Hello.
I had the same problem with me '89 325I. It didn't use (leak) any coolant when the temps were above 0 C. But in the winter, when temps go below 0 C at night it lost some coolant. Not much, maybe 1 liter / month.
I check the cooling system for visable leaks. Radiator, hoses, heater, waterpump, etc. No leaks. Everything was dry. The problem started in the winter of '93/'94. Dissapeared, but came back in the next winter. Still the car run fine all the time. This winter we had some long periods with frost at night and temps of -10 to -18 C. My baby-6 needed 1 liter coolant / week! After a few weeks I didn't like the ticking sound one of the rockers made. I took the car to a local mechanic, told my story and we decided to pressure-test the coolingsystem at night when temps were low. It leaked. A lot! Nothing to see on the outside so it had to be the head-gasket or ...
The next week the mechanic started working on it. Took the head of and had it pressure-tested. The head was cracked..... The crack was between coolant passage and an oil passage. The camshaft was damaged because of the amount of coolant in the oil. I had to mount a rebuild head. Probably the crack opened when temps got below 0 C. It's been three weeks now. The car runs okay but I'm not really happy. It runs a bit rough and is nervous at iddle. Maybe their is more damage done by the coolant.

Advice: have you coolingsystem pressurized at temps below 0 C.

Regards, Twan.
'89 325I
'91 K100LT
'75 240Zzzzeeeee From: gunnar_elmgren_at_se.ibm.com
Date: Tue, 20 Feb 1996 08:03:54 EST
Subject: Re: 325es cold coolant loss
From: Gunnar Elmgren
EMAIL = gunnar_elmgren_at_se.ibm.com

• Phone +46-8-7935442 **** Mobile phone +46-10-6681660 ** Subject: Re: 325es cold coolant loss
  Dale Beuning wrote:

> I've got a puzzler of a problem with the '86 325es. Last summer we >drove her to MN from WA for a vacation, and averaged 97mph through 600m >of Montana. Partway through Mont. the low coolant light came on. >I filled it, it was aprox 2 quarts low, and continued on. >After we returned to WA, it started leaking. We took it to the dealer >(Seattle BMW, great dealer) and had them check the system. They could >not get the system to leak. They pressured tested it to 15 lbs and >drove it on the freeway, left it sit over the weekend indoors, nothing >would make it leak.
> About this time we switched from parking it outdoors to parking it >inside our heated garage. Guess what, no leak. When we parked it >outside overnight, it would leak. When we parked it inside, no leak. > The only thing I can come up with is a hairline fracture that seals >on above 60 degrees. I'd like to appeal to the list for other ideas >on what the heck could cause this leak. Also, it would only leak >a 8" diameter puddle per episode.
>
>Ideas???

Dale,
have you removed your valve cover to check yet? I just had a very similar problem om my father-in-law's '87 320i. It lost coolant under cold wheather conditions, all pressure tests etc were OK. The difference from your symptoms was, that we found most of the missing coolant in the oil sump rather than on the ground.... (If you have coolant in your oil, it will show up as a messy, whitish gunk on the dipstick and inside the oil filler cap.) Anyway, the 'baby sixes' before mid '88 had hex cylinder head bolts, that have a reputation of breaking. I our case, the bolt on the intake side between cyl 5 and 6 was off. This was not enough to cause any compression leaks from the cylinders, but coolant leaked out, and also up through the mounting hole of the broken bolt, and into the oil. (Posted a query on this to the digest a couple of weeks ago, thanks REB for pointing me in the right direction!) We were lucky that the bolt head that came loose didn't cause any harm to the valve mechanism or the camshaft, if you have this problem make sure to check all components in the head for damage. We fitted a new cylinder head gasket, new head bolts (AND a new camshaft drive belt) last saturday, and now everything seems to be OK. After mid '88, different cylinder head bolts (with TORX heads) were fitted. NEVER mount back old style bolts (hex head), get new ones with TORX head from your BMW dealer!

Good luck!

Gunnar Elmgren,
Stockholm, Sweden
'71 1600-2 'vollcabrio'
'75 2002tii
'88 735iA
'76 1502 with 2002ti transplant (donor car) '87 320i (father-in-law's) not loosing coolant anymore