Hoping someone can help with this as I’m out of ideas.

Coolant temp needle drops when I turn heating on

Main thermostat replaced

Dsg inline thermostat replaced

All coolant temp sensors changed

Fresh coolant

New waterpump

0 fault codes , scanned using vcds

All genuine parts

Heaters are HOT when on Hi, but if I turn it down 1 notch it is cooler

CFGB

Any ideas?

Thanks

On 20/08/2025 at 03:36, Gtd80y said:

Coolant temp needle drops when I turn heating on

From what temp to what new temp

I'd expect a drop of 5-10 egrees pretty normal

Bit surprised the guage shows it but....I thought they were pretty sticky around 90 degrees

On 19/08/2025 at 18:36, Gtd80y said:

Fresh coolant

Did you vacuum fill or just pour it into the expansion tank?

When changing the coolant this year I had to use a vacuum to fully fill with the new coolant.

Kev.

May be an airlock somewhere. Could be in the heater matrix

Alasdair

No air lock, tried back flushing heater matrix aswell and flow was good. Needle only drops when heaters go on otherwise stays solid 90

The new thermostat. Was it the correct one. I have heard of different ones for cold or hot climate?

Yes it was a brand new genuine part from TPS, fitted at a VW specialist. The car takes 11 mins to hit 90c driving at 30mph without heaters. As soon as heaters go on it will start dropping. The DSG thermostat was a genuine part aswell from TPS

Might be an over sensitive gauge. Can you use VCDS to check the actual temp and compare with gauge. Mine drops slightly in really cold weather but normally dont notice when heaters are on. On mine which is diesel I think there is an auxiliary heater for cabin. Wonder if its a problem with it somehow if you have one.

Alasdair

If it's still overcooling after all of that I'd question if the work was actually done by the mechanic..did he give you the old parts.