Ford Mustang 1989-1993 Repair Guide

Thermactor Air Injection System



The Thermactor air injection system reduces the hydrocarbon and carbon monoxide content of the exhaust gases by continuing the combustion of unburned gases after they leave the combustion chamber. This is done by injecting fresh air into the hot exhaust stream leaving the exhaust ports or into the catalyst. At this point, the fresh air mixes with hot exhaust gases to promote further oxidation of both the hydrocarbons and carbon monoxide, thereby reducing their concentration and converting some of them into harmless carbon dioxide and water.

All Ford rear wheel drive vehicles, including the Mustang, equipped with the 5.0L engine have a managed air thermactor system. This system is utilized in electronic control systems to divert thermactor air either upstream to the exhaust manifold check valve or downstream to the rear section check valve and dual bed catalyst. The system will also dump thermactor air to atmosphere during some operating modes.

The Thermactor air injection system consists of the air supply pump, air bypass valve, check valves, air supply control valve, combination air bypass/air control valve, solenoid vacuum valve, thermactor idle vacuum valve and air pump resonator. Componentry will vary according to application.

Air Supply Pump

The air supply pump is a belt-driven, positive displacement, vane-type pump that provides air for the thermactor system. It is available in 19 cu. in. (311cc) and 22 cu. in. (360cc) sizes, either of which may be driven with different pulley ratios for different applications. The pumps receive air from a remote silencer filter on the rear side of the engine air cleaner and is attached to the pumps' air inlet nipple or through an impeller-type centrifugal filter fan.


  1. Check the belt tension and adjust if needed.

Do not pry on the pump to adjust the belt. The aluminum housing is likely to collapse.

  1. Disconnect the air supply hose from the bypass control valve.
  3. The pump is operating properly if airflow is felt at the pump outlet and the flow increases as engine speed increases.
  5. If the pump is not operating as described in Step 3 and the system is equipped with a silencer/filter, check the silencer/filter for possible obstruction before replacing the pump.


  1. Disconnect the negative battery cable.
  3. Remove the drive belt from the air pump pulley.
  5. Disconnect the air hose(s) from the air pump.
  7. Remove the mounting bolts and if necessary the mounting brackets.
  9. Remove the air pump from the vehicle.
  11. Installation is the reverse of the removal procedure.