What pressure in engine cooling system should be and what to do if it is not normal

What the pressure in the cooling system is for and whether it is needed at all

Pressure in the cooling system of a car and what it affects – one of the popular topics of internet car hooligans, though by incandescence of passions it is certainly far from “oil rubbing” or discussions of the cooling system “warm up – not warm up” type. Nevertheless, this question is important and interesting, and I would like to dot all the i’s in it.

The boiling point of water at atmospheric pressure is the well-known and canonical 100 °С. Ethylene-glycol antifreeze at the same conditions – 105-107 °С. However, as the boiling point of the coolant becomes higher under increased pressure, it purposefully creates pressure of about 1,2-1,5 atm in the engine cooling system. Due to this, the boiling point of the antifreeze moves to 120-125°C and even higher, and “hot” motors (which became the majority in the last 10 years) successfully maintain a stable temperature without the risk of the coolant boiling under normal conditions.

Pressure above atmospheric is the norm for the cooling systems of 99.9% of modern engines. Its main and only task is to ensure that the antifreeze does not boil if the engine operating temperature is higher than the coolant boiling point at atmospheric pressure. Boiling generates an abundance of vapor, which prevents the pump blades from pumping the fluid efficiently, and the vapor bubbles, standing as a barrier between the fluid and the surface it washes over, dramatically worsen the heat dissipation. These two processes are closely related, mutually supporting each other and progressing rapidly. The result is rapid engine overheating, which does not stop immediately, even after shutting down, and for this reason is rarely entirely without consequences.

As a matter of fact, the operating temperature of internal combustion engines has been increasing throughout their evolution, and this process continues even now. Conventionally, the “stages of growth” can be designated as follows:

  • “80-85 °C” (long-gone temperature characteristics peculiar to mid-20th century engines)
  • “95-105 °C” (characteristics that have been the norm for the last few decades and are still relevant for relatively simple engines)
  • “120-130 ° C” (temperatures at which the most advanced modern engines operate, which are at the peak of fuel efficiency and environmental standards)
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These figures are approximate, given simply to understand what values we are talking about. There are exceptions, where it is “the other way round”, but they are rare and only confirm the rule.

Now we are interested in an early period of development of automobile industry – those 80-85 °С. As we see, this temperature is lower than the boiling point of water at atmospheric pressure, and the more so – lower than the boiling point of antifreeze under the same conditions. So, these engines did not need pressure in the cooling system? Exactly right – it was not there!

The old clay days – the era of engines with an open cooling system! There were, of course, plugs in the radiators of cars of that period, but they did not provide tightness, but served only to prevent splashing water when the car was shaken on potholes. All the rest did not differ essentially from the modern engines: the pump also rotated and carried liquid with its impeller in a circle through the engine jacket and radiator, and water expanding during heating was displaced in the compensatory volume, which was the upper tank of the not filled up to the end radiator.

Despite decent overall power, these engines operated in the mild conditions of low rpm and little power taken off each liter of cubic footage. The blocks and heads were cast-iron, massive, with large amounts of oil in the crankcases, with large radiators and constantly rotating cooling impellers mounted directly on the pump pulley or crankshaft, without any temperature sensors or viscocouplers. Therefore, even at the maximum load the temperature of water in the cooling system without pressure did not approach a hundred degrees, and the serviceable engine did not boil. And even at initial stages of malfunctions (not fully opened thermostat, low liquid level, partially clogged radiator, etc.) the problem did not arise at once – the engine had a large stock of “meat” and it was not so easy to bring it to steam spewing.

However, the flipside of the medal and the inherent satellites of the characteristics of such engines were fuel voraciousness and low environmental friendliness. These two points subsequently required reforms in engine engineering, and the engines became smaller, eat less, produce more per liter, and operate at higher temperatures. Open-type cooling systems disappeared, giving way to sealed systems – temperatures increased, and antifreeze pressure took over the main role in protecting it from boiling.

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Accordingly, under the hood appeared such detail as a cap of the expansion tank with a calibrated valve, which had a big responsibility – to keep the pressure at a strictly marked limit. In case of its excess in the case of malfunction in the cooling system – to open and release steam and antifreeze outside in order not to burst the hoses and radiators.

However, despite the fact that in the cooling system after the introduction of pressure nothing has changed fundamentally, except for the shift of temperature to a higher zone, many motorists have mistakenly considered pressure as a necessary condition for a variety of processes. On auto forums one can often meet statements that if there is no pressure in the system due to a failure or absence of the expansion tank plug, the pump cannot work properly, the thermostat does not open, the engine cannot reach operating temperature (!) and other similar fantasies.

It is not so. The pump circulates the fluid and does not know what pressure it is under or without pressure. Circulation quality is only affected by the integrity of the impeller, belt tension, cleanliness of the channels in the radiator and the viscosity of the antifreeze. The thermostat opens only on the coolant temperature and nothing else. When the antifreeze reaches the thermostat opening temperature, the thermostat will open even if the pump does not rotate at all.

Yes, raising the operating temperature of engines has become one of the inevitable measures to meet today’s requirements for environmental friendliness and economy. But the pressurized cooling system also has two very significant disadvantages…

The first is an increased risk of antifreeze leakage. While a car is new, there are no problems, of course, but with age, weak points begin to appear in cooling system. Spring clamps become weaker, rubber connectors lose their elasticity and get covered with cracks. Plastic elements (transition connectors, connectors, bodies of thermostats and so on) become brittle and fragile. And where it’s thin, it tears. Coolant pressure starts pushing it out at the first opportunity. The “aged” cooling system is unpredictable in its surprises, which price is rather high – if it does not “croak” from overheating of the engine, you have at least to pay for a caravan, as you will not go far without antifreeze even after cooling down.

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The second lack is a kind of the first. Modern motors have practically no “meat” wherever you can poke, not excluding a heat capacity of cooling system. The raised pressure accelerates the antifreeze on asphalt at the slightest leakage, and where the old engine (even with the cooling system working under pressure, let alone the open one!) would hold for some time, losing the liquid gradually, the modern engine loses it at a dangerous rate. Or rather, the rate is the same, but the result is different. The cooling system of the modern B-class car contains twice less antifreeze, than even classic “Zhigul”, and if each of cars loses one liter in half an hour, then the first car will lose 10 %, and the second will lose 20 %. The “vitality” of the car decreases proportionally, and the risk of overheating consequences increases proportionally.

Is it possible to fight it? It is possible, but difficult. “By the way, veteran Gazellists can recall quite a long history from the late 90s when the quality of assembling was such that even skillful drivers couldn’t stop the antifreeze leak for months. Only by unscrewing the cork of the expansion tank and switching the cooling system to the “without pressure” mode it was possible to get rid of endless blue puddles on the asphalt in the morning… But such trick could be used only with ancient ZMZ engines, which primogenitors were working without water pressure.

Unfortunately, it is impossible to transfer hermetic system of cooling to the open variant on modern automobiles, in order to avoid overheating. Therefore, having got the car with age of 7-10 years and/or with large run, it is highly desirable to conduct total replacement of all cooling system – at least, all rubber hoses, clamps, most plastic details (transition hose joints between hoses, etc.), thermostat and cork of expansion tank. But even with use of good quality nonoriginal articles, such procedure turns out to be rather expensive, and rare buyers of used automobiles dare for such preventive measures without obvious breakages.

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Working pressure of a cooling system. You cannot just set an expansion tank from VW!

The reason of tank bursting is not only in bad quality of plastic, but often in not working drain valve in the lid of the tank. Overpressure occurs and the weak tank bursts, like a fuse, while keeping the rest of the system intact!


I, like many people, changed the tank on my VW a little over half a year ago. Before that, 2 tanks had burst, but the cracks were small and the antifreeze was not all gone. The first thing I did was put together a test bench and measured the pressure at which the drain valve went off. The valve worked at 1.8 atm. But this is not VW… The working pressure for our taz is 1-1.2 atm! Finished the tank cap to 1.2 atm. There is an instruction at the link above.

After 6 months the antifreeze is slowly running out. I topped it up a couple of times. In winter, at one point the antifreeze went out completely. At least the ECU warned me about such surprises. The overheat alarm went off beforehand and I didn’t overheat the engine. Here we go…

– First I changed the thermostat hose, it was leaking. Didn’t help. – Found antifreeze in the cabin under the mats – changed the heater radiator and spigots Didn’t help. – Changed all the clamps and pulled all the hoses. Did not help – Emulsion appeared on the dipstick and under the engine filler cap (could not be seen before). Opened the engine – replaced the gasket in the cylinder head.

Only main radiator and a part of pipes remained alive… No more antifreeze went out. In between repairs, I drove the car with the plug unscrewed to get less antifreeze.

The car is 3.5 years old, 76,000 miles. Of course all these problems could not come out only from the new tank, but one way or another, I think he forced them. The cap has now been reworked to 0.8 atm! I know a lot of people don’t screw it on at all, but the antifreeze under pressure boils later, I’d like to take that into account too.

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A bit of theory

Antifreeze Felix Carbox G12 – Start boiling point was 110 ° C, above the requirements of the regulations on 5 degrees. (taken from this test)

Boiling point/start of boiling (from the article Methods of testing of coolants O. M. Goltyaev, Candidate of Physical and Mathematical Sciences, Deputy General Director of JSC “TECHNOFORM”)

The boiling point is the upper limit of the automotive antifreeze operating temperature range in the cooling system. Boiling of the coolant in a working automobile is a very unpleasant event associated with the forced car shutdown, refilling of the evaporated coolant and possible defects in the engine.

The boiling point of the working coolants at atmospheric pressure is 107-109°C for the 50% concentrate solution, 108-110°C for OZH-40 and 115-116°C for OZH-65. But usually the car cooling system generates increased pressure (1.5 – 2.5 atm) due to a special valve on the expansion tank lid. Because of this, the boiling point of the coolant rises to 120 -135°C. The graph shows isobars depending on the boiling point (°С) of the coolant depending on the concentration (% vol.) at various external pressures (1 bar ≈ 1 atm).

Judging by the stated boiling point of G12 – 110 °С without pressure. According to this graph – the density of the concentrate is 82%. All approximately, I hope I have not miscalculated anywhere. Write if you have any objections.

See graph 1.2 atm. ~ 140°C 0.8 atm. ~ 130°C 0.5 atm. ~ 113 ° C 0 atm. ~ 110 °С

Normally the threshold for turning on the second fan is set at 115 ° C. Mine is 97. I think 0.8 atm is good enough. Let’s see how long it lasts.

p.s. Give thought to your car!

UPD : Great implementation of testing the cap, without removing the tank from the car. Nipple is cut into the thin return hose with a tee and a small piece of hose.

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