DIAGNOSING ENGINE OVERHEATING
by Reiner B.

Continued from Part 1

ENGINE OVERHEATING DIAGNOSTIC TIPS & TRICKS

It should be noted that while engine overheating cannot always be prevented or avoided, drivers are often presented with several warning signs that if observed, can limit the damage that comes with engine overheating, or even prevent a disastrous occurrence of engine overheating altogether.

However, since the causes of engine overheating are not always immediately apparent, there are several diagnostic checks most car owners can perform themselves to diagnose and identify issues/symptoms that can develop into serious engine overheating and even catastrophic engine failure if these warning signs are ignored or not attended to in a timely manner.

To help you interpret the warning signs of a potentially fatal episode of engine overheating, use the following questions (and the answers to them), to help you avoid a misdiagnosis, starting with-

Is it necessary to top off the coolant level regularly?

Since there is no such thing as a perfectly sealed engine cooling system, some coolant loss is to be expected. Typical sites of coolant loss is past radiator caps that have lost some of their spring tension, expansion tank caps that have less than perfect seals (or have not been tightened properly), seepage past hose clamps, and pinhole leaks in radiators, heater valves/heat exchangers, and the mechanical seals in water pump.

On vehicles that are in a good to reasonable state of repair, these coolant losses should not amount to much, and topping off the coolant level with a few ounces of coolant every several weeks or so should not be an issue.

However, if it becomes necessary to top off the coolant level every day, or even every few days, there is a serious leak present that MUST be found and repaired without delay to prevent the possibility of catastrophic engine failure due to fatal overheating.

Are there no visible fluid leaks?

On some applications, it can be very difficult to spot coolant leaks, but one way of making it easier is to add a few ounces of brightly colored food coloring to the cooling system when the engine is cold.

The next time the cooling system is hot and pressurized, the food coloring will leave a stain at and around the site of the leak, which makes it easier to find the exact site of the leak.

No fluid leaks, but the heat gauge shows an abnormal reading?

Note that an engine can overheat fatally even if the temperature gauge shows a below normal reading. Here is how to find out what is going on if the coolant boils out, but the gauge shows a normal, or even a below-normal reading-

WARNING: Use extreme caution during this step to avoid sustaining burns and scalds, and REMOVE ALL JEWELLERY AND LOOSE CLOTHING THAT CAN BE CAUGHT IN DRIVE BELTS, PULLEYS, AND OTHER MOVING PARTS.

• Let the engine cool down- preferably overnight. Check that the coolant level is up to its proper mark, start the engine, and allow it to idle. Locate both the radiator hose that enters the top tank on the radiator, and the bottom hose that channels cool water back to the engine. While monitoring the heat gauge, periodically touch both hoses, but bear in mind that the top hose will feel hotter to the touch sooner than the bottom hose will.
• The purpose of this exercise is to determine if the thermostat is working or not. Since the thermostat is a valve that regulates the flow of coolant, the bottom radiator hose should slowly heat up as the engine heats up. In practice, heat is transferred to the thermostat by convection, and if the thermostat works as intended, it will open when the coolant that is in contact with it reaches a preset temperature.
• At this point, you will feel a sudden increase in the temperature of the bottom radiator hose, which should be nearly as hot as the top hose is when the thermostat opens, which allows coolant to circulate through the engine.

WARNING: It is critically important to monitor the temperature gauge during this step, since the coolant temperature switch monitors the temperature of the coolant in the cylinder head regardless of whether the thermostat opens or not. Thus, if the bottom radiator hose does not heat up or reach the same temperature as the top hose by the time the temperature gauge indicates a normal reading, the thermostat is stuck closed, and by definition, defective.

NOTE#2: If all the other components of the cooling system work as intended, the electric radiator fan should start within a few seconds of the two radiator hoses reaching the same temperature.

WHAT TO LOOK OUT FOR AT THIS POINT

• When the radiator fan starts, the temperature gauge should show a normal reading. However, if it shows an abnormally high temperature, the thermostat is defective in the sense that it opens either too late, or not completely.
• As soon as the radiator fan starts, it will begin cooling down the coolant that is flowing through the radiator, and it will shut off automatically when the coolant reaches the desired temperature. However, the drop in temperature should be reflected on the temperature gauge; if the gauge does not reflect a drop in temperature and the fan keeps on running, the fan is either not rotating at the correct speed, or the thermostat is not closing. The latter condition does not allow the coolant enough residual time in the radiator to cool down, and the engine could suffer fatal overheating even if the coolant circulates freely through the engine via a fully open thermostat.
• Note that if the thermostat does not close, it is possible to see an abnormally low coolant temperature at low road and engine speeds because the coolant is circulating slowly, and has sufficient time to cool down in the radiator. However, as the engine speed rises, the water pump spins faster, causing the coolant to circulate at a rate that does not allow sufficient time for the coolant to shed heat effectively. In this condition, it is almost certain that the engine will seriously overheat at some point.

NOTE #1: Coolant circulation issues always require replacement of the suspect thermostat, since thermostats are NOT repairable.

NOTE#2: While it is possible to replace the thermostat on many applications on a DIY basis, it should be noted that the procedure almost always requires removal or disassembly of other, unrelated engine components. If you have doubts about your ability to perform the replacement, the better option is to refer the vehicle to a competent repair facility for professional assistance, since it is easy to make mistakes if you are not skilled at this level of car repair.

Does the engine run rough upon start up, and then smoothes out after a few seconds?

If this happens, it is almost certain that coolant is being pushed through leak paths in the cylinder head gasket(s) or cracks in the cylinder head when the cooling system is hot and pressurized. If the leak is small, the coolant is partially burned and/or expelled through the valves when the engine is running. If however, the leak is bigger, the residual pressure in the cooling system forces coolant into the cylinders when the engine is turned off.

Thus, when the engine is started again, the coolant in the cylinders fouls the spark plugs on gasoline engines, or inhibits the combustion process on diesel engines, with the result that the engine runs rough until all the coolant is expelled from the cylinders.

Note though that in severe cases, so much coolant can collect in the cylinders that the engine can be prevented from turning over, and in some cases, connecting rods, and even crankshafts can bend or break if the engine is forced to turn over.

This type of problem is common when cylinder head gaskets blow for whatever reason, and the only reliable, long-term remedy is removal of the cylinder head(s) and replacement of the cylinder head gasket(s). Note however that this procedure is best left to professionals who have the skills and knowledge to both diagnose and recognize the root cause(s) of the blown head gasket(s), as well as additional damage to other engine parts and components that may have been caused by the initial overheating that had caused the gasket(s) to blow.

Partial head gasket failure is a major cause of engine overheating since combustion pressure is allowed to enter the cooling system, which can force the coolant out of the cooling system either past the radiator cap seal, or through a burst radiator hose.

NOTE: The surest way to detect leaking cylinder head gaskets is to perform a chemical test (or to have it performed by a competent technician) in which a reagent is brought into contact with the coolant while it is circulating through the engine. If exhaust gasses are present in the coolant, the reagent will change color, usually from blue to yellow. Unlike other tests, such as leak down and pressure tests, this chemical test can detect minute leaks in head gaskets that other tests cannot.

HOW TO RECOGNIZE A BLOWN CYLINDER GASKET

There are several ways to detect a leaking cylinder head gasket. Here is how-

• Even if the engine is not misfiring, a white, milky residue under the oil filler cap is a sure sign of a blown head gasket. Note that the absence of this residue ( which is an emulsion of engine oil and coolant) in the oil pan or elsewhere in the engine is not necessarily an absence of evidence of a blown gasket, since the engine oil and coolant do not always mix when head gaskets blow
• If two adjacent cylinders have lost an equal amount of compression, the gasket is leaking between those cylinders, or the cylinder head is cracked between those cylinders.
• Note however that gaskets do not always blow between two adjacent cylinders. In some cases, the leak path opens up between a cylinder and the coolant jacket around the cylinders, or between a cylinder and an oil passage. In the latter case, the engine may exhibit smoking or a hissing sound when the dipstick or oil filler cap is removed while the engine is running.
• Excessive pressure in the cooling system that causes the coolant to boil, froth, or bubble is a sure sign that combustion pressure is leaking into the cooling system. If this condition has not already caused the engine to overheat, it will do so very soon

Does the electric radiator fan work?

It may be that the electric cooling fan does work, but it may not rotate at the speed required to cool down the engine coolant sufficiently. In these cases, the airflow through the radiator that results from normal, high speed driving may be enough to keep the engine cool, but under slow driving conditions, both the fan’s rotational speed, and the amount of time the fan is spinning is critical for efficient engine cooling.

If you have access to a code reader, use it to measure the fan’s high and low speeds, as well as the temperature of the coolant when the fan activates. Also, scan the OBD system for codes that relate to the fan’s operation, and resolve these codes in the order in which they were stored. OBD II codes that may be present will relate to the fan motors’ speed, as well as codes that relate to the operation of the fan switch, which has the purpose of activating and deactivating the electric fan based on the temperature of the coolant.

Is the viscous fan serviceable?

Viscous fans use the shear strength of a fluid to transfer power from the hub to the fan itself, much like how a torque converter uses the shear strength of transmission fluid to transfer power from the engine to the transmission.

If over time, the fluid leak has leaked out of the viscous clutch the fan may no longer be rotating fast enough to create an adequate airflow through the radiator core. One way to test such a clutch is to see if it rotates freely if it is spun by hand. If it does rotate freely, it is defective and must be replaced. If the clutch is serviceable, it must only rotate after a considerable resistance is overcome, but in no circumstances must the fan rotate freely- not even for a partial revolution.

Replacing a viscous fan is generally a relatively straightforward affair, and most DIY mechanics should be able to perform the replacement without encountering undue difficulties.

Is the engine making strange noises?

Most causes of engine overheating are silent, in the sense that they don’t generate sounds or noises that could alert the driver to a problem with the cooling system. In fact, hissing, whistling, or whooshing noises almost always appear when the engine has already fatally overheated. These noises are the sounds of superheated steam escaping under high pressure.

Nonetheless, there is one exception to this rule, and it involves the condition of the water pump. While water pumps generally have reasonably long service lives, their support bearings start to wear out when coolant starts to leak past the mechanical seals that contain the pressurized coolant.

The first sign of a worn water pump is usually a rumbling noise that is caused by coolant in the bearings that washes out the bearings’ lubricant) that increases in intensity as the engine speed rises. Note that this is not always accompanied by coolant leaks when the engine is running. In the early stages of water pump failure, some coolant may sometimes be seen dripping from the pump only after the engine is switched off, and while the cooling system is pressurized.

If a noisy water pump is not replaced in a timely manner, the bearings can fail catastrophically at any time, which usually causes the pump internals to disintegrate, but with the added bad news that the radiator and other components can be destroyed by the drive belt when t is flung off its pulleys at high engine speeds.

OTHER CAUSES OF ENGINE OVERHEATING

It is perhaps worth noting that engine overheating can occur through causes that do not directly involve faults and defects in the cooling system. These causes could include the following-

Blocked/restricted exhaust systems

A restricted exhaust system prevents the efficient extraction of exhaust gasses, which causes excessive backpressure to develop, which in turn, can raise cylinder head temperatures on some applications. Note though that although this type of problem will usually set dedicated fault codes on OBD II equipped applications. OBD I equipped and earlier vehicles, on the other hand, do not have these “early warning systems”, and may experience serious engine overheating issues due to exhaust system issues.

Incorrectly set ignition timing

This mainly apples to older applications that have adjustable ignition timing; over advanced ignition timing can raise combustion temperatures to the point where the cooling system cannot cope with the added heat.

Driving in the wrong gear

This applies to both new and old vehicles, and particularly vehicles that have manual transmissions. Driving while in an appropriate gear, such as driving in top gear when towing uphill, lowers the engine speed, which in turn, lowers the rotation speed of the water pump. Under these conditions, the use of large throttle openings in an effort to maintain speed raises the combustion temperature, which cannot be effectively shed by the low rate at which the coolant circulates.