Home ReviewsRefund
Home/Honda/Prelude/Honda Prelude III (1986-1989)/Repair manual/Testing & Diagnostics/Fuel Evaporation System
Contents Section: Testing & Diagnostics All sections

Fuel Evaporation System Honda Prelude III

Testing & Diagnostics 8 illustrations ~1756 words

DESCRIPTION

Evaporative system minimizes the amount of fuel vapor released into the atmosphere. All models are equipped with a charcoal canister, purge control valve, 2-way valve, and connecting hoses and lines.

Accord models use a thermo valve to control vacuum to purge control valve, while Civic models with Electronic Fuel Injection (EFI) use an Electronic Control Unit (ECU) and purge cut-off solenoid valve. EFI Accord and EFI Prelude models use a fuel cut-off valve and liquid/vapor separator to prevent liquid fuel from entering the 2-way valve.

Scheme 1

Scheme 1: DESCRIPTION
Scheme 1: DESCRIPTION

CARBURETED ENGINES

When fuel tank pressure exceeds a preset value, 2-way valve opens and allows excess vapor to vent into canister. If excessive vacuum forms in fuel tank, 2-way valve opens and outside air enters fuel tank through canister. Fuel filler cap acts as a safety valve if evaporation system fails.

Thermo valve closes and allows manifold vacuum to open purge control valve when engine coolant temperature exceeds preset temperature.

When engine is not running on Accord and Civic, air vent cut-off valve closes float bowl vent to secondary bore and opens vapor vent to canister. When engine is running, manifold vacuum opens air vent cut-off valve to secondary bore and closes off vent to canister. Vacuum holding solenoid stabilizes vacuum on air vent cut-off diaphragm.

Scheme 2

Scheme 2: CARBURETED ENGINES
Scheme 2: CARBURETED ENGINES

Scheme 3

Scheme 3
Scheme 3

When engine is not running on Prelude, air vent cut-off solenoid valve opens outer air vent passage. This allows float bowl vapor to vent into canister. When engine is running above 500 RPM, air vent cut-off solenoid valve and inner vent solenoid valve open float bowl inner and sub-inner air vent passages to air cleaner. On hot engine starts, when coolant temperature is above specified temperature, delay timer delays opening of inner air vent passage to ease hot engine start up.

When engine is not running on Accord and Civic, fuel cut-off solenoid valve shuts off main and slow primary fuel metering passages to carburetor bore. On Accord, when ignition is off, power valve fuel passage to carburetor bore is closed by power valve control solenoid and check valve.

FUEL INJECTED ENGINES

When fuel tank pressure exceeds a preset value, 2-way valve opens and allows excess vapors to vent into canister. If excessive vacuum forms in fuel tank, 2-way valve opens to allow outside air into fuel tank through canister. Fuel filler cap acts as a safety valve if evaporation system fails.

When coolant temperature is above a preset value, purge cut-off solenoid valve on EFI Civic or thermo valve on all other models, along with the ECU, directs manifold vacuum to purge control valve to draw vapor into throttle body port. When coolant temperature is below this temperature, no vacuum is applied to purge control valve.

Scheme 4

Scheme 4: FUEL INJECTED ENGINES
Scheme 4: FUEL INJECTED ENGINES

MAINTENANCE

Inspect system and test components every 60,000 miles or every 5 years. Check all hoses and lines for deterioration, damage, and obstructions.

THERMO VALVE & PURGE CONTROL VALVE

  1. Ensure vacuum lines are properly routed and are not damaged or obstructed. With engine coolant temperature below a designated thermosensor operating temperature, disconnect upper vacuum hose from purge control valve. Refer to «THERMO SENSOR OPERATING TEMPERATURE»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#fuel-evaporation-system) table for specific temperature.
  2. Connect vacuum gauge to hose. Start engine and let idle. Vacuum should not be present. If there is vacuum on EFI Civic models, test purge cut-off solenoid valve. See «PURGE CUT-OFF SOLENOID VALVE»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#fuel-evaporation-system) test. If there is vacuum on all other models, replace thermo valve. Retest thermo valve. THERMO SENSOR OPERATING TEMPERATURE Application Operating Temperature °F (°C) Accord 131 (55) Civic 1.3L 122 (50) 1.5L Carbureted Engine 104 (40) EFI Engine 149 (65) Civic CRXSi 149 (65) Prelude Carbureted Engine 158 (70) EFI Engine 131 (55)
  3. On all models, if there is no vacuum, remove vacuum gauge and reconnect hose. With engine coolant temperature above specified temperature, disconnect upper vacuum hose from purge control valve. Run engine at idle and observe vacuum gauge. On EFI Civic models, vacuum should be present, after 5 seconds of operation.
  4. If there is no vacuum on EFI Civic models, test purge cut-off solenoid valve. See «PURGE CUT-OFF SOLENOID VALVE»(/honda/prelude/iii-1986-1989/remont/testing-diagnostics/#fuel-evaporation-system) test. If there is no vacuum on EFI Accord or EFI Prelude models, replace thermo valve and retest.

Scheme 5

Scheme 5
Scheme 5

CHARCOAL CANISTER

  1. Connect tachometer to engine. Start and warm engine to normal operating temperature. Remove fuel filler cap. Remove canister purge air hose from frame rail and connect vacuum gauge. (Scheme 6) Raise engine speed to 3500 RPM. Observe vacuum gauge. Vacuum should appear within one minute. If vacuum appears, remove test equipment, install fuel filler cap, and reconnect hose.
  2. If no vacuum is present, or does not appear within one minute, stop engine. Disconnect vacuum gauge. Install fuel filler cap. Remove canister. Inspect canister for damage. If defective or damaged, replace. Repeat test.
  3. If canister is not defective, disconnect upper vacuum hose from purge control valve port on canister. Connect hand vacuum pump to purge port. Apply vacuum. Vacuum should remain steady. If vacuum drops, replace canister. Repeat test.
  4. If vacuum remains steady, reconnect hose to purge control valve port. Start engine and let idle. Purge port vacuum should drop to zero. If not, replace canister. Repeat test.

Scheme 6

Scheme 6
Scheme 6

2-WAY VALVE

  1. Remove fuel filler cap. Remove fuel vapor hose from liquid/vapor separator pipe on EFI Accord and EFI Prelude models, or vapor hose from fuel tank to 2-way valve on all other models. Connect vacuum/pressure pump with gauge to hose. Slowly draw vacuum while observing gauge.
  2. Vacuum should stabilize at .2-.6 in. Hg as 2-way valve opens. Reverse pump and gauge connections. Slowly pressurize vapor hose. Pressure should momentarily stabilize at 1.0-2.2 in. Hg as 2-way valve opens.
  3. If valve does not respond as outlined, check hose connections for leaks. If hose connections are okay, replace valve. Repeat test.

Civic EFI

  1. Disconnect 8-wire connector at control unit. Control unit is located at right side of firewall in engine compartment. Disconnect vacuum hose No. 19 from control unit. Connect vacuum pump/gauge. Try to pull vacuum. No vacuum should be present. (Scheme 7)
  2. Using jumper wires, short battery voltage between top and bottom terminals of 8-wire connector. (Scheme 7) Apply vacuum to hose. Vacuum should now be present. If valve does not operate as described, replace solenoid valve.

Scheme 7

Scheme 7
Scheme 7

Accord & Civic - Carbureted Engine

  1. Remove air vent cut-off diaphragm hose from carburetor. Connect vacuum pump/gauge to hose and try to draw vacuum. There should be no vacuum. Turn ignition on and draw vacuum. Vacuum should hold steady.
  2. If no vacuum can be drawn with ignition on, check for leaks in hose connections. If hoses are okay, check for voltage at vacuum holding solenoid valve connector at control unit (Black/Yellow wire of 8-wire connector). Control unit is located at right side of firewall in engine compartment. If voltage is present, replace vacuum holding solenoid valve. Repeat test.
  3. Start engine and let idle. Vacuum should be present. If not, check for blockage in hose. If no blockage, replace vacuum holding solenoid valve (located in control unit). Repeat test.
  4. Turn ignition off. Vacuum should drop to zero. Disconnect vacuum pump/gauge from vacuum holding solenoid valve hose and connect to air vent cut-off diaphragm. Draw vacuum. Vacuum should remain steady. If vacuum decreases, replace diaphragm. Repeat test. Remove vacuum pump/gauge. Replace hose.

Prelude - Carbureted Engine

  1. With engine coolant temperature below 158°F (70°C), remove upper hose from air vent cut-off solenoid valve and connect vacuum pump/gauge to solenoid valve. Turn ignition on. Draw 4-5 in. Hg. Vacuum should hold steady.
  2. If vacuum is not present, check for voltage at solenoid valve. If no voltage, replace air vent cut-off solenoid valve. If there is voltage, check that there is voltage at control unit connector Blue wire with ignition on. Control unit is located at right side of firewall in engine compartment.
  3. If voltage is not present at Blue wire of control unit connector, check Blue wire between ignition coil and control unit. Check that there is voltage at control unit connector Black/Yellow wire with ignition on.
  4. If voltage is not present, check wire. Check continuity between good ground and Brown/Black wire of control unit connector. If continuity is not present, check wire. Also check thermosensor, vacuum switch A and A/C switch.
  5. Start engine. After delay of 5-6 seconds, vacuum should drop to zero. If not, check for voltage to solenoid valve. If there is voltage, replace air vent cut-off solenoid valve. If no voltage, check wires and components previously outlined in steps 3) and 4).
  1. Remove air cleaner and filter element. Disconnect 3 hoses at inner vent solenoid valve. Plug 2 ports. Connect a vacuum pump/gauge to other port. (Scheme 8) Turn ignition on. Apply 10 in. Hg. Vacuum should hold steady.
  2. If no vacuum, check for voltage at inner vent solenoid valve. If no voltage, replace inner vent solenoid valve. If there is voltage, check that there is voltage at control unit connector Blue wire with ignition on. Control unit is located at right side of firewall in engine compartment.
  3. If voltage is not present at Blue wire of control unit connector, check Blue wire between ignition coil and control unit. Check that there is voltage at control unit connector Black/Yellow wire with ignition on. If voltage is not present, check wire. Check continuity between good ground and Brown/Black wire of control unit connector. If continuity is not present, check wire.
  4. Start engine. Vacuum should drop to zero. If not, check for voltage to solenoid. If there is voltage, replace inner vent solenoid valve. If no voltage, check wires previously outlined in step 3).

Scheme 8

Scheme 8
Scheme 8
  1. With engine coolant temperature below 104°F (40°C), remove connector from control unit. Control unit is located at right side of firewall in engine compartment. Using an ohmmeter, measure resistance between Blue/Yellow and Brown/Black wire terminals. Resistance should be above 1000 ohms.
  2. Run engine until warm and check that cooling fan runs at least once. Measure resistance again. Resistance should be about 200 ohms. If thermosensor resistance is not as specified, replace thermosensor and retest. If resistance is infinite, check wires to thermosensor for opens. Repair wiring. Repeat test.
  1. Remove connector and vacuum hose No. 2 from control unit. Connect vacuum pump/gauge to hose. Using an ohmmeter, measure continuity between Black/Yellow and Brown/White wire terminals on connector. There should be continuity.
  2. Slowly apply vacuum while observing ohmmeter scale. When vacuum increases above 3.9 in. Hg, switch should open and ohmmeter should show no continuity (infinite resistance). If not, replace vacuum switch A.

More articles — Engine Performance