BMW E30 325i Coolant Loss Mystery

Hi all,

I'm really stumped with this one and could use some suggestions. 1987 E30 w/ the M20B25. It's using a lot of coolant and I can't for the life of me figure it out. Here are some notes:

• No overheating, no loss of power, etc.
• No visible external leaks underhood. I've never even smelled coolant burning off.
• Cooling system appears to hold pressure fine. I put the overflow tube into a bottle and it hasn't collected anything.
• Head gasket seems fine. No coolant burning in the exhaust, good compression and leak down numbers, verified that no combustion gases are present in the coolant. Oil level stays constant and oil always looks clean.
• No signs of coolant under the dash, in the carpet, etc. Haven't smelled anything inside the car. 
• With a recent timing belt service I replaced the water pump and most hoses...no change in symptoms. 
• Just survived a 1500 mile road trip with no problems other than constantly filling up on coolant.

At this point I feel like I'd just be throwing parts at it. I may pull the radiator and try a replacement but I don't have any justification for that other than it's old. 

Help! 

The culprit:

My WAG is the radiator has a pinhole that leaks at pressure. 

My nephew's 1990 325is had a small crack in the overflow bottle, and apparently it was hard to see any witness mark trails anywhere.

Coolant leaks can be really tricky to track down. Look over everything really carefully and if you can find a way to pressurize the system and hold it there while you look around. 

You need a pressure tester.  Pump it up and wait.  Then start looking if the pressure starts dropping.  The coolant HAS to be going somewhere.

noddaz said:

You need a pressure tester.  Pump it up and wait.  Then start looking if the pressure starts dropping.  The coolant HAS to be going somewhere.

I did rent a pressure tester and it seemed to hold pressure decently well. I pumped it up to just around 15 psi (I think radiator/overflow bottle cap was rated for about that) and looked around for leaks. Couldn't find anything!

Do it with the engine running and the coolant circulating and up to temperature.

In reply to iceracer :

Good point. I didn't try it running. 

I was tracking down a similar leak on my E36 years ago. Finally, I had a light on at night and I could seen that a very fine spray was coming from the top of the radiator cap or overflow bottle (can't picture the exact setup now).  I replaced the part and the leak stopped. 

My guess would be pinhole leak in the radiator or lower radiator hose, or possibly a leak in the heater hoses on the engine-side of the firewall. They're buried back there so you wouldn't necessarily see it and could just be dripping under the car (unless you've looked). You also wouldn't necessarily smell it since its in the engine bay and dripping antifreeze doesn't smell as obviously as it does when coming out exhaust or boiling into a gas.

It's an E30 and it's leaking. I see no mystery. 

Sorry, I have an E30 vert with a coolant leak, so I had to try and be funny. If you you're not laughing, you're crying. 

Had a very similar problem with a mystery leak, on my Volvo S60.  Went through the same checklist.  Turns out it was the coolant recovery tank, and new cap, that cured it.  I went OEM, no problems since then.

In reply to irish44j :

This makes sense and is what I suspect. I keep checking these areas after long drives but can't find any sign of even a small leak. I did replace the rad hoses and heater hoses the last time I did the timing belt but haven't seen any change. My money is on a pinhole leak in the radiator.