I recently did similar work on my 95 E300D, and ended-up with similar symptoms. In my case it turned out to be trapped air in the system. Took close to 300 Miles, over an 6 day period to get rids of it. Coincidentally, my 1 year old Rad cap gave-up at the same time, would not keep pressure.

Try the following: 1) squeeze the top of the radiator hose when its hot. Should be pressurized. If not than its the cap. 2) Squeeze it when its stone cold. Is it soft and spongy, do you feel air bubbles?

So if your problem is trapped air, be patient, it will escape on its own.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

If I am not mistaken the key to all these problems are to jack up the car in the front as high as you can and bleed the system that way...Had the same problem and after the previous procedure it is fine....

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83 300CDTurbo 307,000
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http://www.peachparts.com/shopforum/showthread.php?t=125099

The temperature no longer hits 100C, having flushed the radiator again, but I will try putting my other thermostat. Keep you posted on that.
While I'm at it I think I'll also put in a new radiator since the one currently in it was left for a while with old antifreeze in it and not driven. I'm guessing there's some rust on the inside.

When you say to back fill it through the port on the head, you are referring to the top port directly to the front left (as looking at the engine from the front of the car) of the valve cover? The one that the top radiator hose plugs into?

If so, then, yes, this was filled totally. I did not think to measure the amount of coolant I put in, but I will certainly do this when I change the thermostat.

Why would you jack up the car? The return line from the top of the radiator to the reservoir is already the high point of the cooling system and therefore the air is constantly escaping through there and will release when the cap is taken off. What procedure are you referring to?

Simple....take a look at where the thermostat housing is....take a look at where the top of the head is.........thats where the air pocket is....not the top of the radiator. that helps force th eair buble forward and out of the head.....

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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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"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

AN incorrectly installed thermostat will cause overheating - not sure that it would stop at 100'C though?

Remove the thermostat - check it's orientation when removing it -was it installed correctly?
Test the thermostat in a saucepan of water on stove, 100'C thermometer, note temperature thermostat starts to open,
Check your other thermostat too.
Reinstall the better thermostat in the correct orientation.
Report to us the results of your investigation and test drive engine temperatures too.

Ironically, I installed the new thermostat and water pump to get engine temp up, my old thermo was broken open all the time. Well, I got what I wanted right?

I put in the other thermostat correctly...the old one was in correctly after all...and no result. I tried jacking up the front end as high as it would go and running the car for 10 minutes with the reservoir cap off, and still no luck. Temp the same, even a little higher since the air pockets haven't cleared out yet this latest drain and refill of the system.

So probably my radiator then? Here's what puzzles me:
Lower radiator hose to thermostat housing=cool, which means radiator is working
Upper radiator hose, block to radiator=very hot, which also seems right.
Thermostat housing=hot, right.
Thermostat to block connection=very hot, which all seems right.
Where is my problem? Just not enough flow through the radiator maybe?

Also, could someone explain the best bleeding procedure? I've read to jack the car up, to run it with the res. cap off, to do nothing and wait for it to go out, and on and on. Are there any bolts or plugs I have to remove to do it? What is the best way? I feel like maybe I'm missing something on this part and maybe an air pocket is plaguing me.

I have had good success by using the upper radiator hose to refill the cooling system. I disconnect the hose at the radiator, bend it back and up, and add the water/antifreeze through the hose instead of the coolant reservoir. When I filled at the reservoir, I always had a problem with trapped air. But it always seemed to "burp" pretty quickly, at which time the reservoir would empty itself. So the fact that your problem has lasted so long causes me to suspect that perhaps your thermostat is bad. (I am assuming that your reservoir is still at the proper level.)

The reservoir is slightly above the recommended fill level.

It can't be my thermostat, because I have put in two different, new thermostats, both with the exact same results.

My suspicion is the radiator (not a high enough flow rate through it) and so I will replace that this weekend. Then I'll refill with the exact right mixture coolant/antifreeze, in the right amount, fill with the best procedure (I did use your method of the top hose as that does work well), and hope for the best.

Did you install a genuine Mercedes water pump or some other version?

It was not a MBZ part, but it was a german pump (I forget what brand, I could tell you if you really want to know), and it had the metal fins and everything---not like the crappy ones with the plastic fins---appeared identical to pump already installed which was original equipment.

The reason that I asked is that there have been a number of reports on the forum regarding poor performance of non-OE water pumps. The problems had to do with the impeller design, which resulted in poor coolant flow.
It's difficult to believe that your problem is being caused by the radiator, considering that the engine was running too cool before. That demonstrated the radiators cooling ability. It doesn't make sense that it would loose that ability all of a sudden.

What brand of non-OE pumps showed problems. Do you know? I always use LASO brand pumps. I agree with tangofox007 about radiator - it would not suddenly stop working properly (unless you used stop leak or something awful in the system
I once had a suspect radiator so I took out the thermostat and the engine took a long time to warm up (of course!) and ran cool as a cucumber even with A/C on and a hot day it never approached 100 deg C. Installed a new thermostat and everything was fine, so the old thermostat was sticking or not opening fully when it should have. Try that as a test, it's easier (and cheaper) than a new radiator! PS you can reuse the gasket if its new just don't tear it taking it off though.

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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 don't know.

I've been thinking about this again.

If the engine was running too cool prior to all of this work, then the radiator cannot be the culprit. The radiator does not go bad overnight.

I am assuming that you had no cooling problems and the engine would never get up to 100°C. prior to all this work, correct?

And, you have installed two different thermostats, with the same results. So, this basically rules them out.

The only other items are the water pump and the fill procedure.

These engines can get some air in them and I have also found that it's best to fill the engine via the upper radiator hose after you disconnect it from the radiator.

Then, take it out for a drive. See where the temperature stabilizes. It may climb up to 100°C. and then settle back to 90°C. after more air is returned to the tank. Make sure the heat is on full during the drive.

If you still cannot get it down near 90°C., then you have two possibilites:

1) The pump is not up to par (very doubtful).

2) The gauge is not reading correctly.

To prove #2, you would need one of those infrared temperature probes to check the temperature of the coolant in the upper hose. If you read 90°C., then you know it's either the gauge or the sending unit.