Replaced W115 radiator, now engine overheats????
 

Noticed my 220D had a small leak from the radiator (otherwise always ran at the proper temp. Replaced the rad this morning with a good used unit from another 220D and now the car won't stop overheating?

I've recently overhauled the cooling system, replacing many hoses, the thermostat, and the water pump. I noticed the top radiator hose was not getting warm.

What am I doing wrong?? I've opened up the systems on both this 220D and my 240D multiple times for services and each time I simply fill em up and they run just fine?

Could the used radiator be the culprit?

Air bubble in the system or the used radiator is clogged.

Two things changed; first the radiator and second the coolant was exchanged. If we assume that nothing else failed coincidentally then either the radiator is bad or their is air in the system.

Try lifting the front of the car or parking it nose up on an incline, on a cold engine remover the radiator cap, start the engine, turn the heater on and see if you can bleed out air.

Not running straight coolant, correct?

Radiators can clog both from the inside and the outside. Look closely at the exterior fins for blockage. Use a light at night to see if you can shine it through the fins. When the engine is hot check the radiator for cold spots, they indicate an internal blockage.

Good luck!!!

Beyond the Air bubble issue it is rare because new Thermostats can be no good. Remove the Thermostat and run it. If it does not overheat without the Thermostat that is likely the problem.

Note that before installing Thermostats I used to test them to make sure the opened properly.
When I got lazy and did not test a new Thermostat and of course that is the one that is when one failed and I had to go through the work of draining out coolant and so on and it was in a Van where the Thermostat Housing was more difficult to get at.

Note when I was working as a Mechanic I always had to test them be for installing them. You don't want to overheat the Customers Engine.

Thought I would update this for anyone reading in the future.

Let the car sit over night with the rad cap off - facing up hill and made no difference.

Pulled the thermostat cover off to check the t stat and a HUGE air/coolant bubble came out.

Put the t stat cover back on and engine runs at the perfect temp now.

Confirm that you have circulation. If so then you have a **** radiator if not then you have air in the system