Troubleshooting - Cooling System
Symptom-driven guide to diagnosing overheating, overcooling, coolant loss, and cooling fan problems on the Porsche 924, 944, and 968.
Difficulty: Moderate3 min readApplies to: 924 · 944 · 944S · 944S2 · 944 Turbo · 968
Introduction
This troubleshooting guide is intended to help identify and repair problems with the cooling system. Symptoms covered:
- Overheating
- Overcooling
- Loss of Coolant
- Improper Cooling Fan Operation
Engine Overheats
| Possible Cause | Checks | Procedure |
|---|---|---|
| Improper coolant level | Check coolant level and fill if necessary. | N/A |
| Cooling fans, thermofan switch, or fan relay not operating correctly | Check cooling fan operation using COOL-01. | COOL-01 |
| Air in cooling system | Fill and vent cooling system using COOL-02. | COOL-02 |
| Head gasket | A leaking head gasket can allow cylinder gases to pressurize the cooling system, void the area around the cooling jackets, overpressurize the system, and cause the reservoir cap to relieve coolant — resulting in overheating. This may look like an external coolant leak initially. Identify with a compression test or cylinder leakdown test, or have a radiator shop test the coolant for the presence of exhaust gases. | N/A |
| Faulty thermostat | Remove and test the thermostat using COOL-08. | COOL-08 |
| Wrong thermostat | When Porsche changed the water pump on later 944s, the depth of the snap ring groove from the thermostat sealing surface changed, requiring a different thermostat and sealing ring. Installing the wrong thermostat can cause cooling problems. See COOL-07. | COOL-07 |
| Bad water pump | With some rebuilt water pumps, the pulley can slip on the shaft — particularly with a fresh rebuild where the pump seal is tight. To check: remove the top timing cover and locate the water pump pulley. Mark a line across the end of the pump shaft and pulley using a paint pen or fingernail polish. Start the engine; if the pulley is slipping, the marks will not remain aligned. Correction: place tack welds on opposing sides of the shaft where it meets the pulley to keep the pump balanced. | N/A |
| Faulty coolant reservoir cap | If the cap does not hold the correct pressure, coolant can boil and create voids in the system. Remove the cap and verify it opens at the pressure stamped on it (normally in millibars). Have a radiator shop or auto parts store test it if you cannot borrow a coolant pressure tester. | N/A |
| Radiator internal passages blocked or excess debris on outside | Remove the radiator using COOL-04 and take it to a professional radiator shop for testing and cleaning. | COOL-04 |
Engine Overcools
| Possible Cause | Checks | Procedure |
|---|---|---|
| Faulty thermostat | Remove and test the thermostat using COOL-08. | COOL-08 |
Engine Loses Coolant
| Possible Cause | Checks | Procedure |
|---|---|---|
| External Leak | Leaks often do not appear until the coolant system is fully pressurized, which only happens at operating temperature. The engine compartment heat can make finding leaks difficult. Pressurizing the system cold with a coolant pressure tester is much simpler. Possible leak locations: coolant hoses (radiator, heater control valve, return from heater core at the back of the engine compartment); water pump (may require removing upper timing cover). | N/A |
| Internal Leak | Internal leaks allow coolant into the cylinders or the oil system. Possible locations: head gasket; oil cooler seals; cracked cylinder wall (rare). Head gasket leaks into cylinders produce large amounts of white smoke from the exhaust; remove spark plugs one at a time and look for a wet plug that smells like antifreeze. Confirm with a compression test or cylinder leakdown test. Oil system contamination typically first appears as oil in the coolant (milky fluid) because the oil system normally runs at higher pressure than the cooling system. | N/A |
Cooling Fans Do Not Operate Correctly
| Possible Cause | Checks | Procedure |
|---|---|---|
| Cooling Fans | Cooling fan operation and troubleshooting is covered in detail in COOL-01. | COOL-01 |
Tags:coolingoverheatingcoolantthermostatwater pumpradiator