Cool it! cooling system problems explained

September 18, 2008 by Jon Rhodig

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Its something that almost every ATV or motorcycle enthusiast will encounter at some point in their lives with any particular machine, overheating. Its a condition that can be intermittent or constant depending on the conditions. Manufactures go to great lengths to design a system around the constraints of packaging, manufacturing, reliability and performance and any change in the parameters that the system was designed to run in will most likely cause problems or put strain on the system. I took this from a question I was asked in the ATV forum and expanded on it.

Basically there are three types of cooling systems air, air & oil and liquid, both with distinct differences and advantages. I will break each down explaining the disadvantages and advantages of each system.

Air cooled engine

Air cooling is the oldest type of cooling method, the earliest of combustion engines used this type of system. Air cooling has the distinct advantage of being easy to design and manufacture as well as being a self contained system with no moving parts to fail or wear out. Air cooling uses fins cast to the engines cylinder, head and sometimes block. As the engine moves through the air (or even when its stationary) the fins transfer the heat from the engine to the air. Properties of thermal dynamics allow the engine to cool even while stationary because of air temperature differentials (ie. hot air rises). Biggest downfall the the air cooled engine is its very inefficient in transferring the heat. A lot of factors go into this but the main ones are consistent air flow and the base materials ability to store and transfer heat. Engineers are restricted by these two disadvantages in many ways and its mostly in the performance area as engine temps cannot be evenly regulated. Air cooling is best suited for low performance applications and where weight and simplicity is also a factor.

Air & Oil basically similar to the air cooled only the engineers have added external oil lines to a separate cooler (radiator) that the engines oil circulates through transferring its heat to the air. Main advantage here is the oil gets more direct cooling thus improving its stability at operating temperatures, thus the cooled oil then in turn cools the bearing surfaces. Biggest disadvantage is having external oil lines and an auxiliary cooler is it can be prone to leaking and damage from trail or track debris. Also oil coolers generally require a high volume pump causing parasitic drag on the engine also during oil changes its difficult to flush them out and in rare cases they can be plugged by sludge and other debris suspended in the oil. Overall this system alows engines to produce slightly more power and reliability over straight air cooling.

Liquid cooled engine

Liquid cooling is the most complex and now a days its the most common for any type of engine. Liquid cooled engines have several parts that comprise the system. A circulation pump (known as the water pump), coolant passages in the block and head (known as water jackets), hoses, and of course the radiator. Liquid cooling adds complexity to the design but it also frees up engineers because they parts are modular and can be placed in more convenient places. Also liquid cooling allows for higher performance due to the fact that engine temp can be regulated within a small degree range via thermostats, cooling fans, or the shear efficiency of the radiator itself. Biggest disadvantage is the complexity and cost of manufacture. Also because it requires more parts than an air or air/oil cooled engine the overall weight of the engine package is a lot more. However with proper maintenance liquid cooling will provide reliability and performance where as an air cooled engine you trade one for the other.

Ok so we have established that liquid cooling is probably the best and most common to ATVs now lets look at common problems and solutions regarding cooling systems.

First off with air and air/oil cooled engines there are only two reasons for them to overheat. Poor airflow around the engine and or debris (usually mud) packed into the spaces between the fins. The fix is easy, keep the engine clean and and do not remove fenders or other items designed to channel airflow over the engine. There are other causes for over heating such as jetting and ignition timing and even improper fuel but assuming all that is correct the aforementioned are the main causes.

Now onto liquid cooled engines.

Poor airflow over the radiator: clean the fins and any area in front of the radiator, straighten any bent fins with a radiator comb.

Radiator fin straighener

Poor coolant condition: coolant should be changed every 1-2 years on motorcycles and ATVs its possible that the coolant has lost its ability to transfer heat efficiently or the incorrect type of coolant or tap water was/is being used. I prefer Engine Ice or an 50/50 extended life antifreeze (designed for aluminum engines and radiators) and distilled water mixed with Redline’s(tm) water wetter. The solution would be to do a flush with a radiator flush agent and refill with the correct coolant and water mix. Always use distilled water.

Poorly functioning radiator cap: Believe it or not your radiator cap does need replacing every so often, you can take it in and have it tested to make sure it opens at the correct pressure but its usually just easier to replace it.

Air in the system: If air is trapped in the system (improper servicing of the coolant) Try finding a bleed screw on the cylinder somewhere and unscrewing it slightly to allow coolant to drain and bleed the air out or take the machine to a dealer that can properly pressure (with a vacuum tool) the system and fill it.

Collapsing hoses: Hoses have a tendency over time to get weak, deteriorate inside and can collapse under normal operation. This is hard to diagnose as the hoses could look ok on the outside but be totally corroded inside. Remove and inspect them and if there is any doubt replace them.

Thermostatic fan switch and or thermostat: If it’s equipped you will need to consult your service manual for proper procedure on testing to make sure the fan is functioning properly and that the temp sensor is good.

Head gasket leak: this will usually be shown by white smoke out the tail pipe or bubbles in the coolant when idling with the rad cap off. This is a pretty involved fix and requires a partial disassemble of the motor.

Boysen Super Cooler, available for many models can improve coolant flow thus increasing the efficiency of the system

Broken water pump impeller vanes or broken impeller drive gear: Water pump impellers are made of cheap cast aluminum or pot metal and are usually driven off the primary gear with a plastic bevel gear. They are generally very reliable but they do wear out and I have seen broken vanes from debris floating in the cooling system. You can check the condition by removing the water pump cover and visually inspecting the vanes and by lightly trying to turn the pump you can check the drive gear, if it does not move or has a small amount of play (backlash) it is ok, if it spins the teeth have stripped on the gear. While your at it check the water pump seal for its condition also, this is the main entry point for water in the oil and vice averse.

As you can see there are several unique problems with liquid cooling, most having to do with general maintenance of the system. Usually systems are neglected and several relatively small oversights can compound to make big problems. I make it a rule of thumb to service/check my cooling system at least once a season. Keeping the system in check will insure that you will always have a properly performing machine that will not leave you stranded.