6.7 Power Stroke Thermostat Replacement

6.7 Power Stroke Engine Cooling System Thermostat

All 6.7 Power Stroke engines utilize two separate cooling systems. A detailed account of the cooling system functions and flow paths is available here: 6.7 Power Stroke cooling system overview. This article will focus on the primary cooling system, which is responsible for engine cooling (engine block, cylinder heads, lube oil cooler, and turbocharger). The primary cooling system utilizes a dual thermostat assembly which incorporates two thermostatic mechanisms that open at different coolant temperatures. The low temperature thermostat begins to open at 190 °F and reaches the fully open position at 212 °F. The high temperature thermostat begins to open at 197 °F and reaches its fully open state at 217 °F.

When closed, coolant is returned to the water pump instead of traveling through the radiator circuit. When open, coolant will flow through the radiator before returning to the water pump. By design, there should be always be some degree of flow through the radiator when the engine has reached operating temperature. It is important to note that the thermostats control the same circuit, not separate ones. The dual thermostat design with staggered opening temperatures helps prevent thermal shock (excessively rapid cooling) and provides improved coolant flow rate regulation.

Thermostat Diagnostics

Certain powertrain configurations are known for having flawed or otherwise under-designed cooling systems; the Ford 6.7 Power Stroke is not one of these cases. Ford has a generally positive reputation in terms of engine cooling system design, and from the early 6.9 IDI to the current 6.7 Power Stroke, their cooling systems have always proved adequate. A handful of anomalies and horror stories shared over public forums does not change this fact.

As a generality, a failed or faulty thermostat is going to present itself as an overheating condition. The average person is not on the lookout for signs that an engine is not reaching operating temperature and unless the symptoms are serious enough to trigger a DTC, this condition will go unnoticed. When a thermostat becomes stuck in the closed position, it prevents adequate coolant flow across the radiator and heat will build in the engine coolant until warnings are triggered in the instrument cluster display. When a thermostat becomes stuck in the open position, coolant continuously flows through the radiator and, depending on the conditions, the engine may never reach full operating temperature.

Engine Overheating

The most obvious symptom of a faulty thermostat is an overheating condition. However, before jumping to conclusions, always verify that the coolant level is correct and that there is no contamination of the engine coolant. Coolant contamination points to much larger problems than a faulty thermostat. Additionally, verify that the engine cooling fan is functioning properly. Some overheating conditions experienced at idle in high ambient temperatures could be the result of insufficient airflow across the radiator. Additionally, take the following into account before replacing the cooling system thermostat:

1. Verify that the primary cooling system radiator is clean and free of build up; dirt and road grime accumulation can have a significant impact on heat dissipation across the radiator because it acts as an insulator, not a conductor.
2. If the coolant level is low, a leak test should be performed to find the source of a leak and/or verify that the system holds pressure.
3. Use a scantool to verify that all temperature sensor readings are normal and there are no abnormalities. Read each coolant and oil temperature PID after allowing the engine to cold soak overnight; the coolant and oil temperatures should read very close to the ambient temperature after a proper cold soak.
4. If an engine begins to overheat, immediately reduce engine load and turn the cabin heater on full. Take note as to whether the cooling fan has kicked on or not; it should kick on long before any engine overheating indications.

Engine Not Reaching Operating Temperature

Not all vehicles equipped with the 6.7 Power Stroke have a temperature gauge that displays actual temperature, so low operating temperatures often go unnoticed. Cool ambient conditions and driving patterns, such as short trips at low speeds, can prevent an engine from reaching operating temperature. But what if an engine never reaches operating temperature, even in high speed, high load scenarios?

Such is the issue with the vehicle featured in the procedures below. Even towing a 15,000 lb trailer at 55 mph in the dead of summer, the operating temperature never exceeds 176 °F. And under normal, unloaded driving conditions, the coolant temperature hovers around 172 °F. Replacing the thermostat brought the normal operating temperature up to 190 °F and around 196 °F under the same towing conditions - about a 20 degree increase across the board. Case in point, thermostats can fail in more than one way.

Applicable Parts Lists

[1] - Part can be reused if inspected and deemed to be in good condition with no defects.
[2] - Engine coolant can be reused if in good condition (uncontaminated) and collected in clean vessels. Old, cloudy, or otherwise poor quality engine coolant should be replaced. Do not mix coolant brands and/or types.
[3] - Concentrated engine coolant must be mixed with distilled water at a 50/50 ratio. Prediluted engine coolant is ready to use, do NOT add water.

How to Replace the Thermostat on a 6.7 Power Stroke Engine

Click any thumbnail to view fullsize, detailed image

• After allowing the engine to cold soak (i.e. sit long enough for the engine coolant to reach ambient temperature), remove the degas bottle cap for the primary cooling system (upper driver side of engine compartment). Do not attempt to remove the degas bottle cap nor work on the cooling system until the engine and engine coolant has cooled.
• Locate the radiator drain valve (petcock) on the driver side, lower portion of the radiator.

• Attach a section of 5/16 inch ID heater hose or chemical tubing to the drain valve spout. Place the other end of the hose in a clean container suitable for recovering engine coolant; in total, 3 to 4 gallons of coolant are going to be recovered.
• Open the radiator drain valve by rotating the handle 180 degrees counterclockwise. Note that the first 90 degrees of rotation is going to unlock the valve and the second 90 degrees of rotation is going to cause the valve to move outward and pop off its seat.

• Locate the upper radiator hose for the primary cooling system.

• At the radiator connection, pull the quick-connect locking mechanism upwards to disconnect the upper hose from the radiator fitting.
• Place a shop towel/rag beneath the fitting to catch any remaining engine coolant when it is removed in a future step.

• When properly unlocked, the radiator hose locking mechanism will fall into detents molded into the fitting (pictured).

• Locate the upper radiator hose locking mechanism at the thermostat housing and unlock it in the same manner described in the previous steps.
• Pull the radiator hose upwards at the thermostat housing followed by away from the radiator connector to remove the hose.

• Remove the (3) thermostat housing bolts with an 8 mm socket. Be extremely careful not to lose one of the bolts. Also note that the front-most bolt (closest to radiator) is shorter than the other (2) fasteners.

• Remove the thermostat housing.
• Remove the primary cooling system thermostat.

• Ensure that the thermostat gasket sealing areas are clean and free of debris or remnants of the old seal.

• Very lightly coat the new thermostat housing seal with silicon grease; excessive application may contaminate the engine coolant, so use sparingly.

• Install the new thermostat so that the gasket seats appropriately. The thermostat vent hole must be installed towards the passenger side of the engine. If looking at the thermostat from over the driver side fender, this is the 12 o'clock position. If looking at the thermostat from the front of the vehicle, this is approximately the 9 o'clock position.

• Reinstall the thermostat housing. Recall that the front-most bolt is shorter than the other (2). Torque bolts to 89 in-lbs in a criss-cross fashion.
• Reinstall the upper radiator hose, ensuring that both locks are properly engaged. Tug the hoses to ensure that the connections are secure.
• Verify that the radiator drain valve is closed, then refill the cooling system through the degas bottle.
• Reinstall the degas bottle cap, start engine and check for leaks. Verify coolant level after operating vehicle and allowing engine to cool.