Probable Causes
- Driver perceived problem
- The connection for the shunt line is incorrect.
- Kinked coolant hose or collapsed coolant hose
- The cooling fan is always on.
- Faulty water temperature regulator
- A problem with the fuel heater (if equipped)
- A problem with the auxiliary coolant heater (if equipped)
- A problem with the vehicle's heating system
Recommended Actions
Driver Perceived Problem
The driver expects the engine to be running at a higher temperature for the operating conditions. The following variables will affect the engine temperature:
- Ambient temperature
- Load factor
- Engine speed
- Heater cores for the truck cab
- Fuel heaters
- Transmission coolers
Graphs will be used in order to define expected operating temperatures.
The graphs that follow are for the C15 engine. Temperatures for the C13 engine will follow these trends. The temperatures for the C13 will be slightly lower.
The graphs represent the following specific conditions:
- The fuel heaters are shut off.
- A typical transmission cooler is used.
- No coolant is supplied to the Auxiliary Power Unit (APU).
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| Illustration 1 | g02076555 |
The heater core is considered on when the temperature and the fan speed are at the maximum. (1) The heater core for the Cab is off. The heater core for the sleeper cab is off. (2) The heater core for the Cab is on. The heater core for the sleeper cab is off. (3) The heater core for the Cab is on. The heater core for the sleeper cab is on. | |
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| Illustration 2 | g02076554 |
The heater core is considered on when the temperature and the fan speed are at the maximum. (1) The heater core for the Cab is off. The heater core for the sleeper cab is off. (2) The heater core for the Cab is on. The heater core for the sleeper cab is off. (3) The heater core for the Cab is on. The heater core for the sleeper cab is on. | |
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| Illustration 3 | g02076553 |
The heater core is considered on when the temperature and the fan speed are at the maximum. (1) The heater core for the Cab is off. The heater core for the sleeper cab is off. (2) The heater core for the Cab is on. The heater core for the sleeper cab is off. (3) The heater core for the Cab is on. The heater core for the sleeper cab is on. | |
The Connection for the Shunt Line is Incorrect.
Ensure that the shunt line is connected to the water inlet elbow.
Coolant Hoses
Inspect the coolant hoses. Look for kinked hoses and for hoses that are collapsed. Make the necessary repairs.
Cooling Fan
A cooling fan that is operating incorrectly can cause overcooling. Refer to Troubleshooting, "Cooling Fan Is Always ON", if necessary.
Water Temperature Regulator
A regulator that is stuck open can cause overcooling. Test the water temperature regulator according to Testing and Adjusting, "Water Temperature Regulator - Test".
Fuel Heater
Some fuel heaters use coolant to warm the fuel supply. A malfunctioning fuel heater can lower the temperature of the coolant.
Determine if the vehicle is equipped with a fuel heater. Check the heater for proper operation. Repair any problems.
Auxiliary Coolant Heater
A malfunctioning auxiliary coolant heater can cause low coolant temperature.
Determine if the vehicle is equipped with an auxiliary coolant heater. Check the heater for proper operation. Repair any problems.
The Vehicle's Heating System
The vehicle's heating system may be part of the engine's cooling system. A malfunctioning heating system can cause low coolant temperature.
Determine if the vehicle's heating system is part of the engine's cooling system. Check the heating system for proper operation. Repair any problems.