Thermostat: Description and Operation

Thermostat-3.9L/5.2L/5.9L Engines:

On 3.9L and 5.21J5.9L engines, the thermostat is located beneath the thermostat housing at front of intake manifold. This thermostat has an air bleed notch.

Thermostats installed in the 3.9TJ5.2I-J5.9L engines are of the wax- pellet-driven, reverse-poppetchoke-type. The wax pellet is located in a sealed container at spring end of thermostat. When heated, the pellet expands, overcoming closing spring tension and water pump pressure to force valve to open. Coolant leakage into pellet container will cause thermostat to fail in open position. Do not attempt to free a sticking thermostat with a prying device.

The thermostat controls the operating temperature of the engine by controlling the amount of coolant flow to radiator. On all engines, the thermostat is closed below 88°C (192°F). When coolant reaches this temperature, thermostat begins to open allowing coolant flow to radiator. This provides quick engine warm-up and overall temperature control. The thermostat is designed to provide a minimum engine operating temperature range of from 88 to 93°C (192 to 199°F). The thermostat should be fully open for maximum coolant flow during operation in hot ambient temperatures of approximately 104°C (220°F). Above 104°C (220°F), coolant temperature is controlled by radiator, fan and ambient temperature.

An arrow, plus the word UP is stamped on front flange next to air bleed. The words TO RAD are stamped on one arm of thermostat. They indicate proper installed position.

The same thermostat is used for winter and summer seasons.

CAUTION: Do not operate an engine without a thermostat, except for servicing or testing.

An engine should not be operated without a thermostat, except for servicing or testing. Operating without a thermostat causes longer engine warm-up time, unreliable warm-up performance, increased exhaust emissions and crankcase condensation that can result in sludge formation.