Mk 4 GTi 1.8 not warming up

[denam]

My 2001 Mk4 GTi water temp will not rise above 57 deg C as recorded by my VCDS.
The thermostat and temp sensor have been changed and have no effect.
With the heating turned up to max the car interior gets warm but not hot.
If I switch off the Aircon the engine temp with rise to the normal 90 deg C and the in car heating gets very hot on the max setting.
Switching on the Aircon again causes the engine and saloon heating to drop again.
Has anyone any brilliant ideas?

[denam]

Problem now solved.
The new thermostat and coolant sensor which were both genuine VW parts were both faulty and only served to confuse.
Temp gauge now sits firmly at 90 deg C again.

So the moral  of the story is - do not write off the obvious and assume new genuine parts are fool proof.

[andy68]

Agree with the OEM not being perfect, CTS have always been fine for me, but have had 2 OEM thermostats pack up after 18 months each, also had a Unipart and Q&H stat both lasted 18 months, currently on a Gates one(6months so far) put Stats on my service regularly list now

[Wazzzer]

interesting thread for me as I'm sure my thermostat is starting to play up, on the way back from London the other day the temp was dropping slightly while on the move...

Seems genuine could be the way to go but even then it's not 100% guaranteed

[brewey]

I have a similar problem where the temp drops on the move. Fitted a new coolant sensor yesterday and it now reads even lower. Don't understand it at all.