INTRODUCTION
Before the advent of water cooled systems employed on cars, as is presently the case, in the olden days, cars such as Volkswagen beetle used the air cooling method to transfer excess heat directly from the engine's cylinder heads, and engine blocks, to the environment.
The flaws observed in the VW incessant fire outbreaks, led to other manufacturers, looking for other methods of effectively cooling the engine.
This led to the use of water cooled system of transferring heat from the engine, via a radiator system, and directly to the environment.
In thus system, the risks of car fires has being reduced, and vehicles could go long distances without the fear of sudden car fires.

SYSTEM OF OPERATION
A sides the radiator cooling system, we've the following as part if an engines cooling system:
*thermostat
*radiator cooling fan
*water pump(electric water pump, in the case of hybrid cars)

As the engine gets to it's operating temperature, tremendous heat is generated by the engine, in terms of overcoming frictional forces generated in the engine. It's the overcoming of these forces that translates in to the motion that moves your car's wheels.
To make the engine last longer, a medium is required to dissipate the excessive heat from the car's engine, via the cooling systems, and straight to the environment.
The radiator's upper hose transfers the heat from the cylinder head, and block engine.
Its then cooled by the radiator cooling fan, where its recycled back to the engine, as cooled coolant.

WHY DOES THE COOLANT GETS RUSTY(REDDISH)
*when water reacts with a metallic materia on exposure to oxygen(air), the end product is corrosion. So, if you're using ordinary water as a coolant in your car's cooling system, you're gradually deteriorating the radiators capacity to dissipate heat.
With time, the rust develops into a huge sludge that can block the radiator's cooling fins, and gradually cause engine temperature rise.
Ordinary water contains impurities, that aid corrosion, and influences temperature rise.
*expired radiator coolants, leaves within its trail, sludge.

WHAT COULD A RUSTY RADIATOR SYSTEM CAUSE
*increased temperature rise, that might gradually result into engine damage.
The cylinder head may be warped, if its threshold temperature rating is exceeded.
*an overheated engine, can cause catalytic converter damage.
The reason being as the engine overheats, piston rings and so on, in the combustion chamber looses their sealing abilities in the chamber, causing mechanically relates misfires.
Engine misfires are the major enemies of catalytic converters.

HOW TO KEEP THE RADIATOR SYSTEM EFFICIENT(MAINTENANCE).
*Avoid using ordinary water as coolants for the cooling system. If you're using water, it should be distilled water.
*Use approved radiator coolants for your cars cooling system. Almost all car brands have got their specific radiator coolants.
Toyota has got the SLL(Super long life). Check with your manufacturer to see which is specified for your car.
*Flush, Repair, or replace the radiator system, if it shows signs of extreme rust build up (sludge).
The build up of sludge, gradually reduces a radiator's cooling dissipating capabilities.
*Remove debris that might have being trapped on the radiator's fins.
PS: Radiator coolants are not meant to be used till eternity. They're to be replaced, at least 2-3years. They contain additives that help absorb the heat generated from the engine under temperatures running into thousands of degree Celsius.

Fig1: Radiator system disassembled

Fig2: Sludge build up in the radiator's plastic cover

Fig3: Radiator system manually washed. A tedious process though.

This writeup, is a combination of my personal experience as an automotive engineer, and of researches i have done.

CASE STUDY
We've worked on a car recently with an overheating issue brought in by a client.

WHAT WE DID
*Checked all the radiator system hoses, and water pump for sign of leaks. There weren't any.
*checked the level of coolant in the radiator, discovered it was reddish, as shown on the figures below.
*carried out a steam wash of the radiator and engine block, using pressurized steam wash machine.
*Drained the old ordinary water in the cooling system, and introduced a new one, in order to finalize the engine cooling system flush.
Checked the operation of the radiator cooling fan. It was working efficiently.

CUSTOMER FEEDBACK
"You guys didn't do anything on this car o!. It's still the same thing"

OUR RESPONSE
*Took the car in again, and simulated the customers complaint.
Carried out further troubleshooting, and decided to remove the radiator system from the hood.
*Took to radiator specialist, who dismantled the radiator plastic top cover, and discovered the sludge buildup on the cover.
That was where the real problem was coming fro.. We discovered that as long as we keep on flushing the radiator system, we keep getting rusty solutions from the radiator's overflow tank, and the radiator system itself.
*Carried out the manual flushing of the radiator, mounted it back in its place, and tested the vehicle.
The car's temperature went down, but the reddish rusty solution keeps coming over the radiator cover, immediately it's opened.
The customer decides that since the temperature issue has being solved, he would look at buying a new radiator system, after the recession is down.

Your positive or negative criticism is welcome.

Regards,
TRYGO

@GAZZUZZ. You're right o! That's a better alternative. Some would tell you, Buhari economy de na.

WOULD ADvice you to open up ur water pump , cos from the amount of rust am seeing, its likely the blades of the water pump has rusted and this would reduce the volume of water the pump can effectively displace, thus reduce the heating capacity of the car.
this is a usually overlooked aspect in car cooling systems

@earthrealm. You're totally correct. After the radiator and engine block flushing, we discovered that at some days, when the radiator cover was opened, you'll always get that clayish stuff coming out. The water pump is in good condition, displaced perfectly. No overheating currently. Mind you, the car has a mileage of 250, 853km on it, and has being parked for years. So, probably, like I mentioned in the post about causes of those sludge, it could be that the radiator coolant had expired, or if water was used as its coolant, must have had that same effect.

Appreciate your response.

TRYGO:
@earthrealm. You're totally correct. After the radiator and engine block flushing, we discovered that at some days, when the radiator cover was opened, you'll always get that clayish stuff coming out. The water pump is in good condition, displaced perfectly. No overheating currently. Mind you, the car has a mileage of 250, 853km on it, and has being parked for years. So, probably, like I mentioned in the post about causes of those sludge, it could be that the radiator coolant had expired, or if water was used as its coolant, must have had that same effect.

Appreciate your response.

ok good, you hv tot of that.
people should avoild mixing coolants, thats another cause of sludge. the coolants have different chemical composition, thus may react differently when heated, even though the color may be the same, ie red or green...it doesnt mean that they are the same/compatibilible.
top up with same coolant as was origninaly in the car...or flush and replace with new, if you arent sure...coolant is cheap..less than 4k

@earthrealm. You're right. ABRO coolants are less than 5k per gallon. If you're going for your car's branded coolant, be prepared to spend more.
Thanks for your contributions sir.