On my 1982 300TD (wagon) with about 250k mi, I recently flushed the cooling system and replaced the green coolant it came with with the proper MBZ stuff and replaced the thermostat with a new tested unit and carefully installed it facing the correct way. after driving it around town for a while it burped and I toped it up. Well, I just got back from a long trip and it is misbehaving. On the highway it is running hotter than it used to, around 95C and in the mountains it will run right below 100C. running the heater cools it off by about 5 degrees. The upper radiator hose gets nice and firm. Normally I would think air in the system but I would have thought it would be worked out by now.
The wierd part is that there is a consistent yet intermitant leak of coolant comming out of the radiator cap (or the bypass hose to the aux coolant resevoir?) while running. There is a nice spray pattern of dried coolant radiating from the cap area of the radiator. I have replaced the cap and no change! I also noticed that when hot the auxillary resevoir is almost totally full and stays that way even when it cools down until I open the radiator cap and then it all drains back into the radiator. It is clear that the system is overpressurizing but I cant figure it out. what would cause the coolant resevoir to act this way? Thanks in advance

Is the small vent line between the top of the radiator and the reservoir restricted or kinked somehow? It sounds like it's not venting properly.

craig beat me to it.
it could also be a bad head gasket... but the tank filling up like that sounds interesting. check the lower hose when the tank is full, is it possibly collapsed? also some gunk in either of the lower hoses could cause this.
a stuck thermostat may cause this.
100C is not REAL bad, but if you have a full tank, and air/steam in the system somewhere, it will not cool properly. check all the suggestions, and let us know if you fix it.
John

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none of the hoses look colapsed, I was thinking head gasket at first too but this is all coincident with replacint the thermistat and I even tested it on the stove before I installed the new one so stuck thermistat is unlikely. I'm thinking either some crap in one of the hoses or some really stubborn air in the system.....

It could be as simple as air in the system, but I would also try replacing the radiator cap. It's possible the system is not holding pressure and you are getting a steam bubble in the engine someplace.

Thats a Head Gasket!

Here is a simple test. Take off the rad cap, fill system with coolant, Leave the rad cap off and crank the engine, but don't let it start (just crank immediatly rather than letting the GPs get hot first). If the HG is blown, coollant will shoot from the radiator in strong pulsing spurts. This could also mean a cracked head or block, but that's pretty rare.

Sorry to be the bearer of bad news, but I went through this with a blown HG on my 240D. Identical symptoms, but I hope I'm wrong!

Good Luck, Chris

I was thinking head gasket too but this coincided exactly with changing the thermostat and was not happening before. It hasn't over heated either, it is just running hotter than it used to. I will do the test though.

It could be the head gasket, but I wouldn't jump to any conclusions yet.

Alright,
the overflow hose between the radiator and coolant resevoir is in good shape and has no obstructions. The overflow hose out of the resevoir and onto the road is also unobsructed and working. Cranking the car with the radiator cap open causes no spurting fluid at all. After this started I replaced the radiator cap and still have the same problem. I could understand a stubborn air bubble (would have to be very stubborn at this point) but I can't understand the leakage at the radiator cap or the non-draining expansion resevoir........

A leaking radiator cap would result in lower system pressure. If the temperature inside the engine exceeded the boiling point of the coolant (at that reduced pressure) steam bubbles would form in the engine causing the tank level to increase. That steam would act like an air bubble restricting the flow of coolant, in other words the coolant system could be partially "vapor locked."

The function of the small hose between the radiator and the tank is to keep the (air space) pressure equal between them. If that hose was blocked, the pressure in the radiator could be higher than the pressure in the tank, causing the level in the tank to increase.

Craig,
I agree with everything you just said but:
1) the old cap never leaked before, the new cap also leaks in exactly the same way indicating another cause. (the filler neck is in great shape too)
2) the small hose is not blocked, I just tested it. Also it only equalizes the pressure when the inner seal of the cap is opened by pressure ie not when the system is cold or below the pressure rating of the cap.

I can't figure out why the outer seal on the cap is also leaking, the excess pressure and coolant should equalize between the radiator and the resevoir through the small tube, not to the engine compartment via the radiator cap. Somehow the radiator is maintaining a higher pressure than the resevoir. Any other ideas on how that can happen? This is helping to reason it out, please keep it coming.

No, I'm not talking about the small overflow hose connected to the neck of the tank (under the cap), I'm talking about the small hose connected to the front part of the tank. That line will always equalize the pressure between the tank and the radiator. Are your lines connected correctly?

there are two small hoses, one between the upper part of the radiator filler neck and the side of the resevoir tank itself, and one that attaches to the side of the overflow resevoir filler neck that drains to the road. The first one only comes into play when the lower seal of the radiator cap is opened via pressure as in this picture: http://www.centuryperformance.com/images/tech/radcap.jpg . This has been the arrangement from both before and after this problem started. On my car the radiator and the resevoir both have caps and the radiator is an all metal replacement unit that was already installed when I bought the car. It has worked well in this configuration for at least the last 20k miles without causing the problems I am seeing now.

You do not have the original design installed. The original design is that the upper line from the radiator is always pressurized, and there is no cap on the radiator itself. If the line from the radiator is in the neck above the lower seal of a radiator cap, your expansion tank is operating like an overflow tank and is not correctly vented. I would start by installing the correct radiator.

I appreciate the help but I don't think I'll be starting there for practical reasons. The current radiator worked fine until recently and is a high quality replacement. Again, this is a new problem not coincident with the installation of the radiator. I may explore a way to disable the lower gasket on the radiator cap to return it's function to that of the stock radiator. but none of that would explain the overpressure problem directly.