1979 300SD Coolant Flush and Change

[JasonP]

I have never added or changed coolant in a car before, so this is all new to me.

I would like to flush my coolant system, and then add new, for the hot summer which is already upon us. I have a 1979 300SD, and so far the engine seems to be running just fine - the temp stay a little under, and a little over, 175F when driving on the highway, so I am not worried. I just want to get this done.

* What kind of anti-freeze should I use?
* What should I look for in it (chemicals, numbers, ratings, ...)?
* How do you flush these systems, and with what?
* I assume I am going to do a 50/50 mix with anti-freeze and distilled water when I add it back in, is that correct?

Thanks!

[koan]

Have a look in the library for you model and year, in the engine section you'll find job 20-015 "Cleaning the cooling system", talks about de-oiling, decalcification and derusting, and how to do it, all you could ever want to do.

De-oilling most likely not requred but decalcification and derusting a 30+ year old engine is worthwhile. If you can get the chemicals mentioned in the manual use them, otherwise use a radiator flush compound.

As far as choice of coolant goes, some say only M-B, others say not (let the debate begin!)

[Squiggle Dog]

If you don't use genuine Mercedes coolant (available at the dealer and apparently at NAPA), at least use Zerex G-05 (found at many, but not all auto parts stores). Zerex G-05 is approved by Mercedes and it keeps your engine 10 degrees cooler than the green stuff. Mix it 50/50 or the recommended ratio with distilled water (tap water has minerals which can leave deposits and corrosion).

The dealer sells citrus flush, and Zerex has its own flush in a bottle. I imagine even the Prestone flush would work okay if the instructions are followed. Follow the instructions on the bottle, but basically you drain the system (pull the drain plug on the side of the block--in front of the starter--to get the coolant out of the block), add the flush with hose water, run the car for a bit, then let it drain out again (including pulling the block plug once more). Then fill with 50/50 MB or Zerex coolant mixed with distilled water.

[JasonP]

Thanks, guys! Excellent information!

Squiggle, you once mentioned that this pic of mine showed where to hook up a hose to flush the cooling system. Do I have that right? You can see the disconnected ACC servo in the lower left.

[r5149]

It really bothers me to see cooling systems on these wonderful cars filled with the GREEN STUFF! Very corrosive for the aluminum parts in your cooling system. One of the first things to do when I acquire a MB of this vintage is flush the cooling system with citric acid. I then have noted that if the car was maintained with the GREEN STUFF, there is corrosion & pitting on every aluminum part. I could tell if the previous owner used either MB coolant or G-05, as there is virtually none of this damage.

[JasonP]

So, just out of curiosity, what temperate should my gauge be at? It has been over 100 degrees the past couple of weeks, and I commute daily on the highway, and my temp gauge maxes out around 187 F on these very hot days.

On nicer days, the temp runs just a little over 175 F when it gets warmed up, and stays there.


And can I top the coolant level off with some distilled water, until I add the 50/50 mix?

[r5149]

Sounds like your temp is right where it should be . No worries there. I buy my citric acid by the pound( the amount you should use to mix with a gallon of warm water) from my local bulk food store. I've used both the Zerex G-05 and the MB coolant, & not noticed any difference over the years other than when I disassemble the cooling system.
After changing out the GREEN stuff that a previous owner used, and flushing repeatedly, the system stays clean & runs cooler!
Otherwise, follow the suggestions made by our members. I find the only annoying part of the procedure is getting the block plug in & out.
Good luck & enjoy your 79' SD, I will never part with mine!

[Casey]

If your temperature is consistently off, it's probably your thermostat needing replaced, not coolant.  They're cheap, it's an easy job, and if you have any doubts might as well just replace it.  The problem I had with my 200D was that due to a bad thermostat, the engine ran *too cold*, which leads to decreased efficiency and is harder on the engine apparently.

[cejpat]

I've got a minty '79 300SD ... I've heard that Evans Cooling's lifelong products are highly recommended.

They supply just about all the racing teams.

[oversize]

I've always been worried about chemical cleaners.  Did MB actually recommend one??  If so which one is available in Australia??

[KenM]

The manual recommends citric acid flush Mark, I went through the exercise a while back, best way is to buy the stuff in powder form in 1 kilo tubs over the net, or maybe a chemist supplier etc, it's not

expensive. I did it just the way the manual says but noticed no cleaning effect, from which I think my system was clean to start with.

[oversize]

Supercheap has a special on G05 until 28/11/13. I just got 3 x 5L containers of concentrate for $30ea! Bargain!

[ptashek]

I did it just the way the manual says but noticed no cleaning effect, from which I think my system was clean to start with.

For this to be effective, before flushing with citric acid the system needs to be de-greased with a sodium hydroxide (caustic soda) solution first. I think it's also mentioned at some stage in the manual.

[KenM]

I did it just the way the manual says but noticed no cleaning effect, from which I think my system was clean to start with.

For this to be effective, before flushing with citric acid the system needs to be de-greased with a sodium hydroxide (caustic soda) solution first. I think it's also mentioned at some stage in the manual.

Yep, this is correct. I skipped this part as I figured my system was clean. Curiously, since I did this and filler her up with nice blue MB coolant, I found after a hot run in traffic that my coolant is now all brown and yucky again, obviously loosed up some crud that was hidden somewhere, so I need to repeat the process, this time with the degrease I guess.