Cooling System Trouble Diagnosis (Automobile)

12.8.

Cooling System Trouble Diagnosis

Most cooling system problems fall into three general areas such as overcooling, overheat­ing and noise. The fourth problem, another potential problem, is internal engine overheating. This is visualised as burned valves, and/or scuffed pistons and/or rings, caused by overheating of the internal parts.
12.8.1.

Internal Engine Overheating

This problem is usually caused by rust, scales, or corrosion formation on the water jacket side of the cylinder wall. Rust, dirt, or corrosion particles can deposit in the water jackets. This can act like thermal insulators and slow down the heat flow so that an overheating of the combustion chamber, pistons, rings, and/or valves may take place while the coolant is at normal temperatures. However, usually rust and scale particles break loose and travel with the coolant until they are caught in the radiator and plug the tubes. This causes the coolant as well as the engine to overheat. This type of problem results in a premature need for a valve job, and/or complete overhaul. Also excessive pre-ignition and detonation may take place.
12.8.2.

Overcooling

This problem generally appears in the winter because the heater does not work. Overcooling in a car using a liquid heated choke may show up as a carburettor choke that does not come off. In a modern, computer-controlled vehicle it may show up as an engine control system that does not go into normal operating mode. Sometimes, an observant driver may notice a delayed or an excessively long warm-up, generally caused by a thermostat which is stuck open or missing. There are at least two procedures that can be followed to determine the exact cause of the problem.
(i) A diagnostic chart such as Table 12.2 can be referred. The sequence of checks indicated in the chart follows a logical pattern, assuming that the problem is caused by low coolant level, faulty thermostat, and poor coolant flow through the heater, or faulty heater controls.
(ii) A checklist of probable causes such as Table 12.3 can be followed. These checklists include the most probable causes of a particular problem.
A thermostat which is stuck open or one which fits loosely in its housing allows the coolant to circulate while the engine is cold. The thermostat is usually located inside the upper coolant outlet of the engine. The thermostat is removed, tested for its faults and then replaced.
Table 12.2. Overcooling Diagnosis Chart.

If a heater does not put out hot air while the coolant is hot, it could be caused by poor circulation through the heater core. It should be checked whether the engine is warmed up, by hand feeling the thermostat housing and/or the upper radiator hose or by placing a thermometer into the coolant at the filler neck. Sometimes, poor heater operation takes place due to an air lock in the core which reduces coolant-to-tube contact. Normally, this occurs when the core is placed above the filler cap or the heater hoses go up and down several times. Air locks are usually removed by operating the engine at high idle with the heater flow valve wide open. Severe air locks are removed by disconnecting the heater hoses and running water through the core and/or hoses to bleed the air out.
An overcooled engine can suffer from the following :
(a) Increased cylinder wear.
(6) Dilution of oil due to poor vaporization of petrol.
(c) Greater formation of sludge.
(d) Oil does not thin out properly and fluid friction losses are increased.
(e) Engine does not achieve full power.
(f) Burnt gases, which leak past piston, condense in the crankcase to form corrosive acids in oil.
(g) Lower thermal efficiency i.e. more consumption of fuel.
Table 12.3. Most Probable Causes of Overcooling.

12.8.3.

Overheating

There are several factors that are responsible for overheating. For easy presentation and troubleshooting, they are put under three groups.
(a) Overheating caused by coolant loss.
(b) Overheating without coolant loss.
(c) Overheating caused by factors outside of the cooling system.
The most common causes of overheating are illustrated in Table 12.4. Occasionally it is difficult to determine whether coolant loss caused the overheating or overheating caused boil-over and the coolant loss. Since coolant loss checks are fairly easy to make, this is carried out first as a routine matter. Several manufacturers provide flow-type diagnostic charts such as Table 12.5 for overheating.
The effects of overheating of an engine are as follows :
(a) Detoriation and/or pre-ignition takes place. (6) Possibility of seizure of piston.
(c) More rapid coolant evaporation or in an extreme case, water boils.
(d) Oil may become very thin, possibly causing heavy oil consumption and leakage.
(e) Oil film may be burnt away.
(/) Density of charge entering cylinder may become less if ingoing charge receives too much heat.
Table 12.4. The Most Common Causes of Overheating.

Overheating Due to Coolant Loss.

There are two types of coolant leaks : (i) external, where the coolant escapes to out side of the engine and (ii) internal, where the coolant escapes to inside (combustion chamber or crankcase) of the engine. Internal leaks are often evidenced by water droplets in the oil or discoloured oil. A simple check for this is to touch the dipstick to a hot exhaust manifold. Oil spreads out and produces smoke on a hot surface whereas water sizzles and dances. A greyish, sick-looking mixture of emulsified oil and water is definite indication of an internal leak. An internal leak accompanied by foaming of the coolant is often a sign of a combustion chamber leak.
Many external leaks can be easily located through a visual inspection. Spots with brownish rust or whitish streaks often provide indication of leaks. The following locations are checked for external leaks.
(a) All hose connections.
(b) Radiator seams, core, and drain.
(c) Water pump, thermostat housing, intake manifold, and head gasket edges.
(d) Water pumping shaft seal at weep hole.
(e) Block drain and core plugs.
(/) Heater core, usually evidenced by a wet carpet of foggy mist during defroster operation.
ig) Temperature gauge/sending unit. If the coolant leak is not easily found, then a pressure tester is used to locate the leaks.
Table 12.5. Overheating Diagnosis Chart.

Overheating Without Coolant Loss.
If the system is full of coolant, overheating is usually the result of the following :
(a) Poor coolant circulation through the radiator.
(b) Poor air flow through the radiator.
(c) Poor coolant circulation through the water jackets.
Most overheating problems take place due to a faulty radiator. Normally, the radiator has enough cooling capacity reserve to perform its job. This reserve can be lost through damage or neglect until overheating is indicated. The process of diagnosing this problem becomes easier once the nature of the problem is known.

Poor Coolant Circulation through Radiator.

Radiator flow is difficult to find out. It can be approximately measured on the vehicle with a cooling system analyser or measured off the vehicle using a radiator flow tester. Normally the recommended flow for the specific radiator is not known. Therefore a flow test is necessary to determine if the radiator needs cleaning. This is judged based on experience. A better indication of core plugging can be obtained by removing the radiator and then connecting a water line into the lower outlet while watching the flow of water from the tubes through the filler neck. A reasonable flow of water should be seen coming from all of the tubes. Hand feeling for temperature changes in the core while the engine is running also gives an indication to certain extent of core plugging.

Poor Air Circulation Through Radiator.

The only air flow checks possible in the average shop are to feel the air blast coming from the fan by hand or to place a shop rag or paper in front of the radiator to see how firmly the air flow causes it to cling in place. With experience, these checks become fairly reliable. The air flow through the radiator is generally reduced Ly debris blocking the core, a defective fan and/or fan clutch, missing air seal (radiator support to hood), and/or a defective or missing fan shroud. A defective fan and/or shroud normally causes overheating at slow speeds or prolonged idle periods, while at higher speed operation of the vehicle it is normal.
The radiator is also checked for fin blockage and bent fins which might interrupt the air flow. Modern radiators have very thin tubes. A radiator with severely damaged fins should be replaced or have a new core installed. Also fins are to be checked for any loose section. A faulty fan clutch or belt should be replaced.

Poor Coolant Circulation Through Engine.

A faulty thermostat or water pump is generally responsible for this problem. It can also be caused by a collapsing lower radiator hose. Coolant circulation can be checked by several ways. A cooling system analyser or a flow indicator can be installed between the thermostat outlet and the radiator inlet to measure coolant flow rate. In the other method the upper hose is squeezed and/or the radiator cap is removed and the coolant flow is watched while the engine is accelerated. A noticeable flow should be felt passing through the hose or seen entering the radiator or a hot engine.
It coolant flow is verified, then the thermostat is to be removed and checked. If the engine is warmed up, the thermostat should be open when the thermostat cover is removed. A faulty water pump can also cause little or no coolant flow, and hence is checked. Lower radiator hose collapse is checked by observing the hose as the engine speed is increased. A good hose keeps its round shape and a faulty hose loosens up.
Overheating Due to Factors Outside the Cooling System.
If the cooling system is functioning satisfactorily and the engine still overheats, the problem could be caused by other factors. Some of these factors are presented in Table 12.6.
Table 12.6. Factors, Outside the Cooling System, Responsible for Overheating.

12.8.4.

Noisy Operation

The most common cooling system noises are illustrated in Table 12.7. Some of these are easy to locate, others such as surging or thumping in the water jackets are quite time consuming and expensive to resolve.
Table 12.7. The Causes for Abnormal Noise in Cooling System.