Strange overhating

[Ross]

Regarding a '95 E34/M50
Started it tonight to be greeted by coolant level warning. Level was an inch or so below filler when last checked a few days ago and no leaks or smells noticed since.
I drove perhaps five miles when coolant temp. rose to beginning of red zone.
Immediately stopped, coolant was visible at approximately the same level as before. The hoses were soft so I assumed no pressure in rad.
There WAS pressure when removing cap. The fan was turning and the coolant didn't seem super hot, no steam just overflow from rad. when I opened it. I've only owned this one for a week but have driven it 500 miles w/o any cooling problems.
In the A.M. I will investigate further. The po gave receipt for recent service 2 that included thermostat and anti-freeze.
What is puzzling me is how the hoses could be soft and yet the rad overflows when opened.
Is there something peculiar to this car to make this possible or am I missing something obvious?

[shogun]

The cooling system is under pressure when engine is warm, that is normal. The coolant system can handle more than 100 degree Celsius due to it's mix with 50% antifreeze. The reservoir cap is laid out for such a pressure.

First test you have to do:
is the car overheating only when in traffic jam or also when driving at high speeds?
If always overheating, the thermostat is defective.
If only overheating in traffic jam or when in city traffic:
most probably the fan clutch.
You said the fan clutch was turning. It always turns a bit. But correctly?
Here is the test procedure
http://www.mwrench.com/Whitepapers/FanClutchtest.pdf

Also check: Once the engine warms up enough to open the thermostat, small stream of coolant should flow near the fill cap letting you know the bleeding has been successful:
http://www.nmia.com/~dgnrg/ret2.jpg

[rob101]

could be an airlock try bleeding the system mate, the m50 coolant system is a thing of......... frustration!:p

[madmaxdvl]

Ross,

I've got to go with Shogun and Rob101. I'd check for air first, then go from there with Shogun's suggestions. I feel your pain. I had Vic for 3 days, drove him a few hundred miles and then had an overheating problem. The PO never had trouble (even in Tucson, AZ summer heat), but as soon s I got him, wham! the radiator went.

The coolant in these (if it's the right stuff) is clear, and doesn't have much of a smell. I started noticing a little "weeping" of what looked like water just before the radiator gave up the ghost. The radiators are prone to cracking at the upper radiator hose neck, as well as between the overflow bottle and radiator itself (I believe it's the same radiator for the M50 through '95, someone correct me if I've been misinformed). It may be just enough to let some air into the system. If the car has over 100,000 miles, you'd do well to replace the radiator soon, even if it ends up not being the problem (mine had 125k when it failed).

Also, you said the coolant level warning came on in the morning? Did you check the level at that point, or only when the temp rose? The coolant could expand when heated, so when you saw the level "at approximately the same level as before" it might not be an accurate reading. If the level warning came on, it's either low when cold or you have a busted sensor.

One more thing that wasn't mentioned yet is the water pump. The impeller is plastic in the stock units and can crack, breaking free of the shaft. That can also cause overheating in certain situations (while driving, but not when idling, according to a quick Gogle search). Again, if you've got over 100-150k miles, best to replace that with a revised, metal impeller pump if you have to do other work on the cooling system. All told, I replaced the radiator, thermostat, water pump, radiator cap, and gaskets for around $300 and haven't had any trouble since. I hope you're trouble isn't as extensive, but, as a worst-case scenario, that's not too bad! Oh, and all parts are easy to replace if you can turn a wrench (the fan nut is a b****, but that's it).

[Ross]

I've just topped up the rad. with very nearly a gallon of coolant. Going for test ride.
Thanks to all who responded.
I suspect it will be low coolant level due to insuficient bleeding. Wish me luck.
What REALLY baffles me though is how both hoses were soft yet the rad overflowed. The motor was off. I could resecure the cap the hoses would still be soft.As soon as the cap was removed again the coolant would rush up and out.
Does anyone have an explaination for how there could be enough pressure in the rad to overflow while the hoses are soft?
Also does anyone have an aux cooling fan for sale for a '95 e43? Mine is seized. Don't this is related though as last night was very cool.