Mystery cooling leak? Advice needed...

[SamuelL421]

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

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?

[wonton180]

Sounds like you drive your car regularly, but if you are able, let it sit after driving with some cardboard under it. look for leaks that way. if you don't find the leaks while it's not running, keep the car running, windows down, and pump up the heat (with these frigid temps) to get the coolant circulating.

Personally, I doubt that steam/water vapor will be the only way coolant is escaping your system, so i think looking for drip evidence is the best way.

[StickMon]

If it's dripping onto someplace hot, it may evaporate before hitting the floor, but it will leave telltale crusty residue.

[CarCar]

The thermo sensor connection and the lower radiator hose connection to the thermostat housing are a good guess. Location is near top of motor behind the cooling fan. As you drive the small leak is blown back on the motor and evaporates not leaving puddles. Remove plastic under tray and let car idle for 15 or 20 minutes. You should have puddle then either center under crank pully or drivers side under expansion tank. Best to obtain pressure test kit and use that to find the leak.

[Motionblurrr]

I also drive an e85 3.0i Z4 (~105k miles) that had a coolant issue as well. Except my coolant light never came on and I decided to check out the coolant on a whim (temps were dropping quick in the winter) and I saw that I had almost NO coolant.

Turns out my radiator was cracked. It cracked on the seam where the plastic wraps around the metal on the radiator itself. This was near the expansion tank side too.

So after I installed the new radiator I no longer have any coolant issues. The cracks in the plastic part of the radiator were so small, no wonder I couldn't find any obvious signs of leaks other than being low on coolant all the time.

[The HACK]

When was the last time water pump was changed?

Also, does the motor oil have any "milky" residue?

[Geauxkart]

Originally Posted by The HACK

Also, does the motor oil have any "milky" residue?

I was waiting for this....
You better hope not!

[pungo]

In the past, I've found hard to find coolant leaks using a black light at night. Even the smallest leak shows up pretty well. It may or may not work too well depending on the the coolant type.

[Steeler]

I had a leak on my 04 3 litre and could not detect the leak.
Actually thought it was the blue petcock on the bottom of the rad area leaking ,turned out the expansion tank had a hair line crack.
Have your local rad shop pressurize the system and see what shows up.

[dharper]

dbarton62 may be on the right track. The expansion tank on my '03 had a hairline crack, which allowed coolant to slowly leak out, and then it changed form hairline to total failure. I'd start there since the cooling systems on early Z4's are notorious for eventually failing.