When you start the car you can see that engine RPM is above 1000 RPM close to 1500 RPM. High RPM is normal behavior for the car when the engine is cold because it helps to gradually increase the engine oil temperature, decreasing its viscosity and heating the coolant to help the engine achieve its operating temperature by burning more fuel.
After heating the coolant to normal temperature, the RPM should go back to the values specified in the car’s manual (below 1000 RPM). Higher engine RPM will last longer in winter than in summer because of obvious reasons of cold temperatures in the winter.
The rpm and time values are indicated in the car’s manual. If the speed and/or return time are increased, it means that you should investigate the cause.
What are the causes of high RPM when the engine is cold?
There are quite a few reasons why high engine RPM can occur besides the normal ones.
- Intake manifold air duct. In some engines, the intake duct is closed by a solenoid valve, a pulse motor, or a pulse-controlled solenoid. If these elements fail, the air channel will not be properly closed, and, as a result, a large amount of air will pass through it into the intake manifold.
- Intake manifold pipes. Excess air enters the system due to cracks on the intake pipes, their depressurization, or their bad connection points. This can usually be determined by “a whistle” that is heard from there causing high engine RPM.
- A bad throttle valve. A soiled throttle valve with carbon deposits causes it to not fully close and leads to high engine RPM. Another cause is a hard floor mat that presses the gas pedal and the driver does not realize it.
- A bad throttle position sensor. The main function of the throttle position sensors is to transmit to the electronic control unit (ECU) information regarding the position of the gas pedal at a certain time. If the throttle position sensor is failing the ECU will notice that and, as a solution, will supply the maximum amount of air which will lead to increased engine speed when idling. The speed may also increase when the throttle settings are reset.
- In some cars, such as Toyota, the engine design provides the use of electric motors for forced idling, although their models and management methods are not the same, and they all have a separate management. Thus, the problem of high engine speed at idle can be associated either with its control systemor with their speciffic electric motor.
- A bad idle speed regulator. Another reason why the engine has high RPM on cold. Often a failing idle speed regulator is caused by damage to its electrical contacts.
- A bad mass airflow (MAF) sensor. If this sensor is failing, then wrong information about the amount of air supplied to the engine will also be supplied to the electronic control unit (ECU). Thus, there can be a situation where the ECU will want to open the throttle valve more to increase the airflow. This will lead to increased engine speed not only at cold but in warm conditions as well.
- A bad intake air temperature sensor. If this sensor goes bad, the information received by the ECU from it will be incorrect and optimal RPM and optimal air-fuel mixture cannot be formed. Therefore, increased engine idle speed may occur.
- An inefficient or a failing water pump. You can experience high engine RPM at cold if your car’s water pump loses its performance. Because it pumps an insufficient amount of coolant, heating the cold engine will work ineffectively, and therefore, the engine will work at high speeds for a fair amount of time. You should know the pump has this problem if, after pressing the gas pedal, the heater blows hot air, and after taking your foot off the pedal, the blown air is cold.
- A bad or failing electronic control unit (ECU). Not an often cause, an ECU that does not work correctly because of software issues can result in the engine having and keeping high speed when it is cold. Checking the problem can be very easy if the error is stored in its memory and you could check it with an OBD scanner, but in rare cases the error is not stored, making the diagnosis very difficult.
- A failing thermostat. Thermostat faults are causes of many car faults including the high RPM. When the engine is cold, the thermostat is closed, allowing the coolant to circulate only through the engine. When the coolant reaches the operating temperature, it opens and the antifreeze is additionally cooled by passing through a full circle of the cooling system (also known as”the big circle”). But if the thermostat is not close from the beginning, and the coolant initially moves in this mode, the engine will work longer at higher speeds until it warms up completely.
- Bad coolant temperature sensor. If this sensor fails, then this will send “false” information to the ECU such as the coolant operating temperature is not yet achieved. So the engine will run at increased speeds in order to supposedly warm up the coolant.
What to check for first if you have high engine RPM?
If you experience this problem and realize that increased RPM is not normal, you should first check the throttle valve and idle speed sensor because it can have deposits accumulates on its surface.
The throttle can be cleaned and the problem can be resolved. You can achieve great results using this cleaner
Then check for air leakage, such as the intake pipes and air duct.
Also pay attention to the cooling system, especially to the thermostat and water pump an their symptoms described above.
You don’t need to panic if the engine has high RPM when it’s cold, this is normal if it happens for the short term. The period of high RPM could last longer if ambient temperatures are low. But, if engine speed lasts for a longer period of time or it’s maintained, this is a good reason to perform a check.