Undercooled 240D
 

Now that it's getting down to freezing, my 240D is running cold: at 55 mph, the temperature gauge hovers around the second mark, the one between 100 and 175 (137.5 deg. F., I'm guessing?). I can't figure out what's going on.

Some info: Warm-up time seems no longer than last year's winter weather warm-ups. The thermostat is closed at start-up (no coolant through top radiator hose); come the second mark on the temperature gauge, the thermostat has opened somewhat (pressure inside top radiator hose). The temperature increases to around 175 deg. F. when I get up to 80 and drops to the second mark when I get back down to 55. The temperature appears to increase slightly during idle at stoplights. All through the summer, the temperature generally read just below 175 deg. F. Last winter, the engine would regularly run just below 175 deg. F., regardless of speed. I replaced the temperature sender last summer.

Any ideas as to what I'm looking at would be appreciated.

There is but one component that can make the engine run cold - the thermostat. You need a new one. Jim

The thermostat is dead and stuck open.

x3

Thermostat x4. Also you mean "overcooled", not undercooled. Undercooled=overheating

Thanks all. Apologies for the incorrect term.

Ok, I'll commiserate with you. I have an '81 240d that I bought last year that ran cold. I replaced the thermostat, and it reached 80C, then shortly started running cold again. I replaced the thermostat again, thinking it might have been defective, but that didn't fix it. So either I have happened onto a rash of bum thermostats (one OEM, one AutoZone), or something else is going on. How often do sending units go bad? Can they lose their calibration and just read low?

Actually the thermostat on a W123 Diesel, and most MBs I am aware of, is a mixing valve that operates on the thermal energy of he water coming out of the engine water jacket. It controls the flow of water to the radiator, or the water shorting that circuit and going to the pump, and then it controls the water going into the pump suction from the radiator to prevent a cold slug of water suddenly being pumped into an at temperature or above cylinder head. Because of the complexity the thermostats tend to be a bit more difficult to produce in factories striving to make a thousand for under a dollar. So we get bad ones, quite often. Most of the bad ones run too hot, but they are all bad when they regulate the temperature high or low. Jim

Jim, is it also possible that if put in backwards the pressure behind the t-stat from the block could force it open. Whereas, if installed properly, the coils /spring has to work against that pressure to open.

Make sure the coils go towards the block.

The 616 thermostat is an extremely poor candidate for "reverse" installation.

I'm not sure why you say this. I have managed to put one in backwards on either a 617 or 616, cannot remember which. It got hot that way and the cooling was totally screwed up.