The 2008–2010 6.4L Power Stroke is a twin-turbo, common-rail engine that runs a complicated cooling system, and when coolant flow gets restricted, the whole truck suffers. Like its 6.0 predecessor, the 6.4 uses an engine oil cooler that shares coolant with the rest of the system, and that oil cooler is a known choke point. Add a cooling system that leans on more than one radiator and a lot of hose, and you have a platform where coolant flow problems are common and worth understanding before you start throwing parts at it.
How the 6.4 cooling and oil cooler system works
The 6.4 uses a liquid-to-liquid engine oil cooler: engine coolant flows through the cooler to pull heat out of the oil. That means the oil cooler is plumbed into the same coolant that runs the rest of the engine, and it depends on that coolant staying clean and flowing freely. The 6.4 also runs a more elaborate cooling layout than a typical gas truck, with the primary radiator handling engine coolant and additional cooling capacity for the charge air and other loads. More coolers, more hoses, and more connections mean more places for flow to get restricted or for coolant to escape.
Why coolant flow gets restricted
The enemy here is the same as on the 6.0: casting sand, silicate dropout from old coolant, and general debris that settles into the tight passages of the oil cooler and the cooler cores. When coolant is neglected past its service life, the corrosion inhibitors deplete, material drops out of suspension, and the fine passages start to plug. A partially plugged oil cooler cannot transfer heat, so oil temperature climbs, and a restricted cooling system cannot shed heat, so coolant temperature climbs with it. The two problems compound each other.
The 6.4 is also known for radiator issues. A cracked or leaking radiator, internal or external, bleeds coolant out of the system, and a system low on coolant traps air and overheats. Whenever you are chasing a flow or temperature problem on a 6.4, the condition of the radiators is part of the picture, not an afterthought.
Symptoms of 6.4 oil cooler and coolant flow problems
- Rising oil temperature relative to coolant temperature. On a healthy engine, engine oil temp and coolant temp track fairly close under load. A widening gap points at a restricted oil cooler that can no longer pull heat out of the oil.
- Overheating under load, towing, or climbing grades as the system loses its ability to shed heat.
- Coolant loss, sometimes external at a cracked radiator or a hose connection, sometimes with no obvious puddle.
- Air in the system and inconsistent temps as a low or restricted system traps pockets of air.
- Sluggish heater performance in the cab, a classic sign of low coolant or poor flow.
The oil-temp-to-coolant-temp relationship is your best diagnostic tool here. If you can watch both on a capable scan tool, a large and growing split is the tell that the oil cooler is restricted. This is the same principle that plagues the 6.0, and we cover the mechanism in depth in the 6.0 Power Stroke oil cooler failure guide. The engines differ, but the physics of a plugged liquid-to-liquid cooler are identical.
How to diagnose it
- Monitor oil temp versus coolant temp with a scan tool that reads the correct 6.4 parameters. Note the split at idle and under load.
- Pressure test the cooling system cold and watch for leaks at the radiators, hoses, and connections. The 6.4 has a lot of joints to inspect.
- Inspect the coolant itself. Brown, cloudy, or debris-laden coolant tells you the system is dirty and the passages are likely fouled.
- Check for external radiator leaks and coolant staining at the front of the truck.
Do not condemn the oil cooler on temperature alone without ruling out a low coolant level, a failing radiator, air in the system, or a tired water pump. Confirm the restriction before you commit to the labor.
The fix, done properly
Addressing a restricted oil cooler means replacing the cooler and, just as importantly, cleaning the source of the contamination out of the system. A new cooler installed into dirty coolant is a new cooler on borrowed time. Use a quality Motorcraft-style cooler and fresh gaskets on everything you disturb.
While the system is open, flush it thoroughly to clear the sand and silicate, inspect and replace any radiator that is cracked or weeping, replace suspect hoses and clamps, and refill with the correct specification coolant. Bleed the air out completely, because the 6.4, like the 6.0, will fool you with a false overheat if you leave air trapped in the system. Run it up to temperature and confirm the level holds and the temps track where they should.
Parts you'll need
- Engine oil cooler and gasket kit
- Correct specification coolant, enough for a full flush and fill
- Radiator or radiators if yours are cracked or weeping
- Hoses, clamps, and any O-rings you disturb
- A proper coolant flush
You'll find oil coolers organized by engine in our oil cooler collection, and everything else for the platform lives in the 6.4L Power Stroke parts collection. Organized by engine, confirm fitment for your application.
FAQ
Is the 6.4 oil cooler the same failure as the 6.0? The mechanism is the same idea: a liquid-to-liquid cooler that plugs with debris and stops transferring heat. The parts and layout differ, but the diagnostic approach, watching oil temp versus coolant temp, applies to both.
Do I need to replace the radiators too? Only if they are cracked, leaking, or restricted. But the 6.4 is known for radiator problems, so inspect them carefully while you are in the system. A weeping radiator undoes all your other work.
Why does clean coolant matter so much? The oil cooler and cooler cores have fine passages. Old coolant drops silicate and debris that settles into those passages and chokes flow. Fresh, correct-spec coolant on a proper interval is the cheapest way to keep the whole system healthy.
Where we land
On the 6.4 Power Stroke, oil cooler restriction and coolant flow problems go hand in hand, and both trace back to a dirty, neglected cooling system. Diagnose with the oil-temp-to-coolant-temp split, replace the oil cooler along with any bad radiator, flush the system clean, use correct coolant, and bleed it right. Fix the root cause and you fix the symptom for good.
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Related guides: - 6.0 Power Stroke Oil Cooler Failure and Replacement Guide - 6.4 Power Stroke Up-Pipe and Turbo Gasket Leaks