If the engine overheats ... - Toyota Land Cruiser Prado, 3.0 liter, 2000

How to detect overheating?

The answer seems to be obvious - look at the coolant temperature gauge. In fact, everything is much more complicated. When there is heavy traffic on the road, the driver does not immediately notice that the pointer arrow has moved far towards the red zone of the scale. However, there are a number of indirect signs, knowing which you can catch the moment of overheating and not looking at the devices.

So, if overheating occurs due to a small amount of antifreeze in the cooling system, then the heater located at the high point of the system will be the first to react to this - hot antifreeze will stop flowing there. The same will happen when antifreeze boils, because. it starts in the hottest place - in the cylinder head near the walls of the combustion chamber - and the formed vapor plugs block the passage of the coolant to the heater. As a result, the supply of hot air to the passenger compartment is stopped.

The fact that the temperature in the system has reached a critical value is most accurately indicated by a sudden detonation. Since the temperature of the walls of the combustion chamber during overheating is much higher than normal, this will certainly provoke the occurrence of abnormal combustion. As a result, an overheated engine, when you press the gas pedal, will remind you of a malfunction with a characteristic ringing knock.

Unfortunately, these signs can often go unnoticed: at elevated air temperatures, the heater is turned off, and detonation with good sound insulation of the cabin can simply not be heard. Then, with further movement of the car with an overheated engine, power will begin to drop, and a knock will appear, stronger and more uniform than during detonation. Thermal expansion of the pistons in the cylinder will lead to an increase in their pressure on the walls and a significant increase in friction forces. If this sign is not noticed by the driver, then during further operation the engine will receive serious damage, and, unfortunately, it will not be possible to do without serious repairs.
What causes overheating

Take a close look at the cooling system diagram. Almost every element of it, under certain circumstances, can become the starting point of overheating. And its root causes in most cases are: poor cooling of antifreeze in the radiator; violation of the seal of the combustion chamber; insufficient amount of coolant, as well as leaks in the system and, as a result, a decrease in excess pressure in it.

The first group, in addition to the obvious external contamination of the radiator with dust, poplar fluff, foliage, also includes malfunctions of the thermostat, sensor, electric motor or fan clutch. There is also internal contamination of the radiator, but not due to scale, as happened many years ago after long-term operation of the engine on water. The same effect, and sometimes much stronger, gives the use of various sealants for the radiator. And if the latter is really clogged with such a tool, then cleaning its thin tubes is a rather serious problem. Usually, malfunctions of this group are easily detected, and in order to get to the parking lot or service station, it is enough to replenish the liquid level in the system and turn on the heater.

Violation of the combustion chamber seal is also a fairly common cause of overheating. The products of fuel combustion, being under high pressure in the cylinder, penetrate through the leaks into the cooling jacket and displace the coolant from the walls of the combustion chamber. A hot gas "cushion" is formed, which additionally heats the wall. A similar picture occurs due to burnout of the head gasket, cracks in the head and cylinder liner, deformation of the mating plane of the head or block, most often due to previous overheating. It is possible to determine that such a leak occurs by the smell of exhaust gases in the expansion tank, the leakage of antifreeze from the tank during engine operation, the rapid increase in pressure in the cooling system immediately after starting, and also by the characteristic water-oil emulsion in the crankcase.

Obvious leaks in the cooling system most often occur due to cracks in the hoses, loosening of the clamps, wear of the pump seal, malfunction of the heater tap, radiator, and other reasons. Note that a radiator leak often appears after the tubes are "corroded" by the so-called "Tosol" of unknown origin, and the pump seal leak - after prolonged operation on the water. Determining that there is little coolant in the system is visually as simple as determining the location of the leak.

Leakage of the cooling system in its upper part, including due to a malfunction of the radiator cap valve, leads to a drop in pressure in the system to atmospheric pressure. As you know, the lower the pressure, the lower the boiling point of the liquid. If the operating temperature in the system is close to 100 degrees C, then the liquid may boil. Often, boiling in a leaky system occurs not even when the engine is running, but after it is turned off. To determine that the system is really leaky, you can by the lack of pressure in the upper radiator hose on a warm engine.
What happens when overheating

As noted above, when the engine overheats, the liquid begins to boil in the cooling jacket of the cylinder head. The resulting vapor lock (or cushion) prevents direct contact of the coolant with the metal walls. Because of this, the efficiency of their cooling decreases sharply, and the temperature rises significantly.

This phenomenon is usually local in nature - near the boiling area, the wall temperature can be noticeably higher than on the pointer (and all because the sensor is installed on the outer wall of the head). As a result, defects may appear in the block head, primarily cracks. In gasoline engines - usually between the valve seats, and in diesel engines - between the exhaust valve seat and the prechamber cover. In cast iron heads, cracks are sometimes found across the exhaust valve seat. Cracks also occur in the cooling jacket, for example, along the beds of the camshaft or along the holes of the block head bolts. Such defects are best eliminated by replacing the head, and not by welding, which cannot yet be performed with high reliability.

When overheated, even if no cracks have occurred, the block head often receives significant deformations. Since the head is pressed against the block by bolts along the edges, and its middle part overheats, the following occurs. In most modern engines, the head is made of an aluminum alloy, which expands more when heated than the steel of the mounting bolts. With high heat, the expansion of the head leads to a sharp increase in the compression forces of the gasket at the edges where the bolts are located, while the expansion of the overheated middle part of the head is not restrained by the bolts. Because of this, on the one hand, deformation (failure from the plane) of the middle part of the head occurs, and on the other hand, additional compression and deformation of the gasket by forces that significantly exceed operational ones.

Obviously, after cooling the engine in some places, especially at the edges of the cylinders, the gasket will no longer be properly clamped, which can cause a leak. With further operation of such an engine, the metal edging of the gasket, having lost thermal contact with the planes of the head and block, overheats and then burns out. This is especially true for engines with plug-in "wet" sleeves or if the jumpers between the cylinders are too narrow.

To top it all, the deformation of the head leads, as a rule, to a curvature of the axis of the camshaft beds located in its upper part. And without serious repairs, these consequences of overheating can no longer be eliminated.

Overheating is no less dangerous for the cylinder-piston group. Since the boiling of the coolant spreads gradually from the head to an increasing part of the cooling jacket, the cooling efficiency of the cylinders is also sharply reduced. And this means that heat removal from the piston heated by hot gases is deteriorating (heat is removed from it mainly through the piston rings into the cylinder wall). The temperature of the piston rises, and at the same time its thermal expansion occurs. Since the piston is aluminum and the cylinder is usually cast iron, the difference in thermal expansion of the materials leads to a decrease in the working clearance in the cylinder.

The further fate of such an engine is known - a major overhaul with block boring and replacement of pistons and rings with repair ones. The list of work on the block head is generally unpredictable. It's better not to bring the motor to this. By periodically opening the hood and checking the fluid level, you can protect yourself to some extent. Can. But not 100 percent.
If the engine still overheats

Obviously, you should immediately stop on the side of the road or at the sidewalk, turn off the engine and open the hood - this way the engine will cool faster. By the way, at this stage in such situations, all drivers do this. But then they make serious mistakes, from which we want to warn.

Under no circumstances should the radiator cap be opened. It’s not for nothing that they write “Never open hot” on traffic jams of foreign cars - never open if the radiator is hot! After all, this is so clear: with a serviceable plug valve, the cooling system is under pressure. The boiling point is located in the engine, and the plug is on the radiator or expansion tank. By opening the cork, we provoke the release of a significant amount of hot coolant - the steam will push it out, like from a cannon. At the same time, a burn of hands and face is almost inevitable - a stream of boiling water hits the hood and rebounds into the driver!

Unfortunately, out of ignorance or out of desperation, all (or almost all) drivers do this, apparently believing that they are thereby defusing the situation. In fact, by throwing out the remnants of antifreeze from the system, they create additional problems for themselves. The fact is that the liquid boiling "inside" the engine still equalizes the temperature of the parts, thereby reducing it in the most overheated places.

But some people manage to go even further. If there is water nearby, they pour it, cold, on the engine in a bucket - so that it, dear, cools down as soon as possible. The consequences are almost always the same - the head of the block will crack for sure.

Overheating of the engine is just the case when, not knowing what to do, it is better not to do anything. Ten or fifteen minutes, at least. During this time, boiling will stop, the pressure in the system will drop. And then you can start taking action.

After making sure that the upper radiator hose has lost its former elasticity (which means that there is no pressure in the system), carefully open the radiator cap. Now you can add boiled liquid.

We do it carefully and slowly, because. cold liquid, falling on the hot walls of the head jacket, causes them to cool rapidly, which can lead to the formation of cracks.

After closing the plug, we start the engine. Watching the temperature gauge, we check how the upper and lower radiator hoses heat up, whether the fan turns on after warming up and if there are any fluid leaks.

The most, perhaps, unpleasant thing is the failure of the thermostat. At the same time, if its valve "hung" in the open position, there is no trouble. It's just that the engine will warm up more slowly, since the entire flow of coolant will be directed along a large circuit, through the radiator.

If the thermostat remains closed (the pointer, slowly reaching the middle of the scale, quickly rushes to the red zone, and the radiator hoses, especially the lower one, remain cold), movement is impossible even in winter - the engine will immediately overheat again. In this case, you need to dismantle the thermostat, or at least its valve.

If a coolant leak is detected, it is desirable to eliminate it or at least reduce it to reasonable limits. Usually the radiator "flows" due to corrosion of the tubes on the fins or at the soldering points. Sometimes such pipes can be drowned out by biting them and bending the edges with pliers.

In cases where it is not possible to completely eliminate a serious malfunction in the cooling system on site, you should at least drive to the nearest service station or settlement.

If the fan is faulty, you can continue driving with the heater switched on to "maximum", which takes on a significant part of the heat load. The cabin will be "a little" hot - it does not matter. As you know, "steam does not break bones."

Worse, if the thermostat failed. We have already considered one option above. But if you can’t handle this device (don’t want to, don’t have tools, etc.), you can try another way. Start driving - but as soon as the pointer arrow approaches the red zone, turn off the engine and coast. When the speed drops, turn on the ignition (it is easy to make sure that after only 10-15 seconds the temperature will already be lower), start the engine again and repeat everything from the beginning, continuously following the arrow of the temperature gauge.

With some care and suitable road conditions (no steep climbs), you can drive tens of kilometers in this way, even when there is very little coolant left in the system. At one time, the author managed to overcome about 30 km in this way, without causing noticeable harm to the engine.