Ford Tempo/Topaz 1984-1994 Repair Guide



There are 3 basic sources of automotive pollution in the modern internal combustion engine. They are the crankcase with its accompanying blow-by vapors, the fuel system with its evaporation of unburned gasoline and the combustion chambers with their resulting exhaust emissions. Pollution arising from the incomplete combustion of fuel generally falls into three categories: hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).

Some gasoline engines are equipped with an air pump system, positive crankcase ventilation, exhaust gas recirculation, electronic ignition, catalytic converter, thermostatically controlled air cleaner, and an evaporative emissions system. Electronic engine controls are used on various engines, depending on model and year.

If a gasoline vehicle is equipped with a belt driven air pump system, clean air is injected into either the exhaust manifold, or downstream into the catalytic converter, depending on engine conditions. The oxygen contained in the injected air supports continued combustion of the hot carbon monoxide (CO) and hydrocarbon (HC) gases, reducing their release into the atmosphere.

On some gasoline engines equipped with a carburetor, a back pressure modulated EGR may be mounted next to the carburetor on the intake manifold. Vacuum applied to the EGR diaphragm raises the pintle valve from its seat, allowing hot exhaust gases to be drawn into the intake manifold with the intake charge. The exhaust gases reduce peak combustion temperature; lower temperatures reduce the formation of oxides of nitrogen (NOx).

A dual bed catalytic converter is mounted in the exhaust system, ahead of the muffler on most gasoline engines. Catalytic converters use noble metals (platinum and palladium) and extreme heat, sometimes as high as 1,200°F (650°C) to catalytically oxidize HC and CO gases into H 2 O and CO 2 .

The Thermactor system is used as a fresh air (and therefore, oxygen) supply on some gasoline engines. The thermostatically controlled air cleaner housing is able to draw fresh air from 2 sources: cool air from outside the car (behind the grille), or warm air obtained from a heat stove encircling the exhaust manifold. A warm air supply is desirable during cold engine operation. Because it promotes better atomization of the air/fuel mixture, while cool air promotes better combustion in a hot engine.

Instead of venting gasoline vapors from the carburetor float bowl into the atmosphere, an evaporative emission system captures the vapors and stores them in a charcoal filled canister, located ahead of the left front wheel arch. When the engine is running, a purge control solenoid allows fresh air to be drawn through the canister. The fresh air and vapors are then routed to the carburetor, to be mixed with the intake charge.