Coolant Temperature Issues 1.9 Tdi

[UKS3Fan]

Changed coolant temp sensor and thermostat about a year and 10 months ago and have done 40,000 miles since then. Had an oil change couple of weeks ago and was told there is a light coolant leak. Noticed driving the last few days then it gets to 90 degrees then will drop a bit then come back to 90. Checked coolant and had dropped slightly below minimum so topped it up.

Is it really possible thermostat has gone again or more likely to be something else? What are the alternatives. Power is fine, all seems to run well apart from tyres not holding pressure perfectly. Want to avoid spending a lot on it being diagnosed!

 

[QuattroCalum]

Changed coolant temp sensor and thermostat about a year and 10 months ago and have done 40,000 miles since then. Had an oil change couple of weeks ago and was told there is a light coolant leak. Noticed driving the last few days then it gets to 90 degrees then will drop a bit then come back to 90. Checked coolant and had dropped slightly below minimum so topped it up.

Is it really possible thermostat has gone again or more likely to be something else? What are the alternatives. Power is fine, all seems to run well apart from tyres not holding pressure perfectly. Want to avoid spending a lot on it being diagnosed!

I'd be worried if it failed to reach 90 but the fact that it does suggests it's working fine.

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[UKS3Fan]

Thank you, is it not worrying though the it drops from 90 then goes back up to 90 again I thought it should only drop once the engine is switched off for a while?

 

[desertstorm]

If you happen to be watching the coolant gauge a lot you will see this on many cars when it is cold. When the thermostat is closed there is no flow through the radiator. As the engine warms up at some point when the thermostat starts to open there is an inrush of cold water into the engine. This cold water temporarily cools the engine and a drop in temperature is seen briefly on the gauge. The temperature in the cooling system stabilises and the temperature goes back up.
As long as it then stops at 90 there isn't an issue.
You can see the actual engine temperature in the AC diagnostic screen. Rather than the artificial temperature displayed on the gauge. In most cars as long as the engine temp is between 75 and 100 odd degrees the gauge shows 90.
http://mr-fix.info/audi-a6-c5-climatronic-hidden-menu/

 

[UK2004]

I had the coolant temp sensor, thermostat and fuel temp sensor replaced earlier in the year as well as wiring around fuel temp sensor fixed and consumption went back to good levels at last.

Noticed a couple of days ago water in passenger footwell so those drainage holes below the windscreen must be blocked again. Fuel consumption went up last couple of days, is it possible the water flowing into the footwell has damaged one of the three parts I had fitted earlier in the year and consequently I have bad fuel consumption again?