Temperature gauge going up and down like a yo yo

[Vince70]

I wonder if someone can help,
I have an audi 80 2.3 and the other week I put my heater on and noticed the water dropped in the expansion tank by about a litre so I refilled it with anti freeze and all was well.
Today I warmed the car up and all was well then when I put my heater on my temperature when back down to 0%.
I was thinking maybe an airlock or maybe a new thermostat, I put my heater on full hot and undid the bleed screw on the pipe which goes into the heater matrix but nothing came out.
Also I notice when I undo the expansion tank the level goes up a touch but don't know if this is related.
Any ideas would be much appreciated.

[HustleRussell]

You need to bleed it using the bleed screw with the engine running and the heater on. It may take more than one attempt to get all the air out.
If air keeps getting in somewhere then the system is either losing pressure or losing coolant and needs further diagnosis.

[blaineuk]

New thermostat I would think.

[mybrainhurts]

Witchcraft I would think...

[Alucidnation]

Should the temp gauge on the instrument panel be stable, regardless of heating being used and outside air temp?

[Vince70]

The temperature gauge goes up to 90 then when I put the heater on it dropped back down to zero and hasn't gone back up again.
I still seem to get warm air out of the heater but its only warm not hot.
I have checked the oil and water and all seems well without any mayonnaise but the car does steam up when I put the heater on but I put that down to being a cabriolet and not being used in the last 3 months.
When I first put the heater on it looked like steam was coming from the vents but it was pretty damp in there as I steamed cleaned the interior before laying the car up 3 months ago.

[TonyRPH]

Leaking heater matrix.

I had the same issue with a 2.8 A4 a few years ago.

Check for damp carpets around the transmission tunnel.

[Mr Wolf]

take the expansion cap off, allow to run up to temp and ensure the stat opens (feel the pipe downstream of stat)

It should self bleed

[Ozzie Osmond]

Alucidnation said:

Should the temp gauge on the instrument panel be stable, regardless of heating being used and outside air temp?

In colder weather it is not at all unusual to see a fluctuating temperature gauge in some cars,

• engine warms water in block - temp gauge rises
• thermostat opens - cold water floods in - temp gauge falls
• thermostat closes - emgine warms water in block - temp gauge rises
• thermostat opens - cold water floods into block - temp gauge falls
• etc
• etc

[Vince70]

TonyRPH said:

Leaking heater matrix.

I had the same issue with a 2.8 A4 a few years ago.

Check for damp carpets around the transmission tunnel.

I was worried about that but looking at the car I would say its already been done as I can see that the centre console has been removed at sometime and it had a very old rust stain on the carpets which I removed when I steamed cleaned the interior and it never came back and the carpets seem dry.

I can't see any leaks on the carpet but now that your saying the heater matrix I wonder if maybe the previous owner may of bodged it up somehow.