1998 E36 (328i) slow coolant loss

I have an E36, 1998, 328i. Sometime after 108,000 miles I noticed a slight coolant loss. Amount of loss seems dependant on mileage driven between checks, not ambient temperatures.
I have recently replaced the coolant cap. And I have checked the hoses. There does not appear to be a visible leak; no puddles on the floor.
Earlier in 2006 the thermostat and related gaskets were replaced. The engine warms up as it should and never overheats.
I noticed micro stress cracks in the coolant overflow reservoir. Seem like they have been there a while, but maybe not forever.
There was a mention in this NG recently about the reservoir cracking and leaking.
Any help? TIA!
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its not a BMW but I have just sources a similar problem with my sons car.
about 2 months ago I replaced the cylinder head gasket.
not immediately after but say 2 weeks it started loosing water but only on long runs.
my son would top it up and he would report back saying system is OK and holding water. I did not appreciate he was only doing short, under 2 mile runs, at that time. He would then drive a 10+ miles and find a lose in water but could not see any signs of a leak under the engine.
This past weekend I managed to get my hands on the car to give it a check over. I ran the engine up to very hot and hey presto...........top hose slow dripping. I must not have given it a good tightening. He could not see this because it was dripping onto a hot engine and evaporating. And it was only evident with the engine above thermostat opening temperature. The water system pressure would have contributed to forcing water out.
so my sug is while you don't see any leaks they just might be there.
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A pressurized leak test is best to pinpoint where the leak is. You may also check after the car has run for a while and without shutting off the engine. This keeps the pressure.
Another thing that may go slowly and leak is the waterpump, if you haven't replaced it already.
Lez Pawl wrote:

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On my 528 it turned out to be the waterpump - although nothing showed. It probably only leaked when hot and the coolant evaporated off leaving no trace. Found by adding dye to the coolant and pressurizing it. Be careful of anyone who says 'it's the head gasket' without doing this check first.
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Run the engine up to temperature, ideally take it for a fast run then pull up and leave it running, and watch for water or steam coming from anywhere in the engine bay. Rev it up on the throttle to pressurise it. As Dave said, it's tempting in these cases to assume an internal leak (head gasket or crack) but actually that's very unlikely. It's incredible how much water can be lost through a leak you can hardly see. There might even be more than one leak.
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Wasn't able to see anything. About May 1st, FINALLY saw some dripping on the driveway. Radiator was leaking on the driver's side, somewhere above the drain plug and below the hose. Had the radiator replaced, and while at it, the coolant recovery bottle & level sensor. Now that I think back on it, I noticed some "crud" where the metal and plastic meet on the driver's side, top of the radiator, last year. Think it was after the thermostat was replaced, and I assume the system was pressure-tested. There was about 6 months lapse between the thermostat work and the impossible to locate micro leak. That should have cured it. I wish I could have gotten the parts myself & did it, but..... Oh well.
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on about something:

I had a slow leak on my E36, it turned out to be a small hole in the front of the radiator caused by a stone.
The hole was small enough that it only leaked when under pressure, and was evaporating off the hot radiator core leaving no puddle under the car.
Have a look through the kidney grill with a torch, assuming you had antifreeze in the car, the staining will be easy to see if it's the radiator leaking.
Dodgy.
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I had similar on my E39 - loss of water and pressure. Stop the hot engine and the top hose soon 'felt' soft indicating pressure loss. But no sign of coolant ever. Eventually had dye put in the system which immediately showed a weep from the waterpump seal. I'd been told by my main dealer it was a head gasket problem...
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