[Gtd80y]

Evening,

im hoping someone can help me with this issue, my last 2 golfs never did this so I know something isn’t right

Coolant temp needle drops below 90 when I turn the heaters on, I have replaced the following :

Main thermostat genuine
DSG thermostat genuine
2 x temp sensors (Meyle)
Timing belt and water pump genuine
New coolant

the car hits 90c within 10 minutes of driving and the temp needle will only start dropping when I turn heaters on, mechanic can’t figure it out.

2012 Golf GTD cfgb engine
Thanks

[NMNeil]

Have you measured the actual water temperature, and how far below 90 does it drop?

[olduser]

I think what you are seeing is hysteresis in the thermostat.

Without the heater on, the thermostat opens to approximately let sufficient coolant through to the radiator for the engine to be running at about the optimum temperature, bearing in mind the temperature gauge is not accurate.

When the heater starts to take heat from the coolant, dropping the coolant temperature, in a perfect world the thermostat would sense this and reduce the coolant flow to restore the engine to the optimum temperature but there will be some stiction in the thermostat causing it to not move or not move far enough.

In the past engine vibration may have helped the thermostat to move and temperature gauges were relatively insensitive so they masked the problem.

My feeling is, as the ECU is not throwing a code (P0128) the ECU is happy the engine is running efficiently and is not polluting, so no need to worry.

I read, that on some systems, the temperature gauge is driven by the ECU, and the ECU do's not adjust the gauge for minor deviations but holds the gauge at normal running temp until the deviation is significant.

[Gtd80y]

Could it be that the thermostat that was fitted could potentially be faulty? Slightly open?

[Gtd80y]

It will drop down to around 62c, is the new thermostat defective do you think?

[olduser]

If, without the heater everything works as it should suggests the thermostat is OK.

Just a thought - the heater usually picks up hot water from the engine side of the thermostat, and returns the cooled water on the radiator side of the 'stat.
Is the heater water return near the temperature gauge sensor, so that it is measuring the returned (cooler) water from the heater or being unduly influenced by the returned water temperature?