If it does not overheat on the highway then it is not a plugged radiator. The engine uses a lot more power and thus produces a lot more heat when driving on the highway than idling. So if it will stay cool on the highway, the radiator flows plenty well enough to cool the car at idle, as long a water and air flows are adequate.
On my car the radiator is semi-clogged. I can drive around town on a hot day and the temp is rock solid ~80C. Go on the highway at 70mph and the temp is fine, but if I turn the AC it starts to climb. If I go faster, it climbs more. If I slow down, the temp goes down. So the radiator is at its cooling limit at highway speeds (the water pump is new and there is plenty of airflow on the highway).

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Csaba
1972 280SEL 4.5, silver

Just picked mine up.
As well as being only 3 rows instead of 4, it was aslo terribly blocked.
Should run cooler now.

i noticed something else while testing the car... I noticed when running the car... i don't see the water flowing in the radiator. I can usually see some type of current or moving fluid if i look at the radiator with the cap off while testing. I don't really see this on my 4.5? Is this normal? Or is this a sign of a shot pump?

I am pretty sure i am going to buy a new water pump.. just want to hear other's thoughts.

If i buy a new pump.. i see graf and meyle makes one. (around $100 instead of the $400 Mercedes one). Are these fine?

Graf is OK, Meyle not (Even Meyle rep told me that!). I bought a LASO pump. It appears identical to the OE MB pump. There are lot's of bad pumps out there. Even Bosch.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Basic troubleshooting: Blocked radiator?

This will cause overheating at highway speeds. This is because if the radiator is blocked the engine, which at highway speeds needs more coolant flow, is not getting the flow. So it overheats.

The classic sign of a blocked radiator is the lower radiator hose collapsing. This is difficult to asses on these cars because, at least from the factory, the bottom hose has a strong wire frame to keep it from collapsing.

A blocked radiator usually is blocked only in the bottom. Yes, the cores will be clogged a bit, but the real problem is the crud collecting in the bottom of the radiator. It is already full of your transmission cooler so there is not much room there for crud to collect before it starts restricting your coolant flow.

But in this case you may be fighting several problems. I would start with having the radiator boiled out AND replacing the water pump. After that it time to start checking the other water pipes and perhaps the thermostat for a build up of sluge that is restricting your flow.

Blocked rad will also show symptoms when in traffic I would guess, not just in the highway/motorway

Mine is now cured, and I'll now need to reinstall the thermostat as it's now running too cool.

does anyone make a thermostat that opens at a cooler temp than 75c that will fit this car?

You would not want a lower temp thermostat. The D-Jet system already runs the engine rich enough at 75. At anything cooler, it'd be pig rich. You'd probably wash the oil off your cylinders, damaging the engine. 75 is much cooler than modern engines which typically have 90-100C thermostats.

Check out Rialas' pictures - this is why you should have the radiator boiled out, presuming yours is also not a cheaper rebuild (I could have sworn OEM was 3 core but I don't have a 4.5 anymore to be able to check my own. I also don't seem to recall it not taking up the full radiator frame).

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Thanks for the info on the thermostat Tom.

To be clear, mine's a 4 core, but it's a 300se w112, so maybe different to the 4.5.

It depends on how blocked. The engine is under a heavier load on the highway so overheating at highway speeds is a clue that something is not right.

When it reached the point that it is also overheating at idle then that 'something' has gone wrong.

And.... This could be a seeping in the head gasket or some tiny leak in a heater hose. I would wash the engine down until it was as clean as I could get it and then start looking for a small seep.

If nothing else a clean engine is easier to work on.

It's under a heavier load, but it's also getting a huge blast of air just by moving against the wind.

I still think most overheating problems reveal themselves when in traffic, either from the problems you mentioned, or a non working fan....or a slightly blocked radiator.

sorry for the long time inbetween replies.


I have added a new water pump, thermostat and flushed the bock and radiator. The car runs cooler and when moving does not overheat. But when sitting in traffic for an extended period of time the temperature starts to creep up and the car overheats.

I had a similar overheating issue in my 4.5. Changed to the correct thermostat (4.5 should have a 75c thermostat per specs) and issue went away.

I would insure your fan is working-this is classic bad fan clutch. The fan in front of the AC condenser is not for engine cooling-it's so your AC will work when the car is not moving.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT

The other problem at idle can be timing related. The timing gets retarded at idle so that it will run hotter and in turn reduce emissions. Designed that way at least for NA cars. Setting timing so that it is at TDC or even 5deg BTDC at idle (with vacuum) will allow engine to run cooler at idle. With this setting timing should advance to 27-30 BTDC at 3000rpm. This will give best performance and is in line with the standard Euro specs.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5