A3 8L 1.8t overheating

[Miltonov]

So my car thinks it's overheating - AUM 1.8T engine. Changed the CTS, waterp pump is almost new, thermostat is new. There's no smoke coming out of anywhere and there's no coolant loss.

Yesterday I drove 50km and everything was fine. Was checking the temp on the climatronic menu (49C) and it was floating from 88 to 98 when the car was heated up. It took the car about 10 kilometers to get from cold (30 to 90 degrees on climatronic menu). After 50km I stopped the car, went away for an hour, and got back. I took a 2km drive and the coolant temp on the climatronic dash got from 90 to 121 (max) in a heartbeat. I got a low coolant warning light and the bigger vent started spinning. I turned the engine off, went into the store for 20 minutes, came back, turned the engine on and the temp was at 88 degrees. I drove 50km again and the temp stayed normal.

Today I drove 2km. It's about 2 degrees outside and the car went from cold to 90 degrees (according to climatronic) in about 1 kilometer (it took it 10 kilometers yesterday.

Sometimes the temp goes from 30 to 90 very quickly, but gradually in 1-2 kilometers of drive. After that it sometimes falls back to about 50 and gradually gets back to 90 in the next few kilometers. However, sometimes it just shoots up to 121 in about 4 kilometers of drive.

A week ago it went from normal after 100km of drive to 121 in a heartbeat totally randomly. Stopped the car for 2 hours, went on another 100km drive and everything was normal.

I took it to the mechanic some time ago when the dash was showing 121. After a very quick inspection, he said that it doesn't show signs of overheating and it might be a faulty cluster.

Found a guy who owned the same car and he said that I have a bad instrument cluster and that it is a common fault of 8L's. I went through the entire web and didn't find any occasion of A3 8L false overheating because of a bad cluster.

What do you think? What should I do next?

[Magnet]

Hello Matt,

I will have a go at helping, but whether it turns out to be of any use .....?? 

‘Changed CTS’...... cars of this period were often fitted with a coolant temperature sensor ( which talked to the ECU to control fuel mixture etc.), and a coolant temperature sender which signals the actual coolant temperature to the temperature gauge. Are you sure you have changed the right one, if your model is indeed fitted with two? 

‘Changed thermostat....’ What make? - since aftermarket ones can be unreliable. 

Apologies but Climatronic menu is new to me, so I wonder if you have actually tried running the car with it plugged in to live data so you can get an accurate (confirmation) of the believed fluctuating temperatures. 

You mention you had a ‘low coolant light’ warning, yet the car doesn’t use any coolant. So this sounds like another issue. 

The description of the symptoms would point to thermostat issues, so it would be interesting to have your answer to -what make.

Probably nothing to do with it, but a faulty coolant reservoir pressure cap can give rise to issues, but this normally leads to coolant loss due to the system not retaining pressure. 

Of course, temperatures up to 121 degrees could only be maintained via. the system being under a relatively high pressure.

Last simple test would be feel the radiator core once the car is up to a reasonable temperature to see if there is any significant variation in the temperature across the core - which might point to partially blocked core issues. Hopefully no need to mention the safety issues with carrying out this test since fans can run/start up with the engine switched off. 

Kind regards,

Gareth.