The rubber seals that seal the rad core to the tanks can sometimes only leak for a period after the car is shut down. While hot the seals are good, when cold the cooling system is under vacuum, but in between there can be a leak.

Darn it...I may have this problem. I will only find a puddle under the car (under the expansion tank/coolant cap area) after parked overnight...sometimes. I don't see it all the time and don't see it after it's parked a day at work. The expansion tank is relatively new (maybe 3 years, less than 5k miles) and the radiator is about 1 year old. Could it also be a bad aux fan not running after shutting the car off to cool the radiator further, thus causing a slight overheat and dumps the coolant out the cap? May just drop it off at my indy and let him figure it out :dunno:

I think this would only happen if your overflow tank was full. It should flow out of the radiator cap and into the overflow tank, so that it can be sucked back into the radiator when the engine cools off.

Thanks. I may need a bit more investigating. I've only seen a puddle underneath the car a couple of times in the past 3 weeks. This started happening after I had topped up the expansion tank about a month ago. My question is if the overflow tank was full from me topping up, why didn't it flow back out of the cap immediately afterwards of my initial drives? Why am I only seeing the puddles at 2 random times over a month after parking it overnight :dunno: The random nature is why I thought it may be a bad aux fan.

Just checked the level again yesterday and it's back down to the same level as before I started adding coolant to it about a month ago. Also, I've noticed some evidence that coolant is leaving the cap...so that may be encouraging of a simple or no solution needed :thumbup: I've cleaned up the area around the cap of coolant residue and will need to continue monitoring it :popcorn:

Try a pressure test of your cooling system. Marginally failing parts can be very frustrating to find but a pressure test will help stress them a bit. In my case it was the plastic hoses behind the oil filter housing on the side of the motor had distorted O rings. Does your heater work OK?

Yes, the heater work...if this was directed toward me? I'm planning on letting my indy do a pressure test if the issue persist. Right now, the coolant level stick indicator is just sitting below the neck of the expansion tank. If it remains there without further coolant loss, then I'll just chuck it up as "that's where my expansion tank cap likes to be at".

OP - sorry for the thread jack. Please keep us updated with what you find with your issue? Thanks.

Bentley manual says to pump it to 17 PSI. It should hold for 2 minutes. Pressure runs higher when hot. However testing says lower when you can open the tank and it's cooler.

THAT makes sense. It's difficult to prove, but I get it.

The aux fan is used in conjunction with the AC, there is also an aux pump that helps keep the cabin warm if you shut the engine off is a place such as the drive-thru lane.

That's interesting as that is only 1 Bar and the cap is supposed to hold up to 2 Bar. One of the reasons some think the M54 cooling systems like to self destruct.