CHASSIS AR 2A6530 - Caterpillar

Series Engines Equipped With An Air/Fuel Ratio ControlIf a 3400 Series Engine equipped with an AFRC (air/fuel ratio control) has a misfire and/or backfire, check the diaphragm in the AFRC for damage. When these engines are operated at low idle, there is a negative boost (vacuum) in the inlet manifold. If the diaphragm has a hole in it, oil from behind the diaphragm can be pulled into the inlet manifold and then the cylinders. At low idle, this oil can cause a misfire and/or backfire. At higher rpm the engine will operate smoothly but there will be excessive (too much) smoke during rpm increases.