Overcooling and Overheating
Cooling System On-Board Diagnostics
Modern on-board diagnostic (OBD) systems monitor engine cooling, transmission cooling, motor-generator cooling, and power control circuit cooling.
Circulation of both air and coolant is crucial for mechanical and electronic systems.
Vehicle diagnostic systems monitor coolant and air temperatures in high heat generating systems.
Sensors read signals for coolant level, engine oil temperature, water pump drive-belt tension, engine knock (due to overheating), and ambient air temperature.
OBD systems also measure ambient or outside air temperature surrounding the engine and electric drive train.
Use a scan tool to analyze the system when the malfunction indicator light is on.
Hybrid/Electric Vehicle Considerations
For hybrid gas-electric or full-electric vehicles, OBD reads both coolant and air temperatures as a fail-safe.
The engine ECM monitors coolant and air temperatures circulating through the HV battery pack housing.
It measures temperatures of the motor-generator stator housing, the HV PCM heat sink, and the HV coolant liquid (antifreeze and water solution).
Scan for stored trouble codes and observe real-time operating temperatures from all vehicle sensors.
Temperature Sensor Readings
If a vehicle sits overnight, compare scan tool readings for all temperature sensors (engine coolant, engine air inlet, supercharger coolant, HV coolant).
All temperature sensors should read approximately the same ambient air temperature.
Record all air and coolant temperature sensor readings for both the gas and electric powertrain after warming the engine to full operating temperature.
All scan tool readings at full operating temperature (engine, HV battery, motor-generator, HV PCM, supercharger, turbocharger) must be accurate and within specifications.
An incorrect temperature reading from a sensor suggests the sensor is faulty, or a major assembly may be overheating.
Pinpoint tests are required to verify the exact source of the problem.
Diagnosis Charts
Use a cooling system diagnosis chart to locate difficult problems.
A service manual contains accurate charts for the engine and cooling system design.
The chart helps decide which tests and repairs are needed and their proper sequence.
Cooling System Inspection
A visual inspection can reveal cooling system, mechanical, and HV driveline problems. Look for:
Coolant leaks: Antifreeze solution on the ground under the vehicle.
Loose or missing fan belt: Can fly off the water pump pulley; loose belts may not spin the water pump properly.
Low coolant level: Check the translucent, plastic reservoir tank.
Abnormal water pump noises: Grinding or chattering sounds restrict coolant flow.
Leaves and debris: Obstructions on the outside of the engine radiator can cause overheating.
Coolant in the oil: White and milky oil indicates water and antifreeze mixed with the engine lubricating oil, possibly due to leaking head gaskets or a cracked head/cylinder block.
Combustion leakage into the engine coolant: Air bubbles in the coolant indicate hot exhaust gases entering the cooling system, reducing water pump efficiency and causing overheating/radiator overflow.
Safety Warning
Wear eye protection and stand behind (not over) a spinning engine and high-voltage cooling system fan blade.
Keep hands and tools away from the spinning fan.
Disconnect electric fans before working near or on them.
Cooling System Problems
Cooling system problems can be grouped into three categories:
Coolant leaks
Overheating
Overcooling
Coolant Leaks
Cracks or ruptures in the system allow pressure cap action to expel coolant.
Overheating
Engine temperature is too high, the warning light is on, the temperature gauge shows hot, or coolant/steam are blowing out of the overflow.
Overcooling
The engine fails to reach full operating temperature, resulting in poor or sluggish engine performance.
Coolant Leaks (Detailed)
A low coolant level may indicate an internal or external antifreeze leak.
External leaks appear as wet, discolored areas in the engine compartment or on the ground, smelling like antifreeze and having a similar color.
Redder coolant color indicates more suspended rust particles.
Leaks can occur anywhere, but usually at hose ends or the radiator/heat exchanger.
Internal leaks are caused by a cracked engine block, cracked/warped cylinder heads, or blown head gaskets, contaminating the oil and turning it lighter in color.
Check the coolant level in late-model vehicles at the overflow tank or plastic coolant reservoir.
Avoid removing the pressure cap unless tests/repairs are required.
Only open systems (no reservoir) require removing the pressure cap to check coolant level.
Figure 48-2
Illustrates how engine problems can affect the cooling system.
A - Blown head gasket allows combustion gases to enter the coolant, creating bubbles.
B - Mineral deposits in water jackets prevent proper heat transfer, resulting in overheating.
C - Cracked part or blown head gasket allows coolant to leak into the engine oil, turning it milky white.
Warning: Radiator Cap Safety
Never remove a radiator cap when the engine is hot.
Pressure release can cause coolant to boil and expand into steam vapor, potentially causing severe burns.
Water boils at , and modern cooling systems operate at .
Removing the pressure cap can cause scalding steam to shoot out of the radiator.
Tech Tip: Radiator Filler Neck
Modern sloped hood lines place the radiator filler neck on or near the engine or overflow tank to help purge air from the cooling system.
Combustion Leak Test
Checks for combustion gases in the coolant, indicating a blown head gasket, cracked block, or cracked cylinder head.
Perform when overheating, bubbles in the coolant, or coolant level rise upon starting are observed.
Service Procedure
Place a block and head tester (combustion leak tester) in the radiator filler neck or reservoir.
Start the engine.
Squeeze and release the tester bulb to pull air from the cooling system through the tester.
The fluid in the block tester is normally blue; exhaust gases cause a reaction, changing its color.
A combustion leak turns the fluid yellow; if it remains blue, there is no combustion leakage.
If a leak is indicated, short out spark plugs one at a time, testing the cooling system with each plug shorted.
When the fluid doesn't change color, the cylinder being shorted out has a combustion leak.
Consequences of Combustion Leakage
Very damaging; exhaust gases mix with coolant and form corrosive acids that can corrode the radiator and other components.
An exhaust gas analyzer can detect combustion leakage into coolant.
Coolant in Oil
Water, antifreeze, and oil mix, creating a milky white solution.
Found in the engine oil or valve covers, indicating a coolant leak.
May be caused by a blown head gasket, cracked head/block, or leaking intake manifold gasket (on V-type engines).
Both combustion leakage into the coolant and coolant leakage into the engine oil can occur simultaneously.
Antifreeze in engine oil can cause engine damage: cylinder wall gumming or scoring.
Always correct the engine problem causing internal leakage.
Figures 48-3 and 48-4
Figure 48-3: Illustrates using a bulb to draw a sample of radiator air into the combustion leak tester.
Figure 48-4: Shows an exhaust gas analyzer, used by holding the test sensor over the coolant filler neck and accelerating the engine. LED indicators light when combustible gas is detected, indicating internal combustion leakage.
Modern Scan Tools
Provide temperature sensor readings or operating values, useful for solving engine overheating and related problems.
Common Causes of Engine Overheating
Low coolant level: Due to a leak or lack of maintenance.
Thermostat stuck closed: Prevents almost all coolant circulation, causing rapid engine overheating.
Rust or scale: Accumulations of corrosion debris and acid in the coolant clog the radiator core or build up in the water jackets.
Stuck thermostat: Restricts coolant flow.
Retarded ignition timing: Transfers too much heat into the exhaust valves, ports, and manifold.
Loose fan belt: Reduces coolant circulation.
Bad water pump: Broken shaft or damaged impeller blades prevent normal pumping action.
Collapsed lower hose: Suction from the water pump may flatten the hose if the spring is missing or the hose is badly deteriorated.
Missing fan shroud: Reduces airflow through the radiator.
Ice in coolant: Can crack engine blocks and cylinder heads, block circulation, and cause overheating.
Engine fan problems: Prevent adequate airflow through the radiator or heat exchanger, resulting in overheating and failure.
Open temperature sensor: Prevents the fan motor from energizing to remove heat.
Radiator Inspection and Cleaning
Inspect the outside of the radiator for debris, such as leaves and dirt, which can limit air circulation through the core. If necessary, use a water hose to wash debris out of the core. Spray water from the back of the radiator to push debris out of the front.
Overheating Protection
Some engines are protected from overheating by the engine control module using data from the engine temperature sensor to detect overheating. The computer cuts off the spark to one cylinder at a time.
Ignition timing, throttle response, and fuel injection are also altered to limit top speed.
"Dead cylinders" cool the engine and prevent overheating damage.
Engine Overcooling Symptoms
Slow engine warm-up, insufficient warmth from the heater, low fuel economy, or sluggish engine performance.
Overcooling reduces fuel economy because more combustion heat transfers into the metal parts of the engine.
Overcooling can also cause increased wear on engine or electric drive parts.
Parts are not at full operating temperature, their clearances are too great-the parts will not expand enough to produce the correct fit.
Conditions Causing Overcooling
Thermostat stuck open: Allows too much antifreeze circulation.
Locked fan clutch: Excess airflow through the radiator.
Shorted fan switch: An electric fan that runs all the time increases warm-up time.
Shorted temperature sensor: The electric cooling fan runs constantly, even with the ignition turned off.
Water Pump Service
A bad water pump may leak coolant (worn seal), fail to circulate coolant (broken shaft or damaged impeller), or produce a grinding sound (faulty pump bearings).
Rust in the cooling system and lack of coolant are common reasons for pump failure.
An overtightened drive belt is another common cause for premature water pump failure.
Checking a Water Pump
To check for a bad water pump seal, pressure test the system and watch for leakage at the pump. Coolant will leak out of the small drain hole at the bottom of the pump or at the end of the pump shaft. A leaking water pump must be replaced.
To check for worn water pump bearings, try to wiggle the fan or pump pulley up and down. On vehicles equipped with a serpentine belt, you may need to remove the drive belt and turn the water pump pulley by hand to check for problems.
Figure 48-7
Shows a visual inspection to identify water pump problems.
A - Wiggle the engine fan to check for water pump bearing wear. The pump shaft should not wiggle, and coolant should not leak.
B - On engines that use large drive belts and electric fans, you may need to remove the belt to check water pump condition. Turn the pump pulley by hand to check for roughness or looseness.