Poor Fuel Economy
While the performance characteristics of the 6.4L Power Stroke are impressive, the fuel economy is not. The 6.4L Power Stroke was the first engine in Ford's history to feature a diesel particulate filter (DPF), an emission control device that captures soot from the exhaust stream. As the filter accumulates particulate matter, it requires cleaning. The primary cleaning process is a strategy called active regeneration, which introduces raw fuel into the exhaust stream in order to raise the exhaust gas temperatures such that the material in the DPF can be burned off. This is the source of 6.4L Power Stroke's poor fuel economy - the engine itself is not inherently inefficiency, but the exhaust aftertreatment system uses a significant amount of fuel. More frequent regeneration cycles result in reduced fuel economy.
As tampering with emission control devices is illegal, there is little owners can do in terms of modification. Driving patterns play the largest role in regeneration frequency. Long periods of highway driving are ideal for minimizing filter loading and therefore the need for frequent regeneration cycles. On the contrary, excessive idling and stop-and-go driving conditions will result in more frequent regeneration resulting from repeated loading of soot in the filter.
Cylinder Washing & Fuel Dilution
During the active regeneration process, fuel is introduced into the exhaust stream by means of a post-injection strategy, in which fuel is injected late in the exhaust stroke so that it may exit the cylinder in its raw (or relatively so) form. This method, as opposed to the "9th injector" design that relies on a dedicated fuel injector mounted in the exhaust system, causes fuel to stick to the wall of the cylinder where it is carried into the crankcase.
Cylinder washing is a concern very much related to the fuel dilution of engine oil. While the active regeneration process is in progress, fuel injected late in the exhaust stroke has a tendency to "wash" the cylinder, diluting the thin layer of protective engine oil lining the cylinder. While this hasn't been associated with any significant percentage of engine failures, it's obviously less than ideal for the longevity of the 6.4L Power Stroke.
Excessive Fuel Dilution ("Making Oil")
Fuel dilution is a common and quite significant problem in the 6.4L Power Stroke, as diesel fuel does not provide near the lubricating properties as engine oil. But fuel dilution is also normal and to be expected from this system. In order to prevent possible damage from excessive fuel dilution, owners should check the engine oil frequency and consider changing their oil more frequently than Ford recommends. Many owners follow Ford's "Severe" maintenance schedule regardless of driving patterns, which cuts the oil change in half. If you find that the oil level in the crankcase is increasing, have your oil changed immediately.
Leaking radiators are common and more prominent on early build engines - it seems as though the problem was addressed and eradicated in later production engines. Leaks commonly develop at the ends of the radiator where the crimps holding on the end tanks work loose overtime. New radiators do not typically develop leaks like the early design, although replacing one can be rather expensive.
Fuel-Water Separator Issues
The fuel-water separator on the 6.4L Power Stroke is prone to clogging, often without the owner knowing. As water collects in the bowl of the separator, it mixes with diesel fuel and tends to coagulate, producing a thick, grease-like sludge that may clog the drain valve. When this occurs, the fuel water separator will not drain and, after reaching maximum capacity, will allow water to enter the upstream fuel system. This can cause expensive damage to the high pressure fuel system. In order to avoid these problems, owners should be draining the fuel-water separator frequently; draining at the fuel filter replacement interval has proved insufficient for this design.
As this was Ford's first DPF equipped vehicle, it's worth noting that every manufacturer (including the Cummins powered Ram and Duramax powered GMC/Chevrolet HD) has experienced an onslaught of DPF clogging issues early on. Diesel particulate filters may become clogged when the regeneration process malfunctions and/or fails to keep the filter clean as required. In order to reduce the propensity of DPF clogging concerns, only use ULSD fuel and an engine oil that meets CJ-4 low ash specifications. All other diesel fuels and engine oils are not compatible with the 6.4L Power Stroke and will cause ash to accumulate in the DPF. This ash cannot be burned off through the regeneration process and will require removal of the DPF for manual cleaning.
Wire Chafing at High Pressure Fuel Pump (HPFP)
Chafing of the wiring harness for the high pressure fuel pump (injection pump) is relative to trucks produced prior to August 20th, 2007. After this date, the wiring harness features a protective loom that eliminates this concern. When wiring chafing exposes the wires of this harness, they run the risk of shorting to the HPFP. This is a problem that Ford is well aware of, and TSB 07-26-2 was initiated in response to concerns. If this problem occurs, the new harness part # is 8C3Z-9G805-A, as identified by the service bulletin.
Cracked Exhaust Up-pipe Expansion Joints
A relatively common problem on the 6.4L Power Stroke is cracking of the expansion joints on the exhaust up-pipes. The accordion style expansion joints have a tendency to fail do to continuous vibration and heat cycles. When this occurs, a loud hissing noise will typically accompany a loss of power.
Cavitation Wearing Holes in Front Cover
Cavitation is the phenomenon by which vacuum pockets form, then dissipate rapidly. The collapse of the vacuum pocket consequently causes the surrounding coolant column to crash into the adjacent surface. While the forces of the crashing coolant column and very minor, they happen repeatedly and at rapid rate. The result is the development of holes overtime. While cavitation has not been known to attack the cylinder jackets of the 6.4L Power Stroke, it does commonly wear holes in the front engine cover. The problem is addressed by using a SCA/DCA (supplemental coolant additive/diesel coolant additive) in the cooling system, maintaining the SCA/DCA concentration over time, and performing scheduled cooling system flushes per Ford's recommendations.
Engine Oil Cooler Clogging
Much like the 6.0L Power Stroke, the 6.4L Power Stroke's engine oil cooler is prone to clogging. This is primary do the breakdown of engine coolant and concerns can be reduced with regular cooling system service or the installation of a coolant filter. A clogged oil cooler can cause a check engine light to illuminate, signaling a related DTC and in severe cases the engine will de-rate itself as a protective measure.
6.4 Power Stroke Flaming Exhaust
Ford safety recall 07S49 applies to trucks built before 3/9/2007 and addresses concerns of flaming exhaust due to excessive exhaust heat during regeneration on the 6.4L Power Stroke. The recall updates engine calibration and solves this issue as well as many other DPF/active regeneration related concerns by altering the regeneration schedule. Very few trucks were affected, although a viral video on the web that featured a 6.4L with flames shooting out of the exhaust made the issue seem much more significant. Many suspect the owner of the truck in the video altered the truck to create the flames. Owners should not be concerned, the recall was small and all trucks have the updated calibration, which permanently prevents the problem.