[Barrosco]

Hi everyone,
A few months ago I finally got my M5 on the road after a lengthy engine rebuild process. I bought the car non-running after it had been sitting for 10 years. Problems have been fairly limited, but for the past month or so I've been losing a lot of coolant. Initially I had only distilled water in the system so tracking down a leak was tough. The only place I found water was at the end of the overflow hose by the firewall. After having added coolant, I've just about confirmed the only place I'm losing coolant to is the overflow. Sometimes, after checking the expansion tank after a drive, I can hear plenty of hissing and popping. I put a new reservoir cap on with no change. The odd thing is that on some drives I lose a bunch of coolant, and some I don't lose any.

So, my question is, what may be causing this? Is it possible that my expansion tank could be warped around the cap? Or is it possible that something may be causing the cooling system to be over-pressurizing? My next step is to get a combustion leak tester to see if I'm getting exhaust gasses in the coolant.

Any ideas would be appreciated!

Thanks!

 

[m6bigdog]

Do not drive the M5 with this problem it will most likely only destroy the engine.
Either the engine is overheating (coolant flow problems, radiator, bad thermostat, i.e., boiling coolant) or the head gasket is blown (leak between the cylinder and water jacket).
1) never just use water because it can cause electrolysis between the dissimilar metals in the engine, i.e., corrode the head gasket and water doesn't have a high enough boiling point to keep micro-boiling at the cylinder head which can over heat the head!!
2) Always use the 50/50 mix of coolant and distilled water to make sure the coolant boiling point is maintained and the coolant dielectric properties are maintained.

Diagnose:
1) Check for coolant mixed with the oil in the crankcase, Color of oil will change to milky or cloudy, also if you put oil from the dipstick on a hot surface it will sizzle if there is water in the oil. You will typically get a milky condensation on the oil filler cap with coolant on the crankcase.
2) Use a cylinder leak-down test to see if one of the cylinders is leaking into the cooling system.
3) Cold engine, remove the radiator cap start the engine (let idle) and look for rising coolant level or bubbles in the coolant reservoir. Shut engine off before engine gets warm and repeat several times, after engine cools down.
4) Pressure test the coolant system to make sure it holds pressure.
5) Pressure test the cap.
6) Heat up the engine to normal operating temp, then pull all the plugs and run the starter to see of any coolant/moisture is blown out of any of the plug holes.

 

[Barrosco]

Hi m6bigdog,
Thanks for the response! Most of your points I had already addressed, but I was definitely more concerned about the possibility of a combustion leak due to a crack or headgasket issue. I ended up purchasing a combustion leak tester off of Amazon and ran the test last night. While waiting for the car to warm up, I never noticed any bubbling or unusual fluctuations in the coolant level and the test came back negative, so I'm super thankful for that!

Right after I made the original post I made a little catch can for the overflow and it would fill up quickly. I did notice that the expansion tank neck was getting wet even though it was dry before startup, so I'm certain that the expansion tank is my issue. Should have a new one today or tomorrow. I'll post an update when I get it on the car.

 

[M5touring reg]

This is apparently a common problem.......
a batch of brand new BMW OEM expansions tanks
seems to be faulty....!!?
-

The BMW M5 E34 club

For you people who owns or just loves this car. The BMW M5 E34.

www.facebook.com

 

[Barrosco]

Good news! It ended up being the expansion tank. With the new one installed, I now have no coolant loss/leaks. I was reading about that issue with the new expansion tanks, but the one in my car was at least 10 years old so it wasn't one of the defective ones. It just went bad. I still don't completely understand why its failure mode was initially blowing coolant out of the overflow. Only within the last few days did it start actually blowing out of the reservoir neck itself. I'm just glad it wasn't a big, expensive failure!

 

[m6bigdog]

Failure mode:
The leaking expansion tank prevents the cooling system from developing the required system pressure.
Without the coolant under the 29 psi (2 Bar) of pressure it will boil in the head and the bubbles from the boiling coolant will constantly push coolant out of the overflow tube.
The 50/50 mix coolant/H2O boiling point of 226 deg.F is raised about (3 deg.F for each psi of pressure) 84 degF to 313 deg.F with 28 psi system pressure.

The risk from a cooling system that is boiling coolant is the head will overheat as the boiling coolant will prevent the coolant from adequately cooling the head and the hotter the head gets the more it boils the coolant.
Note: boiling coolant, even though it is hotter than the thermostat opening temp will not typically show as an overheated condition on the temp gauge as the boiling is happening in the head and the boiling coolant can cool as it exits the head and/or hot air bubbles will not heat the temperature sensor mounted on the coolant tube.
The big deal is; once the head is sufficiently overheated the head bolts loose their proper torque, can cause the head to warp and the head gasket looses its seal.
The head/head gasket damage can all happen in a matter of minutes once the coolant is boiling in the head!!