Feedback Carburetor System Chevrolet Nova V

DESCRIPTION

On Nova models, the following systems are controlled by the Electronic Control Module (ECM) to control exhaust gas emissions

1. Fuel Evaporative Emission Control (EVAP) System
2. Carburetor Feedback System
3. Deceleration Fuel Cut-Off System
4. Cold Mixture Heater - CMH

The ECM is the "brain" of the computerized engine control system.

FUEL EVAPORATIVE EMISSION CONTROL (EVAP) SYSTEM

The EVAP system reduces HC emissions by storing and then routing evaporated fuel from fuel tank and carburetor's float chamber through charcoal canister to intake manifold for combustion in cylinders at the proper time.

With ignition off, hydrocarbons from carburetor float chamber pass through de-energized outer vent control valve and into charcoal canister. Also, evaporated fuel from fuel tank passes and is stored in charcoal canister through a check valve on the canister.

With ignition switch in "ON" position and engine not running, outer vent control valve is energized, blocking movement of fuel vapor from carburetor's float chamber. Vapors from fuel tank can still move and be stored in canister.

With coolant temperature above 109°F (55°C) and engine running at 1600-1900 RPM, ECM energizes vacuum switching valve (VSV) which connects engine's manifold to canister purging it of stored vapors.

With engine running above 2290 RPM, throttle position switch closes, vacuum switching valve (VSV) remains energized and fuel vapors are purged from canister into intake manifold. If deceleration occurs, throttle position switch opens, ECM detects change and turns off VSV stopping canister purge to prevent an excessive amount of HC emissions.

When there is high pressure in fuel tank, canister valve No. 4 is open allowing HC from tank to be absorbed into canister. (Scheme 26)and EVAP SYSTEM OPERATION chart.

When there is high vacuum in fuel tank, canister valve No. 4 closes, valve No. 5 opens and check valve in fuel filler cap opens. With these 2 valves open, air is pulled into tank to equalize pressure.

Fuel Evaporative Emission Control (EVAP) System Operation. Scheme 26

CARBURETOR FEEDBACK SYSTEM

The purpose of carburetor feedback system is to maintain air fuel ratio at desired 14.7:1 ratio during normal operation (except warm-up and acceleration).

The carburetor on the feedback control system is designed to operate somewhat richer to set up rich limit of system operation. When a more lean operation is desired, the ECM commands air to be bled into carburetor's main metering system and into carburetor's primary bore. A lean operating condition is therefore easily obtained. The ECM receives inputs from the following sensors

1. Oxygen sensor
2. Vacuum switch
3. Throttle position switch
4. Distributor

The ECM's output consists of a signal to an electric air bleed control valve (EBCV). When energized, the EBCV bleeds air into main air bleed circuit and into slow air bleed port of carburetor. This additional air leans air fuel ratio. When not energized, air fuel ratio moves to rich limit.

With engine running and coolant temperature below 45°F (7°C), thermostatic vacuum switching valve (TVSV) applies atmospheric pressure to vacuum switch B by connecting ports J to L of TVSV. With these conditions, vacuum switch B is off, electric bleed control valve (EBCV) is off and both air bleeds are off. The carburetor is operating toward its rich limits. With engine relatively cold, this is desirable. The computer will not be controlling or influencing air fuel ratios while engine is cold.

At temperatures above 63°F (17°C), with engine running between 1500 and 4200 RPM, the TVSV applies vacuum to vacuum switch by connecting ports K to L of the TVSV. Vacuum switch closes, signaling ECM. Throttle position switch is closed, also signaling ECM. With these 2 switches closed, if oxygen sensor senses a rich condition in exhaust (high voltage - 1 volt), ECM commands electric bleed control valve to be energized, thereby bleeding air into both main metering system of carburetor and intake manifold. This results in air fuel ratio becoming lean.

When air fuel ratio is detected as lean by oxygen sensor (low voltage - 1 volt), ECM will de-energize EBCV and close both bleed ports which results in air fuel ratio moving to its rich extreme. This is closed loop mode operation. To energize EBCV, the ECM completes its electrical circuit on its ground side. (Scheme 27)and CARBURETOR FEEDBACK SYSTEM OPERATION chart below.

Carburetor Feedback System Operation. Scheme 27

DECELERATION FUEL CUT-OFF SYSTEM

This system cuts off part of fuel flow in idle (or slow) circuit of carburetor preventing overheating and afterburning in exhaust system. The first fuel cut solenoid is kept energized by the ECM whenever engine is running, except when throttle is closed with RPM above 2290. This condition will be sensed by ECM when throttle position switch opens as throttle closes.

With first fuel cut solenoid valve de-energized, the carburetor's slow (or idle) circuit fuel is cut off. This occurs when vehicle is decelerated from 2290 RPM. (Scheme 28)and DECELERATION FUEL CUT-OFF SYSTEM OPERATION chart.

Deceleration Fuel Cut-Off System Operation. Scheme 28

COLD MIXTURE HEATER - CMH

This system reduces cold engine emissions and improves driveability during engine warm-up. The intake manifold is heated during cold engine operation to accelerate vaporization of liquid fuel.

The ECM checks alternator (terminal L) to determine if engine is running. It also checks engine's coolant temperature. If engine is running and coolant temperature is below 109°F (43°C), ECM energizes CHM (relay) which applies battery voltage to CMH. The CMH is a multi-element heater ring that is mounted between carburetor base and intake manifold. Once coolant temperature exceeds 131°F (55°C), the CMH relay is de-energized and heater elements turn off. (Scheme 29)and COLD MIXTURE HEATER OPERATION chart.

Cold Mixture Heater (CMH) Operation. Scheme 29

Fuel Vapor Lines, Fuel Tank & Fuel Filler Cap

1. Inspect lines and connections. Look for loose connection, sharp bends or damage.
2. Inspect fuel tank. Look for deformation, cracks or fuel leaks. Inspect fuel filler cap. Look for damaged or deformed gasket and cap. If necessary, replace cap.

Charcoal Canister

1. Remove and inspect canister. Look for cracks or damage. Check for clogged filter and stuck check valve.
2. Using low pressure compressed air, blow into tank pipe and check that air flows without resistance from other pipes. If necessary, replace canister.
3. Clean filter in canister. Clean filter by applying 43 psi (3 kg/cm 2 ) of compressed air into pipe to outer vent control valve while holding other upper canister pipes closed.
4. DO NOT attempt to wash canister. Be sure that no activated carbon comes out of canister. Install canister.

Outer Vent Control Valve

1. Disconnect hoses from valve. Check that valve is open by blowing air through it when ignition switch is in "OFF" position.
2. Check that valve is closed when ignition switch is in "ON" position. Reconnect hoses to proper locations. If valve does not operate, check fuse and wiring connections.

Thermo Switch

1. Drain coolant from radiator into a container. Remove thermo switch (located behind TVSV) from intake manifold. Cool switch to below 109°F (43°C).
2. Using an ohmmeter, check that there is continuity through switch. Heat switch to above 131°F (55°C) with hot water. Check that there is no continuity.
3. Apply liquid sealer to threads of switch and reinstall. Fill radiator with coolant.

Scheme 30

1. Check vacuum circuit continuity in VSV (located on left strut tower) by blowing air into pipe. Connect VSV terminals to battery terminals. (Scheme 30) (Scheme 30): Checking Vacuum Circuit Continuity
2. Blow into pipe and check that VSV is open. Disconnect positive battery terminal. Blow into pipe and check that VSV is closed. If necessary, replace VSV.
3. Check for short circuit. Using an ohmmeter, check that there is no continuity between positive terminal and VSV body. If necessary, replace VSV.
4. Check for open circuit. Using an ohmmeter, measure resistance between positive terminal and other terminal. Ensure that resistance is 38-44 ohms at 68°F (20°C). If not within specification, replace VSV.

Throttle Position Switch

1. Connect tachometer to engine. Start engine and warm to normal operating temperature. Disconnect throttle position switch connector. Connect ohmmeter between throttle position switch connector and ground.
2. Slowly raise engine speed. Check throttle position switch setting speed when ohmmeter shows continuity. Setting speed should be 1600-2000 RPM. Adjust setting speed using throttle position switch adjusting screw.

System Check

1. Check TVSV with engine coolant temperature below 45°F (7°C). Disconnect vacuum hose from vacuum switch. Start engine and check that no vacuum is felt in disconnected vacuum hose. Reconnect hose.
2. Check EBCV with engine warmed to normal operating temperature. Disconnect EBCV connector. Maintain engine speed at 2500 RPM. Reconnect connector and check that engine speed drops about 300 RPM momentarily.
3. Check EBCV with engine warmed to normal operating temperature. Disconnect EBCV connector. Maintain engine speed at 2500 RPM. Reconnect connector and check that engine speed drops about 300 RPM momentarily.
4. Check EBCV with engine warmed to normal operating temperature. Disconnect EBCV connector. Maintain engine speed at 2500 RPM. Reconnect connector and check that engine speed drops about 300 RPM momentarily.
5. With engine idling, disconnect EBCV connector. Reconnect connector and check that engine speed does not change. Disconnect vacuum hose from vacuum switch.
6. Disconnect EBCV connector. Maintain engine speed at 2500 RPM. Reconnect connector and check that engine speed does not change. If no problem is found, system is okay. Otherwise, inspect each part.

Electronic Air Bleed Control Valve (EBCV)

1. Check for short circuit. Using an ohmmeter, check that there is no continuity between positive terminal (terminal closest to lock tab) and EBCV body. If there is continuity, replace EBCV.
2. Check for open circuit. Using an ohmmeter, check that there is no continuity between positive terminal and other terminal. If resistance is not between 11-13 ohms at 68°F (20°C), replace EBCV.

See FUEL EVAPORATIVE EMISSION CONTROL SYSTEM description above in this article.

Vacuum Switch

Using an ohmmeter, check that there is no continuity between switch terminal and switch body. Warm engine to normal operating temperature. Using an ohmmeter, check that there is continuity between switch terminal and switch body. If necessary, replace switch.

Scheme 31

1. Drain coolant from radiator into a container. Remove TVSV and cool to below 45°F (7°C). Check that air flows from pipe J to M and L and from pipe K to N. (Scheme 31) (Scheme 31): Checking TVSV
2. Heat TVSV to 63-122°F (17-50°C). Check that air flows from pipe K to N and L and flows from pipe J to M.
3. Heat TVSV to above 154°F (68°C). Check that air flows from pipe K to M and L and flows from pipe J to other pipes. Apply liquid sealer to threads of TVSV and reinstall. Fill radiator with coolant.

Scheme 32

1. Warm engine to normal operating temperature. Connect voltmeter to service connector (Green connector behind right strut tower). Connect positive testing to Ox terminal and negative testing probe to terminal E. (Scheme 32) (Scheme 32): Checking Oxygen Sensor with Service Connector
2. Run engine at 2500 RPM. Check that voltmeter fluctuates 8 times or more in 10 seconds with 0-6 volts. If so, oxygen sensor is okay. If not, inspect other parts, hose connections and carburetor feedback system wiring. If no problems with these are found, replace oxygen sensor.

Scheme 33

1. Connect tachometer to engine. Start engine and check that is runs normally. Disconnect throttle position switch connector. (Scheme 33) (Scheme 33): Throttle Position Switch Connector
2. Gradually increase engine speed to 2300 RPM and check that engine is fluctuating. Reconnect throttle position switch connector. Gradually increase engine speed to 2300 RPM and check that engine operation returns to normal.
3. If no problem is found with this check, system is okay; otherwise inspect each part.

First Fuel Cut Solenoid Valve

1. Remove 2 wire solenoid valve from carburetor. Apply 12 volts to one of the solenoid valve wires while grounding the other.
2. Check that you can feel "click" from solenoid valve when battery is connected and disconnected. Check "O" ring for damage.
3. Reinstall valve and wiring connector. If problem is found, replace failed part.

See FUEL EVAPORATIVE EMISSION CONTROL SYSTEM description above in this article.

1. Start engine. Check CMH with coolant temperature below 109°F (43°C). Using a voltmeter, check that there is voltage between positive terminal (White/Red wire color) and ground. CAUTION: Voltmeter should be inserted from rear side of connector.
2. Warm engine to above 131°F (55°C). Check that there is no voltage. If no problem is found with this check, system is okay; otherwise inspect each part.

Cold Mixture Heater

Unplug wiring connector. Using an ohmmeter, check that resistance between heater terminals is 0.5-2.0 ohms. Plug in connector.

Scheme 34

1. Check for continuity between terminals 1 and 2. Check that there is no continuity between terminals 3 and 4. (Scheme 34) (Scheme 34): Checking CMH Relay
2. Check for continuity between terminals 3 and 4 with battery voltage applied to terminal 1 with terminal 2 grounded.

See FUEL EVAPORATIVE EMISSION CONTROL SYSTEM description above in this article.

EMISSIONS SYSTEM & COMPONENT TEST PARAMETERS

EVAP SYSTEM OPERATION

CARBURETOR FEEDBACK SYSTEM OPERATION

DECELERATION FUEL CUT-OFF SYSTEM OPERATION

COLD MIXTURE HEATER OPERATION

Nova ECM Wiring Diagram. Scheme 35

See also:
• EVAP SYSTEM OPERATION