Jeep CJ/Scrambler 1971-1986 Repair Guide

Air Injection System


See Figures 1, 2, 3, 4, 5 and 6


All of the gasoline engines used in these Jeep vehicles have, at one time or another, incorporated the air injection system for controlling the emission of exhaust gases into the atmosphere. Since this type of emission control system is common to most of the engines, it will be explained here.

The exhaust emission air injection system consists of a belt driven air pump which directs compressed air through connecting hoses to a steel distribution manifold into stainless steel injection tubes in the exhaust port adjacent to each exhaust valve. The air, with its normal oxygen content, reacts with the hot, but incompletely burned exhaust gases and permits further combustion in the exhaust port or manifold.

Air Pump

The air injection pump is a positive displacement vane type which is permanently lubricated and requires little periodic maintenance. The only serviceable parts on the air pump are the filter, exhaust tube, and relief valve. The relief valve relieves the air flow when the pump pressure reaches a preset level. This occurs at high engine rpm. This serves to prevent damage to the pump and to limit maximum exhaust manifold temperatures.

Pump Air Filter

The air filter attached to the pump is a replaceable element type. The filter should be replaced every 12,000 miles under normal conditions and sooner under off-road use. Some models draw their air supply through the carburetor air filter.

Air Delivery Manifold

The air delivery manifold distributes the air from the pump to each of the air delivery tubes in a uniform manner. A check valve is integral with the air delivery manifold. Its function is to prevent the reverse flow of exhaust gases to the pump should the pump fail. This reverse flow would damage the air pump and connecting hose.

Air Injection Tubes

The air injection tubes are inserted into the exhaust ports. The tubes project into the exhaust ports, directing air into the vicinity of the exhaust valve.

Anti-Backfire Valve

The anti-backfire diverter valve prevents engine backfire by briefly interrupting the air being injected into the exhaust manifold during periods of deceleration or rapid throttle closure. On the 4-134 and all of the 1971 and later American Motors engines, the valve opens when a sudden increase in manifold vacuum overcomes the diaphragm spring tension. With the valve in the open position, the air flow from the air pump is directed to the atmosphere.

On the 6-225 and the 1972 6-232, the anti-backfire valve is what is commonly called a gulp valve. During rapid deceleration the valve is opened by the sudden high vacuum condition in the intake manifold and gulps air into the intake manifold.

Both of these valves prevent backfiring in the exhaust manifold. Both valves also prevent an over right fuel mixture from being burned in the exhaust manifold, which would cause backfiring and possible damage to the engine.


Anti-Backfire Diverter Valve

On the 4-134, the anti-backfire valve remains open except when the throttle is closed rapidly from an open position.

To check the valve for proper operation, accelerate the engine in neutral, allowing the throttle to close rapidly. The valve is operating satisfactorily when no exhaust system backfire occurs. A further check can be made by removing the large hose that runs from the anti-backfire valve to the check valve and accelerating the engine and allowing the throttle to close rapidly. If there is an audible momentary interruption of the flow of air then it can be assumed that the valve is working correctly.

To check the valve on a V6, listen for backfire when the throttle is released quickly. If none exists, the valve is doing its job. To check further, remove the large hose that connects the valve with the air pump. Place a finger over the open end of the hose, not the valve, and accelerate the engine, allowing the throttle to close rapidly. The valve is operating satisfactorily if there is a momentary audible rush of air.

To check the diverter valve on American Motors engines, start the engine and let it idle. With the engine idling, there should be little or no air coming out the vents. When the engine is accelerated to 2,000-3,000 rpm, a strong flow of air should be felt at the vents. If the flow of air from the air pump is not diverted through the diverter valve vents when the engine is accelerated to the above mentioned rpm, check and make sure that the vacuum sensing line leading to the valve has vacuum and is not leaking or disconnected. The diverter valve should bleed air when 20 in.Hg or more vacuum is applied to the vacuum sensing line or when the output of the air pump exceeds 5 psi. When the engine is slowly accelerated, the diverter valve should begin to bleed off air between 2,500 and 3,500 rpm.

Check Valve

The check valve in the air distribution manifold prevents the reverse flow of exhaust gases to the pump in the event the pump should become inoperative or should exhaust pressure ever exceed the pump pressure.

To check this valve for proper operation, remove the air supply hose from the pump at the distribution manifold. With the engine running, listen for exhaust leakage where the check valve is connected to the distribution manifold. If leakage is audible, the valve is not operating correctly.

Click image to see an enlarged view

Fig. Fig. 1: Air pump system as used on the 4-134 engine

Click image to see an enlarged view

Fig. Fig. 2: Air pump system as used on the 8-304 engine

Click image to see an enlarged view

Fig. Fig. 3: Air pump system as used on the 6-225 engine

Click image to see an enlarged view

Fig. Fig. 4: Air injection system components used on 1971 and later in-line 6 cylinder engines

Click image to see an enlarged view

Fig. Fig. 5: Air diverter valve and manifold mounting on the 4-150 engine

Click image to see an enlarged view

Fig. Fig. 6: Air pump mounting on the 4-150 engine

Air Pump

Check for the proper drive belt tension and adjust as necessary. Do not pry on the die cast pump housing. Check to see if the pump is discharging air. Remove the air outlet hose at the pump. With the engine running, air should be felt at the pump outlet opening.


Air Pump
  1. Loosen the air pump adjusting bracket bolts.
  3. Remove the drive belt.
  5. Remove the air pump intake and discharge hoses.
  7. Remove the air pump from the engine.
  9. To install, reverse the above procedure.

Air Distribution Manifold and Air Injection Tubes

It is necessary to remove the exhaust manifold only on the 4-134 prior to removing the air distribution manifold and the air injection tubes. On all the other engines, these components can be removed with the manifolds on the engine.

  1. Disconnect the air delivery hose from the air injection manifold. Remove the exhaust manifold on the 4-134.
  3. Remove the air distribution manifold from the air injection tubes on the 4-134 only.
  5. Unscrew the air injection tube from the exhaust manifold or the head. Some resistance may be encountered because of the normal buildup of carbon. The application of heat may be helpful in removing the air injection tubes.
  7. Install in the reverse order of removal.

There are two lengths of tubes used with the 4-134. The shorter tubes are installed in number 1 and 4 cylinders. The air injection tubes must be installed on the exhaust manifold prior to installing the exhaust manifold on the engine.