Having written a little about the radiator cap and water pump http://www.pitchcare.com/magazine/the-cap-that-keeps-its-cool.html I thought that the cooling system in general might be of interest. There are many engine problems associated with the cooling system that can be prevented by a little maintenance.
Why do we need a cooling system?
At 4000 rpm there will be 2000 controlled explosions per piston per minute, so a 4-cylinder engine will have 8000 of these explosions every minute, each one generating heat. Now, much of this heat is expelled via the exhaust system, but a great deal is absorbed by the engine block which is not all bad - an engine runs more efficiently when the coolant is around 190-210 degrees Fahrenheit. The oil is thinner and engine parts move more freely so there is less metal wear.
Temperatures inside the combustion chamber ensure efficient vaporisation of the fuel, and more complete burning leads to reduced emissions, or in the words of an American friend, "more bang for your buck". So, some heat is desirable but too much is not.
Let's take a closer look at how it all works.
Since we are looking at the cooling system, lets start with the bit that does the actual cooling of the liquid coolant, the radiator.
Commonly, nowadays, made of aluminium (but not always), it consists of a header tank and a bottom tank. The header tank has an inlet port through which passes the hot coolant from the engine. The coolant now passes through a series of flattened tubes to the bottom tank. While passing through the tubes the coolant is cooled by a through-flow of air passing over the surface of the tubes. The flow of air is generated by the fan and this is usually driven by a belt from the engine crankshaft, the same belt (usually) powers the water pump.
In the bottom tank is an outlet port, from where the coolant is forced back into the engine via the water pump to be heated by the engine, where it passes through the thermostat and begins its journey back to the radiator. A nice simple and straightforward cycle, not all cooling systems are this simple but let's stick with this one for now.
Some of the problems you can encounter in a cooling system that cause overheating are:
• Blockages inside the radiator
• Blockages outside the radiator
• The fan and/or pump not turning
• Leaking coolant
• Faulty thermostat
Let's look at these problems one at a time starting with internal blockages.
Blockages inside the radiator.
The engine should be run on a mixture of 50% Ethylene Glycol (antifreeze) and 50% water in most cases. The antifreeze contains inhibitors that help prevent internal rusting of the engine water jacket, rust particles will circulate in the coolant and settle in the bottom tank, when the engine is switched off, to accumulate as solid sediment and, eventually, block off the tubes in extreme cases, restricting or preventing the flow of coolant in the system.
Using water only will result in salts forming in the radiator tubes through electrolysis; these will thicken the walls of the tubes and make them less efficient and restrict the flow of coolant so it enters the bottom tank far too hot. Another double whammy.
Blockages outside the radiator.
These can have a variety of causes and a common one is airborne debris, especially on rough cutters in dry conditions. Rear mounted fans "pull" air through a screen at the rear of the engine covers, airborne grass chaff will be sucked onto the screen preventing sufficient flow through the radiator fins. Finer airborne particles will be sucked through the screen and, over time, will accumulate around the tubes and fins, again cutting down on air flow.
Introduce an operator to machinery and anything can happen, but let's stick to overheating problems. He comes with his bag containing flask, wet weather clothing etc. and puts it behind the seat, not realising (perhaps) that he is preventing air being sucked through the radiator. Sometimes this type of blockage can be more accidental than the above but plastic rubbish bags are often stored there to be used on the operators rounds, since these are light and flimsy they are easily sucked up against the radiator or screen and very effectively block off air flow.
Leaks on the outside can have a variety of causes, and I won't labour the point with long explanations.
Rubbing and chafing are common causes of hose failure; also oil contamination can weaken the rubber compound causing it to burst under pressure. Regular inspection of hoses is essential if failures are to be prevented. Things to look for are the above rubbing and chafing, signs of perishing and cracking, swelling behind the hose clips and hose clips in poor condition.
Core plugs (occasionally) pop out or rust from the inside (if water has been used instead of antifreeze mix).
Fan and/or water pump not turning
Keeping with our "simple" system, we will assume that the pump and fan are driven by the same belt (but this is not always the case). So, it necessarily follows that a belt failure will cause overheating because there is no flow of air through the radiator fins and the pump is not circulating water through the engine, another double whammy.
Pump bearings fail - nothing lasts forever - and, usually, coolant will leak past the seal.
Alternator bearings fail and seize and a belt could burn out.
There is no secret to preventing these things from happening; thoroughly inspecting and servicing is the key to preventative maintenance.
Blockages inside the radiator - can be prevented by using the correct antifreeze mixture.
Coolant additives that temporarily repair leaks will settle in the bottom tank and premature blocking will occur, so leaks must be addressed by a specialist repairer of radiators and these additives should be viewed as a "get you out of jail" measure.
Blockages outside the radiator - radiator fins must be blown out with compressed air and a long nozzle air gun before starting a work period. Initially, blow chaff out the opposite way to which it was sucked in then vice-versa, get right up in the corners of the radiator and alternate from one side of the radiator to the other until no more dust, pollen or chaff comes out. Start the engine and run at full throttle, wait for dust to clear, then start your cleaning of the radiator again, you will be surprised at how much more you will blow out.
Do NOT use water to clean the fins, it is much less effective and most of the debris will be left behind, this will then dry out, solidify around the fins and be impossible to remove without taking out the radiator and (carefully) cleaning with a fan spray on a pressure washer.
Instruct operators not to place bags and/or clothing in front of radiators. This also applies to other equipment or materials that may prevent air flow.
Leaks can be prevented with a thorough inspection of hoses and hose clips. Radiators occasionally get holed accidentally, and there is no guard against this, but radiator condition should be included in the inspection routine as should the core plugs (where possible).
Look for staining by antifreeze on the engine block and radiator that may lead to a more serious leak.
Water pumps often have a tell tale hole drilled in the bottom of the seal housing - look for staining at this point, as it could be that a seal or bearing failure is imminent.
Fan and/or water pump not turning - check condition of the fan belt all the way round, remove if necessary and replace if in doubt, it could save time and money later.
The alternator bearings may fail without warning. Again there is no guard against this but, if it should happen and the belt survives, you must replace it when changing the alternator, the heat will have made the belt very brittle and it will fail soon, very soon.
The Thermostat - there is no inspection routine for the thermostat, it either works or it doesn't. However, if the engine should overheat shortly after start up and there is sufficient coolant in the system, then take out the thermostat, the chances are it will be closed after removal because of the time it takes.
Place the thermostat in a container and pour hot water over it - it should open! Leave it until the water cools and it should close again. This is a fairly cheap part so, if you are in any doubt about its operation, fit a new one.
Drain the system every two years and fill with fresh coolant mixture.
Each October I have the antifreeze checked and adjusted for strength against freezing using a hydrometer. If you have machinery parked outside during the winter (as we do) then please don't think you have "got away with it" because it didn't freeze overnight. Such is the cooling efficiency of the system that the radiator could freeze while it is being used. Remember that cold air flows across the radiator fins and tubes and may well freeze a weak mixture in the radiator and very effectively block it causing the engine to overheat.
Thoroughly inspecting the condition of the cooling system could save you a lot of downtime and much money, it is time well invested.