I just caught this thread. I'm not sure if you remember me mentioning this when we caught up last - but the removal of the radiator and thermostat is necessary to really get it to work well. The problem is, how do you keep circulating the solution for the minimum 15 minutes??? And you really need to do it for an hour or more if it is possible, when things look bleak.
well I don't know if you have a 20 gallon drum, or even if you have a spare 5 gallon drum laying around, but I found them invaluable for things like this.
My family has a few plumbers so for me some things are easy to do / get hold of.
1. 5 gallon drum - large paint tin works.... as you can take the lid off and see all the junk...
2. some 2" or 1 ¾" copper tube. (about 1 foot length cut into 4 pc.)
5. 2" rubber hose - or old radiator hoses
6. small stepladder to sit the drum on in front of the car while it's running.
the citric acid you grab from a home-brew place. citric is used to clean out carboys for brewing..... save $$$ and buy bulk.
cut a 2" dia hole (or whatever the diameter of your copper / pipe) in the drum at ¾ height, and another at ½ height . braze in 2 pieces of the 4 pc of copper into these holes. It helps if you angle the inlet pipe at the top to create a vortex with the inlet flow. make sure the outlet is at 90º though, (inlet at top, outlet is bottom) like a swirl pot - which is essentially what we are making - on a large scale.
Grab your old rad hoses or if you're flush, some irrigation pipe or other material to join the top and bottom hoses to the drum, using the other 2 pieces of copper and some matching hose clamps.
If your rad hoses are smaller, use smaller pipe if need be. so long as it can form a cooling system loop it doesn't matter how you achieve it. All you're looking for is a substitute radiator.... which is what the 5 gallon drum /paint tin is.
Once it's all hooked up, sit the drum on a ladder to elevate it above the thermostat housing & header tank height.... hence small stepladder or barstool.... remember it's going to get HOT, so don't put it on something that will melt.
Fill the drum up past the inlet pipe. Add the citric acid (about 3-5KG) stir it around until it dissolves and start the car. let the car warm up. At this stage, do not give it any revs. all you want is coolant circulation and no air pockets.
The car will warm up, it will take time as there is more coolant circulating in the system.
You can leave it for an hour or so, and it will be fine. while the coolant is circulating, you will notice some bits of rusty crud falling to the bottom of the drum. This is why you want that outlet higher up on the side, so you don't pick up that crud and recirculate it. You can put a stocking over the bottom pipe stub before you fir the hose to it, to act as a further filter.
Once you are happy with the flush - (bear in mind you can drain and repeat it until the coolant is clear, within reason), you must thoroughly rinse flush the system. best way to do this is with a garden hose, straight into the bottom hose inlet, and car off. let it run on a medium flow for about 15 minutes, which is what it will take to thoroughly neutralize the acid.
That's part 1.
Part 2 is more sensitive. It's about the radiator core.
If you have a brass tank / copper core, then you need to be careful when acid cleaning. If you have an alloy core plastic tank, then you need to worry a fraction less, but you do need a different solution to clean each one.
for copper tank you can use zinc chloride - aka soldering fluid. buy off the shelf or make your own with some zinc and muriatic acid. either way - whatever is cheapest and easiest.
fill the core with a diluted solution, soak for a minute, drain, rinse with water, repeat.
If this is not working, then you can use a slightly higher concentrate, but beware.... an old core is an unknown, and any acid wash or dip can pinhole cores - particularly old ones which have seen many years of internally corrosive fluids.
This will ensure that any passages that are not blocked, will flow cleanly. It does not guarantee that any cores which are badly blocked, will be unblocked. Chemical treatments can only go so far.
The copper / brass radiators are usually soldered with a low-temp solder, and it's easy to take the radiator to a specialist rad guy, to desolder the tank and wire through the cores as well as acid dip, resolder and respray. This will get you some extended life and is a better alternative...HOWEVER -
If the core is 10 years old or more, do yourself a favour and have the rad re-cored.
The overheating issue I had was difficult to diagnose. ended up replacing every part of the cooling system, pumps, tanks, hoses, visco coupling... t-stats etc... (but the rad was flowing heaps, and i mean heaps. the fire hose in my work basement proved this) All except the radiator. Why? because the FIRST thing I did was citric flush and pull the rad and flushed that several times, and it looked like it was absolutely perfectly serviceable.
I finally took the rad to get recored as the last straw. - i'd done the acid clean, everything....
So I had my SLC recored by a specialist guy who does serious earthmoving equipment.
He pulled the tanks off, found the core to be 80% blocked (yet I'd already proved it was flowing huge amounts of water - it just goes to show that your heat-soak issues can be mis-diagnosed when you think the radiator is flowing fine..... when in fact it isnt!)
Specialist guy says "no wonder - your core is not right - it's for a stationary diesel engine" He looks up his catalogue, finds 3 different core applying to Mercedes 350/450/500....
1st core is 3 row parallel (factory), 2nd is a replacement modern 3 row offset, 3rd is a 4 row parallel (factory). He checks the specs of each.
The more modern design 3 row offset core has an 80% efficiency increase over the exact replacement 4 core (4 row parallel which was in the car).
not only that, air flow requirement was less than the 4 core.
Dimensionally the radiator can obviously only use a core that fits within the tank dimensions, otherwise youre talking custom...
After he showed me the old core I was amazed at the blockage - in every single tube! some bad, some completely blocked, some simply hanging in there... it was a tragedy waiting to happen.
He silver brazed the tanks onto the new 3 row offset core and repaired the air bleed line in the header tank, I refitted it to the car.
1st start and it got up to temp quite quickly. I started to get a bit worried I had bigger problems...
But the temp of the car stayed right on thermostat opening temp and never budged - It remained that way over an EVIL summer where we had 45ºC temps for days on end. I thrashed the car and it did not even look like breaking a sweat.
It sat just over 85º and fluctuated the tiniest of amounts. I was rapt.
and to top that off, we discovered a tag on the original radiator core - it was from 1975... so that told me 3 years after purchase, the SLC's radiator was replaced..... STRANGE!! I wonder what caused that.... but not bad for a 42 year old car eh? It was looked after, but you can't predict these things.
So moral of the story is, citric acid flush will only do so much, but you really need to get the radiator checked as well, especially if it's a bit old... And even if the core looks like it flows plenty - silicone worms and other crap can block the cores and cause big trouble. So while it may work for a while, pay attention to the way the temperature behaves when you're not doing highway speeds.
So I learned something valuable about older Mercedes copper radiators... they can hold huge pressure, and they will work perfectly fine, until the heat-soak threshold is reached. They are far more capable of coping with bad cooling systems than any other car I have owned to date....
But it can catch you out - It doesn't have to happen on a hot day... in fact for me it happened on a cool night on the way home from work, well after peak hour traffic...
Once it happens, you simply cannot cool the system down as the heat input is exponentially greater than the heat loss across the core. This is why cars overheat when seemingly 'unloaded' with regards to engine use.