temperature and proper running conditions.

REMOVAL & INSTALLATION



Charcoal Canister
  1. For the MFI-Turbo, remove the air cleaner and disconnect the oil cooler pipes.
  2.  
  3. Tag and disconnect the hoses from the canister assembly.
  4.  

If access to the vapor hoses is difficult with the canister installed, wait until the canister is released from the bracket or mounting, then reposition the canister for better access.

  1. Loosen the screw(s) fastening the canister retaining bracket to the vehicle.
  2.  
  3. Carefully remove the canister or canister and bracket assembly, as applicable.
  4.  

To install:
  1. Position the canister in the vehicle.
  2.  
  3. If the hoses cannot be accessed once the canister is secured, connect them now as tagged during removal.
  4.  
  5. Install the canister to the retaining bracket and secure using the retaining screw(s).
  6.  
  7. If not done earlier, connect the lines to the canister assembly as tagged during removal.
  8.  
  9. For the MFI-Turbo, connect the oil cooler pipes and install the air cleaner.
  10.  

Thermostatic Vacuum Switch (TVS)

See Figures 4, 5 and 6

The TVS is located near the engine coolant outlet housing.

  1. Drain the engine cooling system to a level below the TVS.
  2.  
  3. Disconnect the vacuum hose manifold from the TVS. If the hoses are not connected to a single manifold, be sure to tag them before removal in order to assure proper installation.
  4.  
  5. Using a wrench, unthread and remove the TVS from the engine.
  6.  

To install:
  1. Apply a soft setting sealant to the TVS threads.
  2.  

DO NOT apply sealant to the sensor end of the TVS.

  1. Install the TVS and tighten to 120 inch lbs. (13 Nm).
  2.  
  3. Reconnect the vacuum hoses.
  4.  
  5. Properly refill the engine cooling system, then run the engine and check for leaks.
  6.  



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Fig. Fig. 4: Disconnect the vacuum hoses from the TVS



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Fig. Fig. 5: Loosen and remove the thermostatic vacuum switch using a wrench



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Fig. Fig. 6: Common TVS mounting

 
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