Why do engines lose power when they are hot?

There may be many reasons why the engine loses power when it is warm. In petrol cars, this problem appears most often because bad or failing mass airflow sensor, a failing coolant temperature sensor, a gasoline pump not working properly, or air leaks.

Regarding diesel engines, the most common fault of engine power loss, when it’s warm, is because of a bad high-pressure pump.

To better understand why engines lose power when they get hot, especially in petrol cars, it is good to know the conditions of the fuel combustion. At low temperatures, the air is denser, so the amount of air in one cubic meter is greater.

As the temperature rises, its mass decreases, the car begins to choke and the engine’s power goes down. This is true for most petrol cars.

The fuel combustion itself is not responsible for engine power loss when it is hot. For that, we must look into the fuel system and the ignition.

What are the reasons for engine power loss when hot?

The operating parameters of individual parts are the main reasons why engines lose power when they get warm. Problems in the operation of sensors, ignition module, fuel pump, distributor, and high-voltage lead to a weak air-fuel mixture that leads to an engine power loss.

Not only a bad mass airflow sensor and a bad coolant temperature sensor are responsible for engine power loss when it gets warm. Other parts lead to power loss when the engine is hot if they are not working properly.

  • A bad MAP sensor. Like in the case of the mass airflow sensor, when the MAP sensor fails, it can give incorrect information to the electronic control unit (ECU) especially when this sensor is hot. Thus, it is better to check this sensor in this case.
  • Heated throttle valve. Turning off the throttle heating in summer is important for cars where there is a throttle heating. Preheating overheats the air and there will be a weak mixture.
  • Bad or failing gasoline pump. The fuel pump doesn’t need to be defective. First and foremost it is good to have sufficient fuel in the tank to allow proper cooling of the fuel tank because when it is overheated, the pump will not work properly. Also, you can also check it by removing it, it can accumulate dirt. After cleaning it should be ok. But if this does not solve the problem, consider replacing the pump.
  • Air leaks in the intake manifoldIn this case, the air mass flow sensor and the MAP sensor will send false information to the ECU, which will create a weak mixture. Cracks through which extra air is sucked are most often formed on the pipes.
  • Steam-air. It is formed when gasoline boils in the fuel line. The solution depends on the engine design. The main thing is to find a place where this happens and insulate that hot part where gasoline boils.
  • The ignition module. This module overheats because of the accumulation of dirt and can cause the engine to lose power when warm. It requires cleaning along with the engine itself.
  • Failing fuel pressure regulator. Sometimes, when it overheats, it starts to fail. If that happens, you need to check the pressure in the fuel ramp and check the regulator.
  • Bad catalytic converter or oxygen sensor. If the catalytic converter is clogged that is a sign of a power loss to the engine when it heats up. The first thing you can do is to remove the catalytic converter and clean it, it should be enough to solve the problem. However, if this doesn’t solve it then you need to change de catalytic converter. Also, a failing oxygen sensor can lead to poor engine power. Check it with an OBD scanner
  • Small gaps on valves. Valve clearance is reduced when the engine is running, which leads to overheating and a loss of power for a warmed engine. If the valve is overstretched, it may stop closing
  • A failing turbocharger. If for some reason, lubricant does not fit the turbo bearings, it can jam at high speeds, and at high temperatures.
  • A bad or failing mass airflow sensor. One of the most common reasons why the engine loses power when hot is the malfunctioning of this part. When it not functioning correctly, it reads incorrectly the passing air, due to which less fuel is supplied and a weak mixture is formed. The first thing to try is to clean the MAF sensor, if this does not work, then replace it.
  • Bad coolant temperature sensor. When this sensor is hot, a failing coolant sensor generates an excessive temperature for the coolant and starts to add less fuel, meaning the engine will suffer a power loss when it is hot, so it’s ok to check this sensor and the MAF sensor first when dealing with an engine power loss.

In diesel engines, the engine power loss is associated with a failing high-pressure pump. In common cases, the plunger pairs wear, which will lead to an overheating high-pressure pump. Another symptom associated with this problem of the high-pressure pump is that the car won’t start.

You can check the pump by cooling it down with water when the engine is at its functioning temperature. If the car starts correctly, then the pump is on the verge of failure. Solutions are to change the plunger pairs or to change the pump entirely.

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