6.4L: Long List of Failures


With its factory compound turbo system, the debut of common-rail injection on the Power Stroke platform, quick-firing piezo injectors, and 350 hp from the factory, the 6.4L definitely had a lot of promise when it debuted in early ’07. Back then, Ford fans who had already been dealing with the 6.0L’s reliability issues for nearly five years were hopeful the 6.4L would be the great savior for the Power Stroke name. Initial impressions were favorable, but as time has worn on and the miles have racked up, the 6.4L has become notorious for a long list of failures. Many of which cost big money to address.

Today, the 6.4L is often referred to as a 150,000-mile proposition, a throwaway engine, or worse: a ticking timebomb. From its leaking radiators and up-pipes to the high-pressure fuel pump’s propensity to self-destruct, to cracked pistons, this engine is plagued by failures both big and small. Appropriately, it’s the rarest to make it to the 200,000-mile mark before facing something catastrophic. But be careful who you say that to… Despite its frequent and well-documented problems, 6.4L owners who haven’t seen this engine’s ugly side tend to swear by it. So what’s the actual verdict? In the following pages, we’ll explore the 6.4L’s most common failure points and let you decide for yourself.

 Pull Quote: “Not only are the 6.4L’s replacement components expensive, but the cab has to be pulled for most major repairs, further driving up the cost of labor.”

Going from bad to worse, and eventually covering the ugly, we’ll kick things off with the inconvenient failure nearly every 6.4L faces at some point in its lifetime: a leaking radiator. The issue occurs when the crimps that hold the aluminum core to the plastic end tanks stretch. This allows the two to separate. Some attribute the crimp failures to the upper radiator support. They say it allows too much flex to occur at the top of the radiator. Regardless, throughout the 6.4L production run, Ford was never able to rectify the radiator problem.
Ford issued several technical service bulletins (TSBs) regarding the leaking radiator issue. One, in particular, called for the installation of a Venturi Tee. According to Ford, it was to relieve coolant pressure spikes within the radiator. As you can imagine, it didn’t solve the leaking issue. According to most shops around the country, the fact that the radiator splits at the aluminum-to-plastic seams is directly related to pour quality from the factory.
Cracked up-pipes are extremely common on the 6.4L. They typically crack at the bellows (or expansion joints). This causes a loss of drive pressure, a drop in boost, and poor drivability. It also leaves a mess of soot all over the firewall and transmission tunnel, and an audible whistle under the cab. Adding insult to injury, accessing the up-pipes means the cab has to go up in the air. We all know battling seized up-pipe bolts is pretty much a foregone conclusion. Needless to say, it’s a fairly involved (and expensive) fix.
As with any after-treatment-era diesel, emissions-control equipment is prone to failure. The 6.4L was the first Power Stroke to come with a diesel particulate filter. Inevitably, a few growing pains followed. Early issues with active regeneration led to excessive heat being built in the DPF. This led to the infamous flaming tailpipe debacle that prompted a recall. Some early production DPFs even leaked soon after leaving the factory. DPF failures are rampant. Be it from filling the wall-flow style catalyst up due to insufficient regeneration operation or faulty sensors.
As a direct result of the 6.4L’s injecting additional fuel on the exhaust stroke during the regeneration process (which is employed to clean the DPF), the diesel that doesn’t make it out of the cylinders slips past the compression and oil rings and ultimately joins the engine oil down in the crankcase. Of course, diluting engine oil with diesel fuel doesn’t bode well for things like main and rod bearings. Nor the turbochargers or valvetrain.
This is why you’ve heard that 6.4L’s “make oil.” The additional diesel that makes it into the crankcase during regeneration eventually adds up to a substantial amount. In fact, it’s not uncommon to drain five to six gallons (20-24 quarts) of oil out of an emissions-equipped 6.4L during an oil change (note: there should only be 15 quarts). After news of the issue became widespread, most owners began following Ford’s severe conditions service interval of 5,000 miles rather than the normal operating conditions’ recommended 10,000 miles.
The 6.4L Power Stroke’s dual EGR cooler and dual poppet EGR valve were definitely an improvement over the hardware used on the 6.0L. Nevertheless, the EGR system still only functions so long before a problem arises. Extended periods of idling (where wet-stacking occurs) can plug the EGR valve in short order and at very low miles. No matter how you drive, the EGR coolers plug up eventually. The horizontally mounted cooler (the one that’s the first to be exposed to exhaust leaving the heads) usually (and predictably) clogs first.
One negative side-effect of the EGR system (there are many more) is the toll it takes on the aluminum front cover. When super-heated coolant returns through the front cover, cavitation occurs (i.e. flash boiling of the coolant). Over time, the cavitation will eat away at the front cover. Eventually, this will cause a hole to develop. Coolant can sneak through to the crankcase and begin contaminating the engine oil.
Blown head gaskets aren’t as frequent with the 6.4L as they were with the 6.0L, but it still happens. If a 6.4L has been tuned for tens of thousands of miles, don’t be surprised to find coolant residue on the degas bottle. Depending on how it’s driven, a tuned 6.4L without head studs can typically survive 80,000 to 100,000 miles on a 300-plus hp file before lifting a head. Of course, that kind of longevity is directly dependent on the way you drive the truck.
So what keeps the head gaskets alive so long as opposed to the 6.0L? Despite the fact that the cylinder heads are virtually identical between the two engines (which includes the use of four head bolts per cylinder), one major difference is that the 6.4L uses a larger diameter (16mm vs. 14mm) and shorter length head bolts. Still, with enough miles racked up in the 600rwhp range, the added cylinder pressure ends up stretching the 16mm bolts.
Like every other engine in the segment, a quality fastener such as the head studs offered by ARP can make blown head gaskets a non-issue. A studded 6.4L can handle big cylinder pressure. Although, weak spots in the block’s casting (on the lifter valley side of the head bolt bores for cylinder numbers 3 and 5, which are in close proximity to water jackets) can crack. This allows coolant to contaminate the engine oil, and essentially trash the block. Most of the issues associated with cracked blocks stem from head studs being over-tightened. Once more engine builders began observing the manufacturer’s recommended torque specs, cracked block cases declined.
To be sure, there is nothing you can do about a thinly cast area of the block. But there are a few insurance measures you can take when installing ARP head studs. First, chase all threads in the block with a 16×1.75mm tap to clean them up. Second, apply ARP thread sealer to the block threads of each stud that protrude into a water jacket. Third, and most importantly, when threading the studs into the block by hand, bottom them out and then back each one off ¼ of a turn (or 90 degrees). During the torquing sequence, the nuts will inevitably turn the studs. This will ensure the studs don’t dig into the block is key.
A major point of wear in the 6.4L valvetrain occurs at the tips of the rocker arms. Unlike the 6.0L, the 6.4L’s rocker arm ends don’t see the same amount of lubrication. Unfortunately, the engine oil can be heavily diluted with diesel fuel. As a result, what little oil the rocker does see isn’t the best lubricant. You end up with excessive ball pivot wear and the “tick” the 6.4L is notorious for. Equally alarming, it’s not uncommon to find each rocker loose when you pull the valve covers.
As the rocker arm’s ball pivot wears, it will begin to gouge into the valve bridge it rides on. Eventually, the ball pivot will wear completely down within its socket at the end of the rocker arm. And just think, while all this disintegration is taking place, those missing pieces are going somewhere…
Cracked pistons are the catastrophe that keeps on giving for the 6.4L Power Stroke. Regardless of power level, this failure can happen at any time during the engine’s lifespan. In most cases, it occurs in higher mileage engines or engines that’ve seen years of accumulative abuse. The fact that no factory short-block is immune to cracking a piston keeps many 6.4L owners on edge, and perpetually uncomfortable.
It might not be quite as bad as cracking the block, but a cracked piston can certainly render the crankcase irreparable. In virtually every case, the crack begins at the thin edge (or lip) of the piston’s fuel bowl. From there, it spreads across the piston, directly above the centerline of the wrist pin. When it comes to pass, white smoke out the tailpipe and excessive crankcase pressure gives the failure away. On top of your mechanic telling you that you’re going to be down a while.
A lot of the 6.4L’s injection system failures can be traced back to water infiltrating the fuel system. Lack of maintenance whereby the water separator seldomly gets drained allows a greasy, white buildup. Known as coagulation, it forms in the collection bowl, which eventually clogs up the drain valve ports.
Coagulation in the water separator effectively limits the assembly’s water-collecting capacity. Sooner or later water slips by and contaminates the fuel system. Once there, water reacts with the metal it comes into contact with and forms rust.
The 6.4L’s tight tolerance injection system doesn’t tolerate contaminants well. As with other high-pressure common-rail systems, the formation of rust within it creates the perfect storm. It often culminates in a colossal, chain-reaction failure of the Siemens K16 VDO high-pressure fuel pump self-destructing and taking out the piezo injectors with it.
Rubbing salt in the wound (as the 6.4L often does), accessing the high-pressure fuel pump calls for pulling the cab. The K16 VDO is gear-driven at the back of the valley and under a dedicated cover. It’s similar to how the HPOP is enclosed on the 6.0L. You’ve got considerable labor (i.e. money) involved in simply removing it. That’s still before forking over the cash for a new or remanufactured pump and everything else to start fresh.
When the K16 high-pressure fuel pump is headed south it typically throws a P0088 code. Unfortunately, when the K16 begins to self-destruct it’s known to send shrapnel through the rails and into the injectors. The cost of replacing the K16, the rails, the injectors, pulling and cleaning the fuel tank, and everything else the job entails typically starts at $6,000 and goes up from there.
In some cases, K16 debris entering an injector can cause it to stick open. This can melt the piston beneath it. Sadly, in these worst-case scenarios—where K16 failure costs the owner his or her short block—many choose to park the truck and walk away once they’ve seen the repair quote. After all, it’s hard to justify putting $15,000 or more in repairs into a truck. Especially if it’s only worth that much on the used pickup market.
This is a more mild K16-related failure, but one that still calls for pulling the cab. It unfolds when the volume control circuit grounds out. It happens when a hole is worn through the wire sheath of the high-pressure fuel pump harness that passes through the cover gasket. When the bare wire makes contact with the pump, it triggers poor performance or a no-start condition, and P0003, P0004, or P0091 codes.
Ford did issue an updated wiring harness that solves the volume control circuit ground problem (PN 8C3Z-9G805-B). However, if you’ve got a 6.4L that was built before August of 2007, you don’t have one of the lucky engines. When this problem strikes, you’re once again pulling or paying someone to pull the cab to perform what is actually a very simple fix at the back of the valley.

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