Slow coolant loss. No visual leak

[paul_james09]

So I’ve got a slow coolant leak. Maybe topping up the reservoir an inch or so every 100 miles. Only seems to leak on longer journeys. 
Can’t see any signs of leakage no puddles or wet patches nor any pink corroded areas. Pressure test seems to hold pressure. No sign of coolant in oil, exhaust smell in coolant tank or white smoke from exhaust. 

What areas should I check next? What’s all the possible areas that coolant could leak internally? 

Car is a3 8p 2.0 170 TDI CFGB engine 

Many thanks in advance

[Magnet]

Hello Paul,

Sorry to hear of your issue. Dropping an inch in the reservoir in 100 miles is quite significant. 
Simple things first - try fitting a new ( quality) cap to the reservoir. 
Kind regards,

Gareth. 

[paul_james09]

Tried a new cap straight from Audi

[Magnet]

Great! Knowledge of that could have saved me bothering. 

[cliffcoggin]

Is the carpet in the footwell wet? Does the car interior mist up excessively?

[paul_james09]

No moisture inside the cabin. I’ve lost probably 2 litres since the issue has started about 3 weeks ago. I have a feeling it’s either evaporating or being burnt up. Would that much loss being burnt in combustion have a noticeable effect on performance? 

[JMKC82]

Hi Paul,

2 litres in 3 weeks is a lot of coolant, and what do you mean burnt up?  In the cylinders?  That would not be possible (in my opinion).

My trouble shooting suggestion (with extra care of course), is to top up the coolant when the engine is cold.  Get the engine up to temperature (90degrees) with the bonnet open.  Because the coolant system is under pressure and at temperature, you should be able to see 'leaks' or 'steam'.  You may want a second person to rev the engine as the water pump

If you have an undertray, possibly remove that, as that could be catching the coolant?

Again, be careful.

Thanks, Joe

[cliffcoggin]

The water can not evaporate if the system is properly sealed. Nor can it strictly burn, though of course it may leak into the cylinders or air intake and be expelled into the exhaust. Either way the loss should be detected by a pressure test of the cooling system which you said "seems" to be OK. What pressure was the test done at, and how long did it hold pressure?

What is the general condition of the engine and has it been correctly serviced? How many miles has it done? Was the water pump renewed when the cambelt was changed?

[paul_james09]

I believe we tested it at 26 psi for about 20 minutes. Serviced every year by previous owner. I had the timing and water pump done at 75k I’m currently on 105k. No other engine related in the two years I’ve had it. 

[Magnet]

Hello Paul,

Certainly odd, since any leak - external water pump, external gasket leaks, or internal leaks into cylinders , would result in a loss of pressure over a 20 minute hold. 
Having said that, it might be worth checking the water pump for shaft leakage after 30K miles of service before moving on. 
Kind regards,

Gareth. 

[cliffcoggin]

Your pressure test is fine so no obvious answer presents itself.

Have you opened the cooling system recently? Is the heater working? I am wondering if you could have introduced an air lock which, because of its expansion when heated, can cause water to be expelled from the reservoir. Air locks can be stubborn to get rid of.