Coolant Leak

[Nocturnal]

Hello,

I have recently discovered a coolant leak with my 2004 Z4 3.0i.

I haven’t been able to find anything dripping on the ground until yesterday when I pulled into a gas station and found coolant dripping. I opened the engine bay and saw a puddle under the area around the expansion tank.

I was wondering how I can address the issue and test to see if I can find the leak spot. This is my first time doing this kind of work, so any tools recommendations etc would be very appreciated.

 

[Zedebee]

If it’s a new leak, my guess is that the expansion tank has developed a hairline crack. Very common problem, but difficult to see. The crack will be tiny and the coolant forced out at high pressure. Under torchlight it looks like a spider’s thread.

Remove the plastic radiator cover, run your engine up to temperature, then insert a sheet of absorbent paper down each face of the expansion in turn. If it comes up wet, there’s your leak.

You can’t fix it, except by replacing the expansion tank. Fortunately it’s not too difficult and the part is readily available and not too expensive.

 

[enuff_zed]

If not the tank cracked, the last two or three I've done have been the seal in the bottom of the tank where it sits on the mounting plate.
Still need a new tank though as the seal is an odd size and installed inside it.

If the car is an auto then you should also buy the oil cooler thermostat that sits inside the bottom of the tank as they tend to fly apart when you remove the tank.

 

[Nocturnal]

If not the tank cracked, the last two or three I've done have been the seal in the bottom of the tank where it sits on the mounting plate.
Still need a new tank though as the seal is an odd size and installed inside it.

If the car is an auto then you should also buy the oil cooler thermostat that sits inside the bottom of the tank as they tend to fly apart when you remove the tank.

Got it, to diagnose where the leak is coming from what steps can I do? I imagine I want to remove the skid plate and somehow pressurize the system?

 

[enuff_zed]

If not the tank cracked, the last two or three I've done have been the seal in the bottom of the tank where it sits on the mounting plate.
Still need a new tank though as the seal is an odd size and installed inside it.

If the car is an auto then you should also buy the oil cooler thermostat that sits inside the bottom of the tank as they tend to fly apart when you remove the tank.

Got it, to diagnose where the leak is coming from what steps can I do? I imagine I want to remove the skid plate and somehow pressurize the system?

Take the undertray off, preferably with the front of the car up on ramps or axle stands, then run it for a few minutes until hot. In a lot of cases you won't see a leak at that point, but once you turn the engine off and the pump stops running you then localised heating and pressure build up and you'll see coolant escaping. Be aware that if the tank is split you can get a high pressure and almost invisible jet emerging, so watch your eyes. If it's the seal at the bottom you'll get coolant running down and dripping off the blue drain screw.