My 1985 300D has a cooling system with an expansion tank. My 1981 300D does not have any sort of tank connected to the cooling system. I like the idea of 'converting' the 1981 to have an expansion tank. It looks line it would require a different radiator, expansion tank, and accompanying hoses. Has anyone performed such a conversion before? Thoughts…….

So 25 years and 200,000 miles later, the car needs to be re-engineered? Trying to get jump on global warming?

It is interesting to note that, in other makes of vehicles, folks are going to great lengths to convert their cooling systems to eliminate the pressurized coolant bottles. (Late '80 Jeeps, for example.)

My dad is of that school of thought; he hates th eexpansion tanks and tries to get rid of them.

My opinion would to leave it alone but if you want to do it...

You would neet a new radiator that does not have a cap on the top then a new expansion tank and I beleive that there are two hoses that go from the radiator to the tank; one small one at the top that sends from the radiator to the expansion tank and another larger one at the bottom of the expansion tank that sends to the radiator.

Is there any reason why you would want to change it??? Does your radiator have issues???

--Matt

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KI6CCA

Current Stable:
1987 300SDL
1980 450SEL

Out to Pasture:
1982 300D KIA (Rice Rocket vs. 123: He hit me at 80-100MPH I walked away he went to the hospital. Sounds fair to me!)
1985 300D EURO Scrapped (got more money from state than car was worth)
1979 & 1980 300SD Great Cars!

Well....obviously Mercedes it.........

Here is what I am looking at....
On my 1981 300D there is a piece molded in the top part of the radiator that is the mark for the fluid level. The coolant never stays at that level. Whenever I check it is always about 10mm below that mark. It does not seem to lose any more, but it never stays at the fill mark. I discussed this with a friend of mine who has owned dozens of pre-1980's Mercs. He told me that his experience is typically similar. He told me that as along as it is not loosing coolant there should not be a problem.

Now on my 1985 the expansion tank is mounted slightly higher than the top of the radiator. As long as the tank has fluid the radiator is full and more importantly in my mind the water jacket in the engine head is completely full of coolant. This is my concern with the 1981. If the fluid level is lower how full of coolant is the water jacket in the engine head. Perhaps it is OK, but I have much more confidence with the expansion tank that is slightly higher.

Interesting. I dont think that installing an expansion tank will be a big deal or cause any harm. If it makes you feel better than go for it.

BTW; I was thinking just now that you will probably have to replicate some sort of mount for the expansion tank. As I recall it has two metal brackets that it screws into.

If you have any decent junk yards around it probably won't be to bad of a swap. You may wish to have the radiator you get recored (or whatever they do) since you will have it out and limited rush to get it back in.

Good Luck and let us know how it goes.

--Matt

__________________
KI6CCA

Current Stable:
1987 300SDL
1980 450SEL

Out to Pasture:
1982 300D KIA (Rice Rocket vs. 123: He hit me at 80-100MPH I walked away he went to the hospital. Sounds fair to me!)
1985 300D EURO Scrapped (got more money from state than car was worth)
1979 & 1980 300SD Great Cars!

Had the same thought on my 81 300CD last week while at the pick n pull yard. Decided best way was to get a tank from some Japanese import that would place the holding tank next to the radiator and then replace the radiator cap with one that was double sealed which is required with a recovery system. Run the overflow hose into the bottle. Decided against the M.B. tank because as you noted, it would require a new radiator with the hose fitting molded into the lower right side of the tank. I like the tank, not because of no air in the system, but because you can verify coolant with no cap removal. Once done I would then have to get some Italian part in order to cover all the WW2 enemies!

my new 81 240d also, no expansion tank. was thinking of buying an after market tank at Advance Auto, around $10.00. it comes with brakets for mounting, then just run the tube from the raditor to the tank.

bob
1981 240D 181,000

That sounds like a simple solution to achieve basically the same results.

Would this be an overflow bottle or a pressure bottle?

This would work as an overflow blottle.

Most overflow bottles are connected to the filler neck of the radiator, where the radiator cap serves as a pressure relief valve. The bottles on the later model 300D's are pressure bottles. There is a big difference in the two arrangements. You cannot just "connect" an overflow bottle to the radiator.

Well, the cap type is important for this type of setup. It has to be a type that rellieves excess pressure to the overflow tank though one 'valve' and then through a second 'vavle' allows the coolant to be syphoned back into the tank.

Why change a system that has worked fine for 25 years? Whats wrong is it overheating?

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Well,
I found the problem. The radiator top end cap has a very slow leak in a small area where it meets the radiator core. This has been tricky to find because I had never seen the car drip/leak/etc. coolant. I probably have never found it because most trips have been long enough that the coolant has been lost during the driving and no more is lost once the coolant level is at the seam.

I found the leak buy filling the radiator to very top, driving just long enough to get the temperature past 80C, and then parking the car in an uphill position. Now it is hot expanded fluid with no air cushion. This revealed the small leak. The car does not seem to lose any more fluid once it is at the level of the seam.

Can the end cap be removed and resealed or do I need to replace the radiator?