Coolant loss
 

2003 V70 with B5244T3 (2.4T) at 165,000 miles.

About 100ml of coolant is being lost per 400 miles but there are no visible signs of water loss.

All pipes and unions externally dry
Heater matrix and unions externally dry
Engine block core plugs are dry and solid
All five cylinders have the same compression pressure (13 bar) give or take a miniscule amount, and no combustion products are sniffed in the expansion tank (so no head gasket failure)
Expansion tank cap screwed down as tight as it will go and with no crystalline deposits on it or the tank neck and surround
Unscrewing the expansion tank cap several hours after shut down still releases pressure - suggesting that the system is sound

Ideas as to the most likely cause with this engine and installation, please?
The coolant loss is tiny but until a few weeks ago there had never been any.


Cheers,
2.4TSE

I've solved similar mysterious losses by removing and flipping the seal in the expansion cap which has made a better seal again.
I've done similar with pas fluid caps too.

It's free and quick so worth a try.

Done!
Thanks for that idea, cheshired5.

There was a compression ridge on the seal so let's see what now happens.

I had a similar problem a year or two back. Minor, seemingly intermittent coolant loss with no obvious leak. It turned out to be a leak from the radiator!

Nothing leaked with the engine cold or running. However when the engine was switched off and cooling down, a very slight weep appeared. The volume lost was very small and lay on top of the engine under-tray where evaporated, so no trace other than the stain left by the impurities in the coolant (anti-freeze, scale from the water, etc).

I looked for ages but couldn't find anything, and so did the garage. However, with the car raised and the tray off, as the engine cooled down a drip appeared. Mystery solved!

My guess is that there was some degree of differential thermal expansion (or contraction) due to the mix of metal and plastic radiator components. The gap tightened up as the components heated up, and stayed water tight when up to temperature. When cooling down, the components on one side of the gap cooled faster than those on the other side, widening the gap enough to leak. Only my speculation, mind.

Solution was a new radiator! I hope your problem is solved by turning the cap seal over - much easier and cheaper.

Good advice - it is also as well to check for moisture under the passenger side rubber foot mat - if it is damp to the touch and feels quite slippery then you may find you've got a minor leak from the heater matrix connection - the "O" ring seals are prone to leak after the car has done over 150K probably for the same reasons outline by John D above.

The leak begins very, very slowly when cold - the hose connects to the matrix on the left hand side as you look at it from the seat - I couldn't find a leak until having checked several days in a row finally found a drop of green Volvo antifreeze just about to fall from the joint. Subsequent removal of the under-facia revealed a greeny/blue blob of crystalised anti-freeze where the drops had been falling before they found there way to the carpet. As above there was no leak when the engine was running or was still warm.

you mention all pipes checked, including the small hose to the thermostat housing? (this hose only on the turbo engines)

Thanks a lot ASt85 for this idea, I have a very minor coolant loss, I just top off every oil change, but I've been wondering why the floor carpet is wet so have to check it now

1,500 miles on since turning the coolant cap rubber seal upside down, and...

...no coolant loss.

Thanks again for the Top Tip, cheshired5.

My assumption is that the ridge on the side of the seal originally presented to the neck was sufficient to allow coolant loss under running temperature and pressure but insufficient to allow loss as things cooled down a little (so the opposite of your radiator issue, john d!) - hence why I still found retained pressure when checking several hours after shut down.