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Mysterious coolant leak

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coolant leak
17K views 17 replies 12 participants last post by  WreckinCzechE36 
#1 ·
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.
 
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#2 ·
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
 
#4 ·
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?
 
#5 ·
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.
 
#6 ·
The car theoretically runs cooler than before because the overhaul included a fan delete. Thanks though, I'll take a look.
 
#8 · (Edited)
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.
 
#13 · (Edited)
Or are you talking about the hose with two tees in it? The drawing isn't clear which #6 is referring to.....

EDIT: Did some more research....#6 in that diagram is the "radiator and engine return" hose, part #11531738053, but if you're looking for that hose for your 318, that's not the right part.. You have a plastic tube that connects to the heater return hose and fits in the side of the block instead. It's called simply a "connector", part #11531714738. Looks like this: (sorry for the tiny picture). Is that what you're hunting?
 

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#14 ·
If it is on the back corner of the block, you have a head gasket getting ready to go (like was the case with the M-Technic), or that gasket that connects to the climate-control, from the back of the head. Either is a pain to fix. If it is the expansion tank as well, you will see the leak coming out as you rev the car. The cables to rev the throttle body are on the bottom of the TB. It's a pinch to reach in there, but once you're in, you can rev easily.

Can you take a picture of what you saw at the back of the block?
 
#15 ·
If it is on the back corner of the block, you have a head gasket getting ready to go (like was the case with the M-Technic), or that gasket that connects to the climate-control, from the back of the head. Either is a pain to fix. If it is the expansion tank as well, you will see the leak coming out as you rev the car. The cables to rev the throttle body are on the bottom of the TB. It's a pinch to reach in there, but once you're in, you can rev easily.

Can you take a picture of what you saw at the back of the block?
You can throttle back on this one Harry. The leaking coolant part of this thread is over two years old. A couple posts up someone revived it to ask the proper name of a part....
 
#18 ·
Mysterious leak? Just from my own experience if you seriously cant find where its coming from, check the front of the expansion tank. Even though they are made of some tuff ass plastic they do tend to create vertical hairline fractures. This kind of leak might bug the **** outta you cause itll start slow so you keep topping off the radiator and slowly but surely you find yourself filling it up again and again till it becomes a chore. Eventually it'll really crack and your ass cant even make it out the parking lot, and even at this point you might not see the leak cause the front of the expansion tank is covered by the radiator or fan shroud, but basically you cant even see it. Take the expansion tank off fill with water and see if that bitch eventually drips, if it is your in luck cause its a cheap and easy fix. Got my new tank for about 50$ or so and I'll tell ya the tank was cheaper than all the coolant I bought over a years time
 
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