82 500sec euro. Suddenly had an overheat problem, went past 110 and still climbing at idle or running at 50 mph so I bought a new thermostat and put it in. Same problem but---- if I turn on the heater until the temp comes down to a little over 80 on the gage and then turn the heater off the temp stays at 80+ at idle, running, or on restart as long as I don't leave the engine off long enough to cool down. If it's cool I have to run the heater or the temp won't stop climbing. Sounds like an air bubble but once it clears why would it come back at cool down? I read on search but no one said they had this happen. TIA

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K

Make sure that the "air breather" hose from the top of the radiator isn't plugged.

That hose allows for aspiration of air from the engine.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
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44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Just checked, 2 "T"ed hoses open. But there is an air pocket at the top of the radiator, at least while it's cold. No liquid when I took the small air hose off of the radiator top nor from the hose when I took it off of the thermo top. There was liquid coming out of the thermo hole. The air pocket surprised me because the fill tank level is higher than the top of the radiator. I stuck a piece of wire into the hole on the rad top where I took off the hose and it was open.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K

Where you able to find out what was the problem? I have the exact same issue and am trying to narrow down what the cause is. Radiator is good and thermostat was just replaced.

My 73 450SE and 75 450SLC both did the same thing. When is the last time you changed
the coolant? Moreover, drilling (8 ) 1/8 diameter holes somewhat equally spaced in the
thermostat body should help solve the problem. I received that tip from a M-B Tech.
Doing both of those things solved my over heating. Before I did that, I back flushed the radiator, but that did not solve the problem. Good Luck
Old Max

Coolant (MB's own) was just done, as well as radiator and thermostat. I've done a change before on my other car, the 560, but did not have any problems like that.

Drilling 8 holes into a thermostat housing does not sound right? Sounds a little bit like a shade-tree mechanic's advice than anything else.

Anyone else with any advise on what should be done? Sounds like an air-lock in the system somewhere, but I don't know how to get it resolved.

I haven't done anything more to the SEC because of being in a diff city than the car. I may get back to it this summer.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K