Temperature gauge going up soon after new thermostat

[Evie]

New car for me with 70,000 miles on the clock, got it in September. Within 2 weeks the audi overheated, took it to garage, did head gasket test, not that, did thermostat test.. faulty thermostat. Replaced that, all fine. But now about 12 weeks later the temp gauge keeps going up every time I drive the car, it'll go up 2/3 bars for about 30 seconds and then drop right back of to 90 degrees again, it only does this once a day despite how much I drive it.  I took car back to garage who fitted thermostat and they couldn't find a problem, took it to another garage, they did a pressure test thing for the coolant to see if there was a leak, no leak. So assuming now it must be another faulty thermostat getting stuck? Or a crappy sensor?

Please does anyone have any ideas? Or have you had a similar problem? Car still drivable as it doesn't actually "overheat"... 

Thanks in advance for any response. 

[Magnet]

Hello Evie,

I believe your alternative suggestions are sound. What I would ask is did you have the thermostat changed at an Audi dealership? If not, do you know whether the garage used a genuine Audi part or an aftermarket replacement? If genuine Audi part used then I would suspect the temperature sender. If not, then I would certainly suspect the aftermarket replacement since aftermarket component quality can be highly variable. 

Kind regards,

Gareth.

[Evie]

Good morning Gareth, 
 

Thank you very much for your informaitve reply!
 
The thermostat wasn't changed at an Audi dealership, but as I understand it was a genuine audi part used.  Would this mean it's the temperature sensor gone wrong that's held in the coolant tank?  If so, think I'll get that changed and see if the gauge behaves, before spending more money digging back into the thermostat. 

Kind regards, 
Evie 
 

 

 

[Magnet]

Hello Evie,

The   '..........as I understand it it was a genuine Audi part......' to be frank doesn't instil too much confidence in the answer, but of course it is what you have been told, and you must go with it. Or perhaps it might come to proving that to be correct or incorrect.

Air locks? After 12 weeks I would expect this would not be a valid reason for your issue. 

Thermostat? Could be. Sender unit? Could be.

If I were mine, I would be taking the car to another trusted local garage and get them to plug this in and obtain live data on the coolant temperature. This should give you a better idea of the actual running temperature at these apparent 'increased peaks' and allow them to better home in on whether this is a sensor or a thermostat fault. Others on here will be more knowledgeable on whether this live data simply reproduces what the (possibly faulty) sender reads, or if it is an actual temperature. No doubt someone will advise. 

Kind regards,

Gareth

[Evie]

I didn't have much to do the with garage when dealing with the thermostat change as the garage I purchased the car off were sorting it as still under warranty. I would certainly hope it was a genuine audi part and not something aftermarket, no-wonder there might be a problem! 

I'm at a slump now.. I don't know wether to go back and get the garage to check the thermostat or wether to do a process of elimination and get the coolant sensor changed as that would leave only the thermostat to be replaced.  We did a test at a garage on the coolant pipe leaks, and there were none found. 

Garenth, what please is the Sender unit? Is that to do with the thermostat?

Kind regards, 

Evie 

[Magnet]

Hello Evie,

For sender, read sensor.

Now knowing the full facts regarding the reason for the thermostat change, I would seriously doubt if a genuine VAG part was used, since the car sales garage was paying! 

 

Now we need a more experienced input here, but is the thermostat arrangement on this engined model an 'electronic'  type?  If so it equals expensive (and includes the sensor??). 

 

My next step advice here would be to get a price for the part from your Audi main dealer ( have the Reg. No. and VIN to hand) and enquire what type of thermostat type this is - i.e. Includes the full plastic housing and the sensor. You can then check with a local motor factors and get an equivalent price. Perhaps this may then point you in the next direction from which this has come, and where your next move is.

Kind regards,

Gareth.