Any visible leaks at the front (water pump and thermostat)? Another typical fail is the seal on main top hose connection at the side of the block (you can see it with some light under intake manifold).

 

Thanks for the reply

The car was in the garage last week to try and find the leak. They pressure tested it and they replaced a pipe under the inlet manifold they said was leaking, but it hasn't helped.

I'll take it back when I get back home and get them to have another look.

Just seems odd that it didn't lose any coolant on a 1000km drive and then the next morning it's lost about 1 1/2 litres?

 

Question here is whether its actually losing coolant or if there's just an airlock in the system making it look like it is. If the car lost 1.5l overnight when sitting you'd expect to see a puddle somewhere. It is possible to lose coolant through a bad head gasket but hopefully that's not the case here.

Here's something to try. Check the coolant level on the stick in the overflow bottle when the car is cold first thing in the morning. If the level is correct there but the car says its low then do this. Start the engine with the cap off the overflow bottle. The stick should drop. Top the fluid back up so the top of the stick is level with the top of the bottle again. Shouldn't take much. Replace the cap and go for a drive to get everything properly warmed up. The dash warning should disappear. Check the level again when cold the next morning. It might read a little high on the stick but that's ok. Check the level on the stick over the next few days. If it continues to drop then you'll need to go searching for a leak. However it might just stabilize with the high reading on the stick. If so then leave it. It will run fine like this.

 

Pretty hopeful it's not the head gasket, I did park overnight in a friends garage and there was a puddle in the morning so i'm guessing a leak. Don't think it's an airlock as the heater works well when the coolant is topped up (no heat when the level is low)

 

In this instance puddles are good. Best thing to do if you can't find the leak is to buy some of this:

MASTERCOOL ENGINE COOLANT FLOURESCENT ULTRAVIOLET DYE 53609 | eBay

and a UV torch.

 

In this case I was happy to see a puddle. Great idea with the uv dye I'll give that a try when i get back to the UK

Thanks

 

BM's seem prone to these sort of leaks! course having the undertray - I assume yours does have one? Is a problem because otherwise the location of the puddle would indicate where the leak was, from directly above.. At least as you have the puddle, it doesn't look like a headgasket problem - which is very good news! I am assuming that you are sure the puddle isn't from the aircon.......

As Clavurian says, the water pump and thermostat housing are favourite leak areas, I would also check the radiator, these bow down at the bottom in later life and leak, or the header tank, these can split too. You also have items like the heater valve as potential leak spots.
Is there any water in the oil? It may not be the same on the E39d, but on the E60's there is an oil/water oil cooler fitted on the passenger side of the engine bay, near the oil filter, on many cars VW especially, these are notorious for internal leaks.

A little odd that it seems water tight when its hot, as you would reasonably expect to lose coolant when its under pressure hot, but you are saying it is OK hot. Did the garage do the pressure test cold, as well as hot? The UV dye sounds a good idea, although generally a leak will show some sort of "witness" mark where the leak has been, although I do appreciate that the engine bay is rather full to get at to see!!

 

Problem Solved

Thanks to everyone for their help on this.

The problem was the water pump. Only leaking when the engine was cold.

All fixed and no leaks