Mysterious coolant leak

My 1997 328i has been losing coolant very slowly from an unknown location for the last couple weeks. I have just been topping off the expansion tank with distilled water every 3-4 days. Today I got the car elevated for the first time to take a good look at it. I found that the corner of the block where it connects to the oil pan gasket directly behind the AC compressor was wet with BMW Blue. If I didn't know better, I would think it was leaking from the oil pan gasket itself, as the dampness extends rearwards along the mating surface about six inches. I can actually see the liquid bubbling at one point along the line, albeit slowly at a rate of about one tiny bubble every minute. The leak is a somewhat steady drip at startup but slows down considerably and almost stops once the engine is brought to operating temperature. This is in keeping with the fact that it leaks the most when parked overnight or cools down completely. I've felt around and haven't been able to find any obvious sources of the leak, just the manifestation of it in the drip from the front passenger corner. Once I get back to school I'll use some UV dye to see if that helps isolate the source a little better. Just for the record, there is no evidence of oil/coolant cross-contamination and no drivability concerns, so I'm not thinking it would be the head gasket. I understand that this is an extremely vague question, but if anyone has any advice as to where to go from here or experience with this type of problem, your input would be greatly appreciated. Thanks everybody, have a great evening.

I feel your pain I currently have a very small coolant leak but haven't a clue where it's comingfrom. You say there was some coolant near the sump? I'd check the coolant drain plug which is just above the oil drain plug. Other than that, thermostat housing, coolant pipe connections and check the plastic sides of your radiator some times they can leak. Good luck

One thing that comes to mind is the expansion tank leaking and then coolant getting blown back onto the engine. Just a thought.

Thanks guys, I really appreciate it. I'll check that stuff out tomorrow and get back to you. I should add that the radiator, thermostat, housing, water pump, expansion tank, expansion tank cap, and upper and lower radiator hoses were replaced as part of a system overhaul in July. I'm not as worried about those parts, but I'll not count out the possibility of incorrect installation. I guess I'll just keep it topped off until I can figure out a permanent solution. Anyone else have any thoughts?

Does your car run hot at all? I had a very similar problem a few months ago. Turned out to be a hose that runs under the manifold.

My problem was number 14. But I would check all those cooling hose's.


I know you have an M52. But I think that part of the cooling system is the same.

The car theoretically runs cooler than before because the overhaul included a fan delete. Thanks though, I'll take a look.

Check the connection on the upper RAD. hose where it connects to the Thermo housing make it's all the way on and tight.

Yeah, since the leak was not present before your cooling system overhaul, I'd recommend going through and double checking everything to make sure it's on correctly and tight. There's a decent chance it's just something simple like that. I'm wondering if the bubbling along the coil pan mating surface is just the coolant bubbling off from the heat, and not actually coming from inside the crankcase.

Have you considered having it pressure tested?

dont leave out the possibility of the expansion tank cap especially if its never been replaced.