Air Injection System

Because of the many variables under which the engine operates, some hydrocarbon and carbon monoxide gases escape unburnt from the combustion chamber. To burn these gases more thoroughly, a belt-driven pump is used to supply fresh air to an air injection manifold located on the exhaust manifold. The injection of fresh air supports combustion of the hot unburned HC and CO gases within the exhaust manifold.

The Thermactor system, used on California trucks only in 1972 and 1973, and all trucks thereafter, consists of an air pump with relief valve, a check valve, four air injection nozzles (one for each cylinder) connected to an air injection manifold, an air by-pass valve, and hoses from 1972–74. The air pump pushes air through the one way check valve into the injection manifold and through the nozzles. The check valve prevents any backfire of gases into the system. An air by-pass valve is used to prevent air injection during cold choke operation. Pulling the choke cable closes both the choke and the air by-pass valve, venting all pumped air into the air cleaner. This prevents overheating of the rich mixture exhaust. The air pump relief valve vents excess pressure to the atmosphere.

1975–76 trucks use a slightly modified version of the system. The principle and main components remain the same, but an air control valve, incorporating two relief valves, replaces the air by-pass valve. The no. 1 relief valve is activated by a control unit (an engine rpm switch) located on the kick panel beneath the parking brake lever; the No. 2 relief valve is activated by engine vacuum. Below 4,000 rpm (4,300 with automatic transmission), no. 1 directs air to the injection manifold. Above that engine speed, no. 1 opens and vents air to the air cleaner. Number 2 opens when intake manifold vacuum exceeds 6.3 in.Hg, reducing air flow to the injection manifold during low engine loads.

1977–78 1,796cc engines are the same, except that only trucks sold in California have the air control valve, with one relief valve activated by intake manifold vacuum. In addition, trucks sold in California have a vacuum delay valve.

The systems used in 1979 and later are slightly different. 1,970cc engines sold for 49 States use do not have an air pump. Instead, air injection is controlled by exhaust system back pressure. The air injection pipe runs from the air cleaner case to the exhaust manifold. A one-way check valve is installed in the pipe. During periods of negative exhaust pulsation, air is drawn from the air cleaner, through the check valve, and into the exhaust manifold. During periods of positive exhaust back pressure (positive pulsation), the check valve closes and no air is drawn into the exhaust system.

Models sold in California have an air pump, a check valve, and an air control valve. The air control valve has a no. 1 relief valve, activated by intake manifold vacuum, and a No. 2 relief valve, which modulates the amount of secondary air. The No. 2 relief valve is not used with automatic transmissions.