The 6.4 engines are known to wear out the rocker arms at midlife. The rocker arms are prone to breaking or having the tips wear out. This happens because of the way that the rockers lack oil lubrication. The design is the same as the 6.0 powerstroke but the 6.0 power stroke does not seem to have this issue. We have seen the tips wear enough that the push tube falls off. The ball pivots wear and the tips wear until the clearance is just to great which compounds itself because lubrication escapes at the clearances.
High Pressure Fuel Pump
The high pressure fuel pump on the 6.4 powerstroke is pretty solid. The pump has 2 valves that are a part of it. One of the valves is to control volume and the other valve is to control pressure. In most cases when you have to replace the high pressure fuel pump the problem is really one of the valves, but we have no way to replace only the valve. The P088 code is the most common code for the HPFP.
Because 6.4L injectors are located under the valve covers and rail pressures are extremely high any leaks at the injection line connections will cause the engine oil to become diluted with diesel fuel potentially resulting in serious engine damage. Ford requires that the injection line be replaced whenever it is removed.
WARNING: A major cause of 6.4L injector failure is failure of the high pressure fuel pump. When the high pressure fuel pump fails it fills the fuel system with ‘glitter’ – thousands of tiny metallic particles. These particles can cause an injector to stick open which can, in turn, cause engine failure. Before replacing any 6.4L injector perform a test for metallic fuel system contamination to prevent immediate re-occurrence of the failure! Any time a 6.4L engine develops a misfire or abnormal smoke condition it is prudent to cease driving immediately. Continued operation can result in very expensive repair bills.