N14 Base Engine Troubleshooting and Repair Manual – 007-039   Lubricating Oil Thermostat

Oil flow through the oil cooler is controlled by a temperature sensitive bypass valve (1) in the oil cooler transfer housing (2).

The oil thermostat is either open (retracted) or closed (extended), or anywhere in between depending on the oil temperature.

When the oil temperature out of the lubricating oil pump is below 113°C [235°F], the bypass valve is in the open (retracted) position (1).

When the bypass valve is in the open position, 100 percent of oil from the lubricating oil pump bypasses the cooler.

When the oil temperature exceeds 124°C [255°F] out of the lubricating oil pump, the bypass valve extends to close the bypass loop in the cooler assembly. This will force all of the oil to flow through the cooler core before reaching the filter head.

After the oil is cooled below 113°C [235°F], the bypass valve once again retracts to open the bypass loop, allowing oil to bypass the cooler.

Remove the thermostat and the spacer plate. Refer to Lubricating Oil Cooler Procedures 007-003-002 and 007-003-006.

Visually inspect the valve for damage. Any damage to the front face of the valve will allow oil to constantly bypass the oil cooler.

Inspect the lubricating oil cooler transfer housing bypass valve seat. Refer to Procedure 007-047-007.

Suspend the valve and a 149°C [300°F] thermometer into a container of new lubricating oil. Do not allow the valve or the thermometer to touch the sides or the bottom of the container.

Heat the lubricating oil.

NOTE: The thermostat can be checked in an oven.

NOTE: Record the temperature at which the valve is fully extended. The valve must be fully extended to at least 6 mm [0.250-inch] when the temperature reaches 123°C [255°F]. The valve must begin to open at 113 to 115°C [235 to 240°F].

Replace the valve if it does not operate as described.

Install the bypass valve into the oil cooler transfer housing. Refer to Lubricating Oil Cooler Procedure 007-003-026.