Why is the bottom radiator hose cold and the top hot?

Hot top and cold bottom of the car radiator - normal condition only for cold engine after launch. If the internal combustion engine has worked for several minutes, during which the temperature of the coolant has already reached 80–90 degrees (depending on the model), and at the same time the upper radiator pipe is hot and the lower one is cold - this symptom of a problem in the cooling system. However, there are exceptions to this rule.

The main reasons that the lower radiator pipe is cold after warming up is a breakdown thermostat or pump, clogged radiator. This article will tell you how to determine the specific cause of the problem and what to do in this case.

What causes the lower radiator hose to be cold and the upper one to be hot?

Coolant circuit diagram: Click to enlarge.

Whether the lower radiator hose should be cold depends on the path of the antifreeze.

After starting the engine, it first circulates in a small circle, bypassing the radiator, and after reaching a temperature of about 85 ° C, at which the thermostat partially opens, goes to a large one.

In most models, coolant enters the radiator through the upper pipe, after which it is pumped down and returned through the lower pipe to the internal combustion engine cooling jacket.

In those models where the thermostat blocks the flow of antifreeze from the radiator (for example, VAZ 2109), the lower pipe may be cold when the upper pipe is hot and before the engine warms up. If both channels or the radiator inlet are blocked (as on the Lada Granta), when the upper pipe warms up, both the radiator and the lower outlet should quickly warm up.

With a working cooling system, the bottom of the car radiator is cold, and the top is hot only at the moment when the thermostat has already opened slightly, but the antifreeze has not yet reached the lower pipe. When the air temperature is low and the interior heater is switched on in winter, it takes more time to warm up the coolant. The opening of the thermostat in this case occurs through 10 minutes and more after the start of the movement. At low air temperatures, it often happens that the car radiator is hot on top and warm on the bottom, as the antifreeze cools quickly as it passes through it.

Diesel engines warm up much slower than gasoline engines. Some cars with a diesel engine in winter can fully use a large circle only during high-speed driving (over 100 km / h) and with a long “toffee” in traffic jams.

You need to figure out why the lower radiator pipe is not heating up and solve the problem in the following situations:

• the top hose is hot and the bottom stays cold even after high-speed driving in warm weather;
• the radiator warms up only from above, remaining cold from below;
• the coolant temperature readings on the dashboard go into the "red zone" (over 100–110 degrees);
• boiling antifreeze in the expansion tank.

The main reasons why the upper radiator pipe is heated, and the lower one is cold:

Why does an air lock appear in the cooling system: video

• Thermostat stuck in the closed or semi-closed position.
• water pump cooling worn out and can not create the necessary pressure.
• Channels radiator clogged deposits.
• An air lock has formed in the cooling system.

breakdowns of the cooling system are not always reflected in the readings of the sensors. DTOZH (coolant temperature sensor) may not come into contact with superheated liquid and show normal temperature. For the same reason, the radiator fan may not turn on in a timely manner.

Computer diagnostics helps to identify only problems with an electronically controlled thermostat. The problem with the mechanical valve is indirectly indicated by the readings of the DTOZH located on the block, in the cylinder head or in the area of ​​​​the thermostat that do not correspond to the real ones.

It is impossible to operate a car if the car is warming up and the lower radiator hose is cold! This is fraught with a breakthrough of the gasket between the head and the block, disruption and damage to the pipes of the cooling system, misalignment of the cylinder head, scuffing and wedge of the CPG of the internal combustion engine due to overheating!

How to determine the reason why the lower radiator pipe is cold and the upper one is hot

The lower radiator pipe is slightly colder than the upper one, since the antifreeze coming from above cools down as it passes through the radiator. But usually it's about little difference temperatures. When the outlet pipe / hose from the radiator does not warm up at all, this indicates a violation of the coolant circulation and heat transfer in the internal combustion engine.

All common reasons why a cold lower radiator hose does not heat up normally are summarized in the table.

Possible reason	Consequence	How to understand/define
The thermostat is stuck in the closed or half-closed position.	The coolant circulates in a small circle and is not cooled.	The motor heats up quickly, the stove works fine. when driving, the temperature rises above normal, the radiator fan, even in cold weather, began to turn on more often.
Expansion tank cap valve defective.	The pressure in the circuit is not automatically regulated, which may cause air pockets.	The motor warms up worse, even after warming up, the stove does not heat up to 3000 rpm or more without gassing. On a cold engine, the plastic tank can flatten a little, retract.
Clogged channels (lines) of the radiator.	Antifreeze does not pass through the radiator and circulates mainly in a small circle.	The motor overheats, the temperature on the dashboard is in the "red zone". Turning on the fan does not help to quickly reduce the coolant temperature even while driving. The antifreeze in the tank is cloudy, brown or pale brown due to the presence of contaminants and corrosion products.
The impeller of the water pump has developed.	The pump cannot create the necessary pressure, which worsens the circulation of the coolant and the transfer of heat from the motor to the radiator.	The radiator is warm, not hot, the stove heats up weakly, but the pressure in the system rises quickly. The expansion tank swells, seething in it, hissing, passing pipes are possible. At high speeds (from 3000 rpm), the lower pipe may begin to warm up normally.
There is an air lock in the cooling circuit.	Coolant circulation deteriorates, heat transfer is disturbed, swelling and breakdown of pipes are possible.	The motor is warm, but the stove blows cold. Gasping can temporarily solve the problem (not always).

Vehicles with the most common heat transfer problems

On some cars, the situation when the upper radiator hose is hot and the lower one is cold is more common than others. Such models and their "childhood illnesses" are summarized in the table below.

Brand, model	The cause of the problem	How can I fix
VAZ 2106, 2101 and other "Classics"	The low quality of the original components, namely the thermostats themselves, which wedge.	Instead of the original VAZ thermostat valve, use parts from other companies: Metal Incar, Finwhale, Vernet.
	The cold bottom of the radiator in the VAZ "Classic" with high mileage is a consequence of corrosion of parts and clogging of the radiator channels.	Flush the radiator with a special fluid or, if it does not help, replace it.
VAZ 2108-21099, 2113-2115, Kalina	A faulty expansion tank cap valve does not allow maintaining working pressure in the cooling system, which disrupts the coolant circulation, and the stove does not heat well.	Replace expansion tank cap.
VAZ 2110 - 2112, Lada Priora	On the VAZ 2110-2112, the lower pipe is cold, and it blows cold from the stove due to a predisposition to the formation of traffic jams and impaired coolant circulation.	Monitor the condition of the cooling circuit, expel plugs, especially when replacing antifreeze. As a radical solution, you can replace the thermostat with a “six-hole” one (art. 21082-1306010) or from Lada Granta (21900-1306010), adapting the circuit to it.
	The upper branch pipe is heated, and the lower one is cold, due to the production and lack of pump performance.	Install an improved pump, like the Luzar Turbo.
Skoda Octavia	On the Skoda Octavia, the lower radiator hose is cold, and the engine starts to overheat due to a faulty oil heat exchanger gasket.	Replace the oil heat exchanger gasket.
Daewoo Lanos, Sense, ZAZ Chance	Cold lower pipe and radiator, a consequence of early opening or jamming of the thermostat.	Replace the thermostat with a better one with an opening at 87 degrees.
Ford Focus	On a Ford Focus, the bottom of the radiator is cold and the top is hot, usually due to a stuck thermostat.	Replace defective thermostat.
Mazda 3	On Mazda 3, the lower pipe is cold, usually due to a broken thermostat.
Volkswagen Passat	With high mileage, the pump and thermostat fail.	Replace pump or thermostat.
Suzuki Grand Vitara	Airing of the cooling system due to pressure violation.	Drive out air pockets. Replace the cap of the expansion tank, which is often clogged or vice versa does not hold pressure.

The lower radiator hose is cold: what to produce?

If the lower radiator hose does not want to warm up, you need to carry out diagnostics that will help find out the cause of the breakdown:

How to look for the cause of a breakdown if the coolant circulation is disturbed: video

1. When it is very cold outside - first let the motor work and warm upnot including the stove. Perhaps everything is fine, it's just that the internal combustion engine does not have time to quickly reach operating temperature.
2. Do computer diagnostics according to OBD-II, check the temperature value of DTOZH. If it grows slowly, but the lower pipe gradually warms up, a wedge of the thermostat has appeared in the open position. If, after warming up, the temperature continues to rise, but the lower pipe remains cold, a thermostat wedge, radiator clogging, pump breakdown and air jams in the system are possible.
3. To rule out problems with the pump, check it.
4. If the pump is in order, make sure there is no air lock, if there is, try to expel it.
5. On the internal combustion engine warmed up to operating temperature, feel the radiator - it should be evenly warmed up throughout the area. The presence of cold spots indicates the need for flushing.

Do not open the cap of the radiator or expansion tank and remove the pipes for diagnostics and repairs without letting the engine cool down! The superheated steam contained in the pressurized circuit can reach temperatures above 100 degrees and cause burns!

You can fully check the thermostat valve without an electric drive only by removing it. For diagnosis you need:

How to check the thermostat at home: video

1. Partially drain the antifreeze into a prepared container.
2. Remove the thermostat valve (if possible).
3. Place the valve or thermostat assembly in a pot of water.
4. Heat water by monitoring its temperature with a thermometer or multimeter with a thermocouple.

In a working thermostat, the valve should begin to open when the temperature is reached. 80-90 degrees (depending on the model and indicated on the body of the part), when the liquid boils, it opens completely, and when it cools, it closes completely. If this does not happen, the position of the cymbal does not change or the stroke is insufficient - thermostat stuck and it needs to be changed. After replacing the part with a new one, you need to add or completely replace antifreeze.