Jeep CJ 1945-1970 Repair Information

Emission Control Checks



On the F-head 4-134 engine, 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 the 6-225 engine, 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.


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. A small amount of leakage is normal.


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.