A6 Still Overheating

[ostinpawas]

Help.A6 avant on 53 plate(UK) just out of warranty(by 5months)..overheated yesterday on motorway(light traffic..free flowing at 70mph)..got red coolant temp/quant warning so stopped immediatley and got towed home.
Replicated the problem today by driving it for about 15 mins....fans all work etc...temp stabilised at 90C then suddenly and quickly went to 120C with warning...when car stopped temp decreased v. quickly to 90C (with fan running too).
Observed coolant reservoir bubbling..in fact it bubbles into the overflow underneath...
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OK I have changed the Thermostat now, and got same problem...have bled thoroughly etc......heater is getting hot. Temp guage stabilises at 90 and then after about 10 mins(of idle only) or when being driven the temp. guage rises rapidly towards the upper limit...(I shut down)

CAN ANYONE TELL ME WHAT'S WRONG PLEASE?

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edit... have run the car over 1 mile and though the heater is hot initially it cools down even though the temp guage is on its way up to the upper limit?...????any ideas...please.....
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Many thanks

Ostin

[wilharper2000]

I think the bubbling can indicate a blown head gasket, as well as the heater not working properly. Its probably best to get it checked by audi or a specialist as they will be able to tell if there are exhaust gases in the coolant, which would show headgasket failure if this came out positive.

[guyg]

temp stabilised at 90C then suddenly and quickly went to 120C with warning...when car stopped temp decreased v. quickly to 90C

The thermostat is definately working

Observed coolant reservoir bubbling..

120C - the coolant is boiling!

... have run the car over 1 mile and though the heater is hot initially it cools down even though the temp guage is on its way up to the upper limit

Sounds like the water pump [a recurring theme!] Many of the VAG waterpumps have a plastic impeller that becomes loose on the shaft and only spins when it feels like it. Sometimes they break up altogether...

You might get some output from the heater initially because there will be some initial convection and also, if the impeller isn't broken, they tend to grip occasionally when the engine is idling and when cold [not much use really!]

You can probably check this by removing the big hose from the pump and feeling to see if the impeller is in tact and checking to see if it is tight on the shaft.

Hope this is of some help...

Cheers,

Guy

[Cambrai]

Hi Guyg
My A6 may be suffering from this same problem. If I wanted to check the impeller where would I find it? Also where is the water pump situated? My model is 1999 A6 1.8T SE. If it does turn out to be the water pump is it expensive to repair/replace?
Thanks
Steve

[guyg]

Steve, I haven't worked on the 1.8 engine [but I'm sure many others have], so I'm not sure of you can gain any access by pulling a hose off from somewhere near the back of the pump.

If not it might be worth getting a second opinion before getting the work done.

Cheers

Guy

[John540]

My 1.8 petrol had the water impellor fall off the spindle so no leaks just overheated if you drove a bit fast, if you drove at a steady 50 or so down a motorway it would control the heat, yes it is a big job so worth doing the cambelt at the same time.