Disappearing coolant, no visible leak?

[SamuelL421]

I'm looking for some advice from any of my fellow z4 owners who may have had a similar cooling mystery:

I drive a 2003 z4 3.0i with 110k on the clock. Every two days I get the yellow coolant light reappears and then I have to replace maybe 4oz of coolant. I regularly drive about 50mi daily (round trip), usually in stop-and-go traffic. The outdoor temperature has been around freezing or lower most days and I usually have the heater running, not "full-blast" though, just running around 72* and 50% fan.

With that being the case, I began searching for potential leaks all over the cooling system. I searched both from the top of the engine and from underneath with the panel removed. No obvious spots to be seen or drip marks on any hoses/connections for the cooling system.

My guess is that there is a leak somewhere hidden from view? Maybe a tiny crack just big enough for drips/steam to escape through unnoticed?

I'm open to any suggestions, this has been driving me insane the past few weeks... Is my issue similar to any of your experiences? Driving in freezing weather with mysteriously disappearing coolant?

*PS - those of you who get on "Zpost" will notice I posted on there too... A lot of that group is from the southern USA, and with this being (maybe) a cold-weather problem, z4-forum's UK crowd are probably more knowledgeable

 

[Shipkiller]

You are correct that it is a very small leak and is manifesting itself as steam. You should start to see some abnormal corrosion on the aluminum parts of the engine..
Most likely you have an coolant hose O-ring failure allowing some of the coolant to escape as steam.
You stated that you have 110K on the odometer. If this is true then you are due for some coolant system maintenance anyway so it would be a good time to replace the hoses and expansion tank before they fail at the wrong time.

You are incorrect in that most of the Zpost crowd is from the southwest.

 

[konas]

Had similar problem a couple of months ago.
It was the radiator - had several small invisible holes and the coolant was escaping as a steam.

Found after pressure test, (after leaving the car to cool down for several hours)

 

[chris g]

we lose a cup of coolant every month in the discovery - but that is a tiny crack somewhere in the cylinder head i think - sometimes you get the odd whiff of coolant from the exhaust.

I put a sealant in and it seems to have stopped it for now

 

[grayboots]

I've had this on my old Z4. It turned out to be the thermostat housing seal. Remove the radiator cover and look around for signs of staining below the thermostat. When mine was doing this, it was a tiny leak that didn't show at all until the car was up to temperature.

 

[mad4slalom]

can be as stated above or feater matrix but you may have had steamy windows or damp footwells or musty smell inside. on my z3m I was just getting a whiff of coolant when hot but no drips or noticeable water loss , try wiggling your fan if not electric or your water pump pulley, mine was hanging off , so did water punp thermostat and pipes and hoses I could see as they had all done 70k. good luck ,

 

[H9 BJT]

It seems there could be many reasons.... I had the same thing on my 2004 3.0 around a year ago, with 40K on the clock. Turned out to be a cracked water pump housing that needed the entire pump unit replacing.

Good luck with finding and fixing the source.

 

[Sootydog]

I was also loosing a small amount of coolant recently. Turned out to be a very small crack in the rear of the expansion tank. Fill it up, get a torch and squeeze the top hose, start looking for leaks. Once I got the tank out the crack was almost invisible but enough to loose water under pressure. If it is the tank, PM me, I have a really neat way to get it out without crawling under the car.

 

[lotus791]

+1 expansion tank , typical place to look on a aged bmw