I had a sudden overheating in my 1991 300CE, W124, M104 today.
I've had the car a little over a year, about 122k miles. Prior to these past two warm months
the car has stayed 83-90C. Lately with the warm weather and the a/c on and idling in slow
traffic it would creep up to 100C but fall back to 85-90C when traffic picked up. The two aux
fans would only occasionally come on and I would never hear the roar of the main fan.
Coolant level is as it should be. Today on the frwy, it went slowly from 85C right to 120C
before I pulled over. I limped halfway home, pausing to let the car cool. Having the heater
full blast helped some. Both aux fans are on now. The main fan spins along with the idling
engine but again I have never heard it roar full blast. The viscous fan appears to be in good
shape. It will not freewheel when the engine is off. When you turn off the engine, the fan
will almost immediately come to a stop. The action of the fan is not altogether consistent.
Sometimes it will spin 180 degrees, sometimes only a few blades when the motor is off. The
top radiator hose is hot. The air exiting the rear of the radiator is cool though. To get the car
home I let it cool from 7pm till 11:30pm. Driving about 2-3 miles home, the temp went straight
from cold to 110C and only the heater slowed the rise to 120C. Again, with the motor very hot,
the air from the back of the radiator seemed cool. The radiator looks somewhat new. Relatively
clean. I don't think it is original. Any ideas?

TIA

glenmore
1991 300CE

I thought I remember reading a post here in the past week or so that basically said that if you are doing highway speeds the fans are not the things cooling the engine, that it is mostly the air at speed cooling the radiator.
My car also runs at 100 when stopped in traffic for a while with the a/c on.
My guess is that your thermostat is sticking and needs to be inspected at least.
You aren't hearing any horrible noises when the cars running, I think an old toyota of mine made a crazy noise when the water pump went.
I don't personally know how you tell if you have an air pocket in your cooling system but if someone else on this forum knows how to diagnos and correct that I would be interested to hear the explaination.

CDT

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2002suzukiGSF600S
1985 190e 2.3

At high speeds, the car doesn't need the fans to cool it down.

If you are overheating at high speeds, chances are your thermostat is stuck close.

My 2 cents.

Turns out it was a broken thermostat, $7 to fix. I guess the key things to notice were the overheating at speed and a cold radiator and the suddeness of the change. The hot upper hose threw me a little. The thermostat must have been open enough to allow some hot fluid to pass but not enough for the radiator to cool and then recirculate. The brass bridge on the thermostat was broken in two.

Thanks all,

glenmore
1991 300CE

Thermostats always allow some water to pass through, even when 'closed'.

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1986 300E 5-Speed 240k mi.

In my portfolio of failure analysis of automotive parts, I had the opportunity to diagose a thermostat with a failed bridge. This phenomenom was traced to "hammering" in the cooling system. The combination of hot coolant, vacuum produced by the suction side of the water pump, and nucleant boiling produced a hammering effect, effectively destroying the thermostat. This same effect can also destroy the water passages in the aluminum head.

Glenmore, was the bridge thinned somewhat before breaking? Make sure the rest of the cooling system is in top condition, that you're running the proper coolant strength, and that the radiator cap is holding the proper pressure.

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95 E320 Cabriolet, 159K

Glenmore

Glad to hear a new t/stat solved your problem.

This thread will be useful to others who face overheating at speed.

Cheers.

glenmore, I think you said that the tstat was broken but you didn't say that you drained and refilled the cooling system (although you probably lost some fluid when you took out the tstat). but you probably did drain everything and refill it right? I'd wonder where are the broken pieces parts, you know? Are they floating around ready to do damage to a water pump or something...
cdt

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2002suzukiGSF600S
1985 190e 2.3

Kestas, Tunk,

The brass bridge broke right in the middle where it holds the piston. No sign of stretched metal. It looks like the break developed over time as the broken edges are not uniform in color. It looks like the edges of a fractured metal casting. Does not appear to be any missing metal, possibly a few grains. Dodged a bullet when I took the thermostat out. The piston has a small brass fitting held in place by the bridge. The piston was pushed up past the broken bridge and this fitting could have easily come off. It may have been prevented from falling off by underside of the thermostat housing. In hind sight, you should be careful when taking off a suspected broken thermostat because pieces may fall back in. Good thought, Tunk , I will drain and flush. I cheated and just replaced the lost coolant (about a quart) with water. I have some new MBZ coolant but wanted to get some citric acid and do a complete flush.

Cooling system appears to be fine. I'm keeping an eye on the fan clutch. Already bought a new one, $130! yikes!

In hind sight, I think the thermostat was probably malfunctioning long ago as it always took a while before the car reached operating temperature. Now the car gets to 85-90C right away and I can see the thermostat open and stop the climbing temperature needle. Hopefully this will help my gas mileage, as it has always been lousy, in the high teens.

Thanks all,

glenmore