Overheating in an instant

Phil303 · #1 13th September 2015, 11:23 AM

What would cause the temperature to suddenly jump to the max level (120) in less than 2 miles of driving from a warm (not hot) start?

The car had been sat for an hour after a slow run through town of about a mile.

Shortly after setting off the temp warning graphic popped up on the dash and the temp needle was well over to the right. We stopped and gurgling could be heard from the tank.

We let it cool down a bit and set off again - 3 miles later same happened. did this 2 more times, each time limping a little closer to home (still had 55 miles to go though).

Eventually we stopped for good and saw that the tank was now down to below minimum and some had escaped through the cap as the right hand side of the engine bay was wet. My mate said don't drive any further to save the head gasket.

Standing at the front of the car looking back at it, the right hand fan was going mental but the right one was doing nothing. Is that right? Then after a restart both worked in unison for bit.

Someone's girlfriend came out to pick the others up and bring some bottles of water. After topping up the needle stayed at 90 for a few minutes while idling and not driving, so I thought we were okay but then it started to rise.

The AA had been called at this point and were sending a flatbed to pick me up and get me home.

All this was taking place in the middle of nowhere at 2.00am

I made it home at 4am this morning and before I call the garage tomorrow I was hoping to give them some pointers. The water pump and belts were changed (by the same garage) less than a year ago. Could it be a knackered thermostat or temp sensor of some kind?

And what's with the fans? One has a motor, the other doesn't. Should it?

Any and all advice gratefully received.

Yours very tiredly, Phil

mannyo · #2 13th September 2015, 12:29 PM

Was the thermostat not changed with the cambelt?

It does sound like the thermostat is stuck closed, due to the lack of radiator fans. The fan switch is IIRC on the radiator and if its not getting hot then the fans will not work. One will work if you have the climate control on all the time regardless.

Thermostat change officially requires the whole front of the car off again, essentially the cambelt needs to be removed as its in the way of the thermostat IIRC. Thats why you should always change it along with the belt/pump/tensioners etc.

Goran · #3 13th September 2015, 02:37 PM

Quote:

Originally Posted by Phil303

And what's with the fans? One has a motor, the other doesn't. Should it?

The electric motor driven fan is auxiliary and for aircon also.
The main fan without a motor is on the V-belt it should always be turning.
It has a viscous clutch which is heated by block temperature? and is mechanical in nature.
If viscous clutch is off then fan is freewheeling - not much cooling power. If viscous clutch is on then fan will be turning with some force.

I doubt the fans are the problem, Dave is running without his main viscous fan and I think he said his coolant never overheats.

Phil303 · #4 13th September 2015, 02:45 PM

I'm hoping the thermostat was changed with the belts and pump. I'd be surprised if it wasn't, the garage I use are pretty competent and it was them who suggested changing the water pump while the belts were being done before I had a chance to mention it.

If it was and it's failed then maybe they can go back to Audi and ask for a new one. It's less than a year old after all and I don't do high mileage.

The fans were working but none us could understand why one was pretty full on (like jet taking off) and the other was sometimes on and sometimes not. I guess that must have been the viscous one. They both worked, but at different rates, that's what was confusing us - not helped by being dog-tired, cold, miles from home and unable to see much due to being on some dark road near a farm in Suffolk.

Phil303 · #5 13th September 2015, 04:34 PM

Right... if it was an airlock would the following have happened?

Temp reached 90˚ and sat there, which is what's normal.

While I was watching the expansion tank it suddenly started bubbling then spilled over. Would that have been the lock clearing itself?

David's8 · #6 13th September 2015, 07:10 PM

I had a similar problem a few weeks back (see this thread - http://forum.a8parts.co.uk/showthread.php?t=10327 )

The electric fan cuts in around 97deg whilst the viscous fan should run all the time. I also had an issue with indicated temp being around 90 but when monitored by VCDS the temp sensor was showing higher and rising. The 'stat was the problem and had been replaced 5 years before but for a item costing pence it does require radiator/V - (toothed) belt etc to come off. I replaced belt and pulleys but seeing you have had yours done recently you shouldnt need to. But i would check whether the 'stat was in fact replaced when they didd the belt. Sometimes it comes as part of a kit but the components of that kit can include poor quality items. (see my thread).