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. 

[paul_james09]

Been into the garage. Didnt drop under pressure again. They’ve checked common areas including the water pump. Nothing visible. After a road test more leaking. They’ve put dye in the system and told me to go back in a week. 
 

After a little more research I’m starting to think ERG cooler as it’s not always obvious and can still hold under pressure. What would be the best way of testing this? 

Edited October 2, 2023 by paul_james09

[cliffcoggin]

It wouldn't be the first time the EGR has been the cause of water loss, though I am surprised the pressure did not reveal it.

[paul_james09]

So after the UV dye test i can’t see anything around the engine bay. But there is couple of drops showing in the oil when I take the cap off. Still a slow leak and I’ve probably lost a couple litres in total the oil isn’t milky or grey so it’s not all in there 

[cliffcoggin]

A few drops is a few drops too many, there should be none. That points to the EGR leaking its cooling water into the engine. As far as I know the only test is to remove the EGR and examine it.

What concerns me about the EGR diagnosis is that the oil has not become milky or emulsified, which is what I would expect if a couple of litres of water had mixed with it.

[paul_james09]

Back from the garage. 

Ruled out the EGR cooler after that was pressure tested. Ruled out the heater matrix 

Been told to monitor how much I’m loosing at take back in a couple of weeks. 
 

Whats the next logical thing that should be tested/checked? 

[cliffcoggin]

If your garage is only repeating the pressure test it did a few weeks earlier, and which failed to reveal a leak, it seems it has run out of ideas. Time to take the car elsewhere.

Did you or the garage investigate the possibility of an air lock that I mentioned three weeks ago?

[U Patel]

On 10/13/2023 at 5:58 PM, paul_james09 said:

Back from the garage. 

Ruled out the EGR cooler after that was pressure tested. Ruled out the heater matrix 

Been told to monitor how much I’m loosing at take back in a couple of weeks. 
 

Whats the next logical thing that should be tested/checked? 

Hello mate, just read this conversation and I am experiencing the exact same issue. I have been to a garage, which has done the exact same tests as you have. Was just wondering if you got any further with this, if you seemed to find the issue?

 

[paul_james09]

Never did find the cause. It seemd to have stopped for the last few months and reared its ugly head again. The last time I had to top it up was on 106k and I’m now on 109k and have had to top up in the last 300 miles. I think I’ll get it booked into a specialist and diagnose again. Sceptical to throw money at it again after already spending about £1500 and getting nowhere. If you get yours sorted please let me know  

[daz1609]

My brother is currently having a similar issue on his A3. So we no doubt will be going down these routes, however,

Just putting it out there. We have a 2017 VW Crafter that had all the same symptoms - was passing all the test thrown at it, had no end of parts replaced trying to fix it, passed pressure test after pressure test, only to find it was indeed the EGR cooler, the weird thing is it showed no signs of loosing pressure or leakage, but once we replaced that, it sorted our issue once and for all. The reason there was no signs of a leak is the coolant was burning straight off once passing through the cooler, but still doesn’t explain how it managed to hold pressure IMO.

but yeah definitely worth looking at, as we had 2 garages looking at it, one being a VAG specialist.

[Mar4ix]

Maybe worth disconnect egr cooler from cooling system, with adapter connect straight to heater matrix, block with blank egr pipe at exhaust (and probably at egr itself also blank vacuum line with bolt) (I have removed whole system altogether), and drive that way to see if you are loosing coolant. ... There will be warning light check engine due egr malfunction (possible code out of ECU), main thing is to find out would loose coolant after disconnected egr cooling system. 

 

Edited November 17 by Mar4ix