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The Independent Automotive Aftermarket Federation

MAHLE issues best-practice thermostat diagnosis

Date: Wednesday 12 April 2023

Thermal management specialists MAHLE has issued advice around the fitment of part numbers TM 36 103 & TM 37 103, which are both fitted to 1.2L & 1.4L vehicles.

MAHE has created a handy step-by-step guide for these applications to help determine if the thermostat is the original fault and, if replaced, whether the thermostat is functioning properly.

Firstly, technicians are advised to pressure test the cooling system and the сар for leaks and are then encouraged to repair any leaks before proceeding.

These engines use а 105° С (221 °F) thermostat. The ambient temperatures should be within the specified range given.

Next, they should set the HVAC controls in the OFF position and run the cold engine at idle 20° С-21° С (68-70°F) for 15-20 minutes before checking the engine temperature.

The engine coolant temperature should then be checked at idle with the scan tool. The engine coolant temperature (ECT) should be between 105-120°С (221-248°F).

If the temperature is below 105°С and the thermostat is opening early, ог the thermostat seal is leaking, the thermostat and the thermostat seal should be replaced.

However, if the ECT is above 120°С and the radiator inlet is hot, then the technician will need to turn ON the heater and check if hot air is coming from the heater outlets.

If there is no sign of any hot air, then the technician is advised to accelerate the engine several times in order to remove any air from the system and inspect for any blockages in the heater circuit or for any pinched or buckled hoses.

If, when they turn the heater ON, there are signs of hot air, then they will need to check for any blockages, which will need repairing before rechecking the coolant temperature with the scan tool.

No blockages at this stage may mean that the thermostat housing needs replacing.

MAHLE advises that upon realising the ECT is above 120°С and determining that the radiator inlet is not hot, then there may be air in the system and coolant will need to be added to the surge tank if levels are low. After checking the inlet hose, if it is cool, they should inspect for blockages in the cylinder head, the radiator and the radiator hoses.

Any blockages should be repaired, and the coolant temperature should be rechecked.

If there isn’t a blockage, then this may also mean that thermostat housing will need to be replaced and the coolant temperature checked.

MAHLE regularly provides helpful technical tips and advice in its enewsletter, MAHLE Insider, which customers can subscribe to here.