You say that running temp setting inside car at max keeps the engine from overheating. This means that your heater core is working just fine and is actually cooling the engine while your radiator is not doing the job.

This leaves two options (after pump and thermostat changes).

1. Your radiator has build- up in the internal surfaces so bad it cannot transfer heat well enough to keep the engine cool. I do not believe this is the case because you had no issues with overheating at all before the first pump change. Radiator was working fine before pump change and nothing in the pump change job would cause sudden build-up inside the radiator.

2. The other option is your cooling system has not been filled correctly. If they filled it thru the expansion tank I guarantee the system is not even close to be filled correctly. I had a Chevy Tahoe in the past and experienced this issue with the reservoir at good level between min and max markings but the radiator was empty. Took almost two gallons to fill it up.

In your first post you mention the engine temp gaugre goes up and down and I understand this happened fast. This can only happen when the temperature sensor is not covered under coolant at all times. Real engine temperature cannot change up and down that fast.

I looked at at my car and for a big surprise the radiator does not have the normal fill cap on top of it so MB had again come up with the system that requires special tools and procedure to fill the cooling system properly. It probably has a fill port somewhere at low point of the system and air then escapes thru the reservoir. This closed system cannot be filled (in my car at least) thru the reservoir as air cannot get out.

Go to MB service and ask them to correctly fill your cooling system.

Or perhaps konigsteiger has instructions for this?

I am in Arrie’s camp here. I think it is still an inadequate fill and burp. His explanation is solid.

The “air pocket” that I am trying to explain will not clear out from the system. If radiator is only half filled or even less the air just sits in the top of the radiator.

I don’t know how Mercedes builds the radiators but a typical for the cars has heat exchange tubes running horizontally between the sides of the radiator where all tubes connect to the same space. This means coolant flow is parallel flow thru the tubes. If the radiator is not filled correctly you will have coolant flowing only thru the tubes at the bottom of the radiator as the hot coolant returning from the engine drops down to the bottom of the partially filled radiator. Air inside the radiator stays in the radiator unless enough pressure builds up to push it in the expansion tank.

With a lot of air the air removal can be impossible as the large volume of air allows for a “long spring” that reaches pressure enough to just purge a small amount of air but then maintains enough pressure at cooling that no new coolant or very small amount of it is pulled in from the expansion tank. To pull new coolant in also needs a small vacuum in the system and the “long spring” could prevent enough vacuum from building up.

This kind of systems are designed to work totally air free and they do very well that as with liquid the volume runs in and out of the expansion tank as the volume changes.

In OP’s case I’m almost 100% sure his coolant is very low, so low that the level does not reach high enough in the radiator to do hardly any cooling. This is why his inside heater core does a better job as the water pump pumps the coolant from the bottom of the radiator thru the heater core where the fan takes the heat out. Nothing can fill the radiator in front of the car other than filling it up correctly.

I understand what you are saying concerning 'long spring', however,

1) Mercedes placed the expansion tank higher up than the rad. Its designed like this for a reason.
2) There is a vent tube at the top of the rad leading to the top of the expansion tank.

Even filling the engine cold thru the expansion tank will fill the rad. The air pockets are caught up in the block and other areas initially. As you drive, with coolant flow, they move around.

Even if the top of the rad was filled with air, the expansion tank will empty out of coolant due to
1) the feed of expansion tank coolant into the rad and
2) the top vent hose located at the top of the rad and
3) the expansion tank being placed higher than the rad.

If you find yourself with little to no coolant in the expansion tank at any time, then keep topping up.

Ask me how I know this.

So what happened to this urgent case? No replies and many people wanted to help.

Would be at least polite to tell if the issue was fixed and how.