W123 300TD - Cooling System

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Feb 8, 2013
1985 W123 300TD & 1990 W124 300TE
Evening, my old ‘85 300TD OM603 flung up a bit of a problem this evening and wondered if the group could help.

About 6 months back I renewed the rad, top and bottom hoses, water pump and thermostat. I have not renewed that small piece of rubber pipe running off the thermostat housing because I couldn’t find a part but I thought it was ok. All been fine. Anyway…

She always runs just under 100 deg C, fluctuates if put the heater on etc… Tonight I got back and while I sorted out some space in the garage whuxh meant she stood for about 3 mins. When I got back in the car to move it inside the temp gauge had risen to just under 120 degrees and I could see steam emitting from around the thermostat area and was dripping onto the hot bits causing the steam. Turned engine off and waited to cool down. The top hose was like it was pressurised with air, hard as nails, it’s new remember…

After about an hour the hose had gone soft again, and the coolant level had dropped a little.

Oil checked and ok.

Can anyone suggest what’s going on here? Ta, Dot


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It sounds like it is running a bit hot all of the time - these should be running mid to high 80s iirc - not close to 100.
Was it running that hot before you replaced everything? If no then maybe you have introduced an airlock?

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