Here's my situation:

After doing an overhaul of my cooling system (changing water pump, thermostat, and extensively flushing block and radiator) my temperature rose to a consistent 100C, either at highway speeds or city driving, but with no needle bouncing or erratic behavior.

The only problem with this is that the block is not ridiculously hot to the touch, the radiator is barely warm to the touch except at the top black plastic cover (where it is very hot), and the coolant in the reservoir is lukewarm at best.

Furthermore I can remove the cap of the reservoir when it is purportedly 100C in the system without any pressure escaping, much less heat.

--The hose connecting the block to the radiator at the top is very hot, as is the top of the radiator.--

I drained the radiator again, suspecting a slight blockage in there, flushed it, refilled, and this time bled the system. But this only dropped it to about 95C when driving at city speed and 98C at idle. Probably 100C at highway still.

I don't suspect the gauge since it is behaving smoothly, so there must be some blockage in the system, right? I really don't want to have to replace the radiator, it seemed to flow fine when I flushed it, could there be a blockage in the engine?

I need to fix this since this is my daily driver car, and I don't want to damage the engine with this high heat. Thanks everyone who helps me out with this car, I wouldn't be able to do it myself without y'all.

hell mine doesn't even quite get to 80 c on the highway....................but then I just got it working that well yesterday.................

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I forgot to add:

I bled the system by starting the engine cold with the reservoir cap off and running it at operating temp. a while, let me know if this is wrong, it's what I read on some threads. The car is a 300cd turbo. I'd love it to be an air pocket.

I don't think the aux. fan is coming on at 100C, but I'd have to check further to be sure. I know at least it was working some 6months ago.

Will 100C (if it is in fact at that temp) hurt the engine, or is that within the acceptable range for very temporarily?

On my car, 120 C is redline. During the hot summer months when I'm doing 75 - 80mph on the interstate, my temp stays around 100 C. I've never had any problems. However, what I personally consider normal operating temp. on my paticualr vehicle is a little bit over the 80 C. mark. Cheers.

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1984 300TD Wagon--Astral Silver--Retired
1983 300TD Wagon--Dark Blue--Retired

I am having a similar problem. I just replaced the water pump, thermosat, belts and hoses. Now my top hose and top of the radiator are hot, but the top hose is not hard as it was before and I can remove the radiator cap right away. The bottom hose is noticably cooler than the top, which also is not hard. I was wondering if I installed my thermostat backwards? Maybe this is now normal? Temps hold consistently at 81-82.

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)

I highly recommend you perform the following procedure. If it works, it will save you from purchasing a new radiator:

Pull the radiator out of the vehicle. I know, it's a PITA, but, it's worth it.

Hold the radiator up to the sun. Look through the fins. See all that crap?

You mission, should you decide to accept it, is to remove every bit of that crap which is blocking airflow in a big way.

Use compressed air, a chemical cleaner that foams, and a whole lot of patience and repetitive cleansings from the rear side of the radiator to attempt to flush as much of that crap out as you can. It takes a good two hours and it won't be perfect, but, you'll drop the temperatures at least 5°C. and maybe even 10°C. if the radiator is quite bad.

Unfortunately, there is no tool that I could find that can be stuck into the fins to clean them. I think even pipe cleaners are a problem, but, I haven't tried it.

If this does not solve the issue, then the radiator is partially plugged internally and a new radiator is probably going to be required to significantly drop the temps.

Brian, I don't want to hijak 83's thread, but we both just did similar work, and we both now have similar results. I am running 20 degrees cooler than him, however. But my concern is that I never saw the soft upper hose before and being able to remove the radiator cap right away.
Th only thing I can figure is I changed something when I put everything back together. Well, you can't put in the water pump incorrectly, nor the fan or belts or hoses. The only thing I can think of is maybe I wasn't paying attention and put the thermostat in backwards. What symptoms would a backwards thermostat cause?
I have another gasket so I am tempted to check it out.
Your thoughts?

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)

I would not remove the thermostat. If you had it backwards, the temps would climb well over 81-82°C.

My guess is that the new thermostat allows the engine to run a bit cooler than the previous one and, therefore, pressure has not built up in the system.

Take it out for a good run uphill when the ambient temperature is above 80°F. I'll bet it develops some pressure under those conditions.

Take PEH advice on this one: If it ain't broke, don't fix it.

An interesting point is raised here. Shouldn't the side of the thermostat with the spring be facing towards the block entrance so that it is pushed open as the coolant heats/pressurizes?

Would a backwards thermo cause a temp like 100, like mine? Also I could try putting in a different thermo if that one is bad, I actually happen to have one, but the one in now is a behr, not likely defective.

I take it if the thermostat is not the problem I will either have to clean or get a new radiator?

Alright, I'll leave it alone.

The thing is my old thermostat ran at around 84 or so and the upper hose was always hard and the system was pressurized.

Maybe it is running cooler because I did repeated flush's after using Prestone Flush? The PO apparently hadn't touched the coolant in years as it was rust water that came out initially. Probably why the water pump started weeping. Man, whoever replaced that pump before used gobs of black silicone and a "made in Japan" water pump with odd looking fins - they weren't the cast ones that the German manufacturers use. The only reason I replaced the thermostat was I was in there already doing the water pump etc. and decided to do it as a PM.

So, the upper and lower hoses being soft to the touch, and the lower one feeling considerably cooler than the upper one is nothing to worry about? Just being over anal about my old benz's I guess Man, my kids think I'm nuts messing with these old things all the time when I have "nice cars" sitting on the sidelines

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)

The lower hose should be cooler than the upper hose. If it's not, the radiator is not doing a very good job!

That's the correct orientation, but not for the reason stated.

I believe that the aux fan is energized on the basis of refrigerant temp in the a/c system, not by engine coolant temp.

"But this only dropped it to about 95C when driving at city speed and 98C at idle. Probably 100C at highway still".
Something is wrong with the above scenario.
That engine should not run 100 C unless its driven up a steep hill, on a hot day in the desert.
I suspect you have installed a bad thermostat. There has been reports on this happening with new thermostats, Bosch or Behr? I don't recall but you can search the subject and read it.
I would not drive an OM617 that acted that way unless I used an infrared thermometer to prove my gauge reading was incorrect!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

"I bled the system by starting the engine cold with the reservoir cap off and running it at operating temp. a while, let me know if this is wrong, it's what I read on some threads. The car is a 300cd turbo. I'd love it to be an air pocket."
Did you back-fill the system using the port on the head as the inlet?
You might just have a bad thermostat and no water in the head (aka air pocket)
Did you measure how much water you put into the system? I add approx. 1.75 gallons. (one gal antifreeze and the rest purified water) About 1/2 gal remains in the heater and connecting pipes.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!