The internal combus-tion engine is a heatengine. It has to makeheat to make power. Ifit makes and retainstoo much heat, howev-

er, performance declines rapidly, andthe engine can destroy itself. That’swhy it needs a cooling system.

On the other hand, if the enginedoesn’t make and retain enoughheat—runs too cold—performance,economy and emissions control alsosuffer. In this case, the cooling systemkeeps the engine from running toocool. For optimum engine operation,the cooling system must regulate en-gine temperature within a preciseand narrow range—not too hot andnot too cold. In this installment ofMastering the Basics, we’ll look atfundamental cooling system opera-tion, as well as ways to identify somecommon problems.

Get It Really Hot!When we talk about engine operatingtemperature, we’re referring to aver-age coolant temperature. For mostlate-model engines, this is in therange of 230° to 250°F. Thermostatsare engineered to open in the rangeof 180° to 200°F. Heat transfers tothe coolant as it circulates throughthe engine and the radiator then

maintains the 230° to 250°F average.We learned in grammar school,

however, that water boils at 212°F atatmospheric pressure, at sea level. Ifwe put the water in a sealed containerand increase air pressure to 15 psiabove atmospheric pressure, we canraise the boiling point to about 250°F.Then, if we trade the plain water for amixture of half water and half ethyl-ene glycol and put it in the same con-tainer with 15 psi of air pressure, weraise the boiling point even higher—toabout 265°F. (We’ll use the nominalvalue of 15 psi, but engineers designfor pressures from about 10 to 20 psi.)That’s 53°F above the boiling point ofplain water at atmospheric pressure.It’s also above the required tempera-ture range of a modern engine.

Mixing water with ethylene glycoland pressurizing the cooling systemensure that the coolant remains a liq-uid at temperatures well above theboiling point of water. This brief ex-planation of engine temperature andcoolant boiling points illustrates theimportance of the cooling system as atemperature control system.

The cooling system controls tem-perature through heat transfer. Heatalways moves from a hot object to acooler object, so the cooling systemtransfers heat from the engine to thecirculating coolant. The coolant then

transfers heat to the airflow throughthe radiator. Temperature control andheat transfer are based on systempressure and coolant circulation.

A Closed-Loop SystemTo think of the cooling system as justa pressurized container for coolant ismisleading. In a simple container,pressure is equal in all directions.Cooling system pressure, however, isaverage pressure, usually measured atthe radiator return tank. Actual pres-sures vary throughout the system be-cause without a pressure differential,there could be no coolant circulation.Coolant circulation is vital to heattransfer and temperature control.

Pressures are highest at the waterpump outlet and in the water jacketsnear the combustion chambers. Systempressure is lowest at the water pumpinlet. Pressure is generated by the wa-ter pump and controlled by variable re-strictions, like the thermostat, theheater control valve and valves in theradiator cap. The system also has fixedrestrictions, such as orifices in the wa-ter jackets and passages in the radiator.

Coolant Expansion& ContractionLiquids cannot be compressed, butthe volume of any liquid will increaseand decrease slightly as it’s heated

BY KEN LAYNE

COOLING SYSTEM BASICS

63August 2003

and cooled. This also is a key factor incooling system operation. The waterpump creates system pressure to cir-culate the coolant. Coolant expansiondue to heat raises pressure through-out the system and helps to maintainit above atmospheric pressure.

If the system were completelysealed and airtight, the expandingcoolant would soon rupture the sys-tem’s weakest point—usually a hoseor radiator seam. Fortunately, the ex-panding coolant has somewhere to

go—the overflow, or expansion, tank.A pressure relief valve in the radia-

tor cap opens at the designed pres-sure (15 psi in our example) and letscoolant flow through a hose to theoverflow tank. When the coolant coolsafter the engine is shut down, it con-tracts in volume and creates a slightvacuum in the system. A vacuum re-lief valve in the radiator cap thenopens to let coolant flow from theoverflow tank back to the radiator.

Many late-model vehicles have

coolant recovery tanks that are morethan overflow reservoirs. They’repressurized to full system pressurewhen hot. You can recognize thesepressurized coolant reservoirs by thelabels and warnings on their caps. Thecap is not a simple plug or a twist-offcap. Additionally, the radiator in sucha system does not have a pressure cap.

Start Troubleshooting:Circulation CheckCooling system service begins with aninspection of all components fordamage and leakage. And don’t over-look the water pump drivebelt. Thewater pump creates pressure thatcauses coolant to circulate. Unwantedrestrictions in the radiator, the engineor hoses can block circulation, how-ever. The basic way to check circula-tion is to look for cooler spots in thesystem.

Run the engine until it’s fully warmand the thermostat is open (or shouldbe open). Then shut off the engineand run your hand over the radiatorcore from inlet tank to outlet tank tofeel for cool spots. Do the same withall the hoses. The radiator and hosesshould be uniformly warm. Any spotscooler than others indicate restric-tions, or blockage, in the system.

This old-time method still workspretty reliably today, but cooling sys-tem components often aren’t com-

64 August 2003

Heat always moves from a hot area to a cooler one. Therefore, heat movesfrom the hot engine parts to the colder coolant and from the now heatedcoolant to the airflow through the radiator.

Hold the infrared analyzer probe about an inch above thecoolant in the radiator filler to sniff for hydrocarbons,which would indicate a blown head gasket.

You can identify a pressurized coolant reservoir by thewarning on the cap. Use the same caution when removingit as you would with a radiator cap.

Heat TransferFrom CombustionChamber toColder Coolant

AirflowThroughRadiator

Drive Belt

RadiatorCooled Coolant

Fan

Thermostat

WaterPump

Pressure Cap

CombustionChamber

WaterJacket

Hot Coolant Heat TransferFrom Coolantto Airflow

Ph

otos

: Ken

Lay

ne

pletely accessible on late-model vehi-cles. A more precise way to check foruneven temperature is with a non-contact optical pyrometer, whichlooks like a small pistol with a laseraiming beam. Aim the laser dot at thearea where you want to check thetemperature and get an exact readingon the instrument display.

System Pressure ChecksSmall leaks or internal leaks may notshow up under simple inspection, butthey will show up under pressure.Such tests also verify the ability of thesystem to maintain the specified pres-sure, which is critical to temperaturecontrol. Pressure-test equipmentconsists of a small air pump with agauge, a short hose and adapters to fitvarious radiator filler necks and caps.

Pressure loss may occur through aradiator cap that doesn’t maintain aproper seal, so always include the capin your test sequence. Remove thecap from the radiator and attach it toa test adapter. Dampen the cap gas-kets to help them seal. Slowly operatethe pump until the reading on thegauge stops increasing. This is thepressure at which the cap relief valveis opening. Repeat the test two orthree times to verify the results. Re-

place the cap if the relief valve opensbelow the minimum specified pres-sure or 2 to 4 psi above the maximumpressure.

Next, move your pressure testequipment to the radiator. With theengine cold, add water, if necessary,to bring the coolant level to 3⁄4 inchbelow the neck of a downflow radia-tor or 21⁄2 inches below the neck of acrossflow radiator. Attach the pres-sure tester and operate the pump un-til the gauge indicates the specified

system pressure. The gauge readingshould hold steady for at least twominutes, preferably longer. If pres-sure drops, pump the tester to main-tain pressure and inspect for leaks.

Leaks aren’t always obvious. A pin-hole leak may require higher pres-sure before it shows itself. You can lo-cate most leaks with a test pressure ofabout 15 psi, which is safe for mostsystems.

If a leak exists at a high point in thesystem where air can accumulate with

65August 2003

Aim the laser beam from the pyrometer at the point where you want to checktemperature and read the measurement on the instrument display.

The crud on this radiator cap is a stop-leak compound gonewrong. Maybe it plugged the cooling system leak because itsure did a good job of plugging the relief valves in this cap.

Pressure testing a cooling system to about 15 psi will revealmost leaks. Overpressurizing the system may cause dam-age to the radiator or other cooling system components.

the engine off, it can bedifficult to spot. Apply asoap-and-water solution tosuspected leakage points,such as hose connections,with pressure applied tothe system.

Burp ItTrapped air is one of thebiggest problems with mod-ern cooling systems. Air orgases can enter the coolingsystem from an air leak atthe water pump inlet, at ahose connection or from acombustion chamber leak.Interestingly, air also canenter the system past aworn seal on the waterpump shaft. Because thepump inlet can operate atnegative pressure due tocentrifugal force, air can be drawn pastthe shaft seal. This invisible air inlet ishard to pinpoint, but if the process ofelimination discounts all other possibili-ties, remove the water pump and checkits shaft seal.

The most common source of air inthe cooling system is an improperdraining and filling procedure.Coolant drain and refill is not as sim-ple as opening the drains, letting theold stuff run out, pouring in newcoolant, warming the engine to openthe thermostat and then topping up.Many engines require specific air-bleed procedures, and some coolingsystems have special air-bleed valves.

Coolant refill and recycling ma-chines have made coolant replenish-ing easier and eliminated many air-bleed problems. If you don’t havesuch a machine, however, you canfind air-bleed procedures for mostcars in a good manual.

Nothing Lasts ForeverEven “permanent” antifreeze isn’tpermanent. Long-life coolant withfive-year change intervals has beenon the market for several years, andmany such systems are due for ser-vice. Basic inspection may revealcoolant that’s contaminated by rust,

oil and dirt, but even coolant thatlooks okay may have lost its effective-ness.

Specific gravity is a good measureof a coolant’s freeze protection. En-gine coolant should have a specificgravity greater than 1.0 (plain water);the higher the specific gravity, thebetter the freeze protection.

Draw a coolant sample from theradiator or overflow reservoir into acooling system hydrometer. Severalkinds of these instruments are avail-able, and they come with conversiontables that translate specific gravityreadings into freeze-protection tem-perature ratings.

After many years of service, headgasket materials can lose strength orcollapse from constant temperaturecycling. Most head gasket failures startwith localized hot spots around com-bustion chambers, however. Poorcoolant circulation, faulty system pres-sure and air in the system combine toform steam pockets in water jackets atcombustion chambers. Steam doesn’ttransfer heat, and the engine suffersfrom localized overheating near thehead gasket. Excessive heat weakensthe gasket, and combustion pressuresoon blows it out.

Customer complaints leading to

head gasket failure diagno-sis usually start with over-heating and loss of perfor-mance. If you suspecthead gasket failure, startyour troubleshooting byrunning the engine untilthe thermostat opens.Bubbles or turbulence atthe radiator filler neck in-dicate extra pressure in thecooling system, most likelyfrom a combustion cham-ber leak.

Use the pressure testmethods outlined earlierto check for a pressuredrop in the cooling system.One common leakagepoint is through a blownhead gasket. Pressure test-ing works both ways, sonext do a low-pressure air

leakdown test on suspected cylinders.An air pressure drop in a cylinder andbubbles in the radiator are prettystrong indicators of a blown gasket.

To wrap up the diagnosis, hold theprobe tip of your infrared exhaust an-alyzer an inch or so above the coolantlevel in the radiator filler neck withthe engine running. Be very carefulnot to get the probe tip into thecoolant. If hydrocarbon measure-ments exceed about 200 to 400 ppm,exhaust may be leaking into the cool-ing system. Purge air from the sys-tem, drive the car and retest. If youfind over 250 ppm on the second test,do a high-pressure leakdown test onsuspected cylinders, following all nec-essary safety precautions.

Most cooling system problems arecaused by a loss of coolant or a loss ofpressure, or both. Logical trou-bleshooting operations, based on sys-tem principles, will identify thesource of these problems.

The author would like to thank JackSparks of The DMV Clinic for the infor-mation he contributed to this article.

66 August 2003

Impeller? What impeller? A combination of iron and aluminum en-gine parts and a cooling system filled only with water destroyedthis water pump, and greatly diminished its pumping ability.

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