Coolant temp indication
There’s an ongoing issue with my father’s 2007 VW Polo 9N GTI with the
1.8 turbo engine. The EML keeps coming on with the fault code ‘P0116 coolant temp range/performance’. There are no driveability issues, except sometimes the coolant gauge sits between 90 and cold when at motorway speed and the heating can take a while to get hot. The cambelt and water pump have been replaced using genuine VW parts, and the CTS has been changed twice (the second time with a genuine VW part) along with the thermostat (non-genuine).
The light can stay off for months but always comes on when cruising on the motorway. I have run the engine up to temperature from cold to until fan cuts in (engine idling) and monitored the temperature via the value blocks, but no issues are noted with the temperature. The fan kicks in around 105°C. The only other thing I have noticed is that when I replaced the above parts it was very difficult to bleed the system.
I worked at VW for a while and never had any issues bleeding the cooling system. To me, all the symptoms point to a stuck-open thermostat, but I have replaced this. I might fit a genuine VW thermostat to eliminate this. I know this system has an auxiliary pump that runs when the engine is off and an ECU for the cooling fans – could they be faulty and causing the fans to run all the time?
Russell Harding
Given the fault code, my first action would be to check the live data and, having confirmed that the sensor is reading correctly, replace the thermostat, as you have done. Having observed that the heater takes a while to get up to temperature, this would rule out electrical connection problems. Even if the fans were running constantly, the coolant flow should be impaired by the thermostat sufficiently to heat the engine up to operating temperature before it is affected by the airflow through the radiator.
Having experienced new parts that have been defective, I would re-inspect the thermostat, as logically this is the only component that would allow the engine to run below temperature if not operating correctly.
We then received the following after our initial reply…
I replaced the thermostat with a genuine VW one. The old thermostat (I say old but three months!) had dislodged from its centre hole which meant it was stuck open. It proves that paying the extra for genuine parts is definitely worth it! I have sent you a picture of the defective pattern part and when comparing the two the genuine one had a larger spring.
The temperature now stays around 90°C when driving with hot air from the heater and so far no warning lights. Thanks for your help.
Russell Harding