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Carbureted Fuel System GENERAL INFORMATION On these vehicles, the engine management system is considered part of the emission control system. The major components include the carburetor(s), feedback control system, the air injection system, a throttle control system and the EGR system. The system consists of sensors and switches that feed information to the Electronic Control Unit (ECU), which will then operate several solenoid valves to maintain the ideal air/fuel ratio under all conditions. As useful as the tests found in this section are, the first step in repair or service to engine management systems is still to gain as much information as possible about the problem; when and under what conditions it occurs. At highway speed? At idle only? Only under heavy load or hard acceleration? Wet weather? Defining the problem will eliminate many systems from consideration and possibly point to the affected system. Before diving into an extended electrical diagnosis, take the time to review the basics. Check every vacuum line for cracks or leaks. Check every electrical connector for corrosion or loose pins. Quite often, simply unplugging and reconnecting a connector will break up corrosion on the pins and restore the circuit. Watch out for poor grounds, particularly if the car has experienced major bodywork. COMPONENT TESTING Air Injection System The purpose of this system is to supply oxygen to the exhaust stream at a point in the exhaust manifold that is hot enough to burn off some of the hydrocarbon emissions. The main component is an air suction valve. The valve is spring loaded to stay closed, with engine vacuum supplied to a diaphragm that reduces the spring pressure and allows the reeds to open. The ECU regulates the engine vacuum to the diaphragm by operating a solenoid valve. 1. With the engine at normal operating temperature and at idle, remove the air cleaner and listen for a bubbling sound at the air suction port. There should be no sound at idle, meaning the air suction valve is closed. 2. If the noise is heard at the air suction port, disconnect the vacuum hose at the air suction valve and connect a vacuum gauge to the hose. There should be no vacuum. If there is vacuum and the noise stops, the problem is in the control system. If there is no vacuum and the bubbling sound is still there, the air suction valve is defective and must be replaced. 3. To test the valve, draw a vacuum at the air suction valve diaphragm and listen for a bubbling sound at the air suction port. If no sound is heard, the air suction valve or diaphragm is faulty. Throttle Control System
