Why can a car slow down and speed up for no apparent reason?

Article updated with new information on April 27, 2023.

Why can a car slow down and speed up by itself? The car’s loss of acceleration can be due to many reasons, but today we want to talk about the most common. When a car speeds up or slows down for no apparent reason, it is often due to idling problems.

As you know, idling is the minimum speed of revolutions per minute (turns or turns per minute) to which an internal combustion engine adjusts to remain in stable operation without the need to activate an acceleration mechanism or fuel intake. When idling is rough, the driver notices the car slowing down or speeding up senselessly.

If a car slows down on its own, it is usually because it does not control the engine revs well. Therefore, the mechanics speed up or slow down for no apparent reason; and not only that. In some cases it also stays accelerated or goes out unless we step on the accelerator.

What is the IAC valve and how does it work?

For the idle to be stable there is usually a part (known as IAC or idle valve) that is responsible for controlling the intake air. The IAC is controlled by the ECU, it regulates the amount of airflow into the throttle body to achieve idle speed. If your car slows down, it may be due to a problem with this valve.

The operation of the IAC valve is very simple. At low temperatures, the ECU sends a signal to the IAC to allow a greater flow of air to pass through the engine’s combustion chambers. This allows the idle to be raised to around 1,200 rpm, instead of the 800 or 900 rpm at which an idle engine usually rotates, quickly raising its temperature.

Why does it break down and what are the symptoms of a bad IAC?

Like any other component of a car engine, the IAC valve is subjected to continuous work, which generates wear and can end up leading to a breakdown due to dirt or obstruction.

Luckily, it is not a very expensive part to acquire, since you can find new spare parts online at prices that range between 20 and 50 euros, depending on the manufacturer.

The specific symptoms that this valve fails are:

• Unstable idle: when the valve is in poor condition, one of its most frequent consequences is that the engine has an unstable idle, tending to shut down. This may be because the IAC is clogged or dirty, staying in the closed position, which causes the amount of air that reaches the engine to be low, even stalling it at idle. It can also occur on the move and in transitions between gait and gait.

• Vibrations – Vibrations are another direct consequence of a failed IAC valve affecting airflow at idle or running, causing alternations in the airflow to the engine and consequently erratic mechanical behavior , even generating unpleasant vibrations that leak into the passenger compartment of the car.

• Oscillation in revolutions: the opposite case to the first point can occur. If the IAC valve is clogged or faulty and stays in the open position, the air flow to the combustion chambers will be excessive, which will force the ECU to inject more fuel to balance the mixture and to raise the rpm to carry out the combustion. . This means greater wear on the engine and high fuel consumption.

How to fix the problem that the car slows down and speeds up for no apparent reason?

The best thing to always do in case your car’s engine speeds up and slows down uncontrollably and for no apparent reason is to go to a workshop where a qualified professional can check the vehicle, carry out a diagnosis and repair the possible fault.

Even so, if you have some notions of mechanics and you prefer to try to solve this problem caused by the IAC valve, here are some possible solutions in this case to the slowdown of the car:

• Dirt problems: If the problem is related to dirt on the idle valve, you can remove it and clean it before reassembling it.

• Disassembly problems: in the case of a mechanical problem, the part must be disassembled to clean it, reassemble it and apply a little lubricating oil before reinstalling the component in the vehicle.

• Problems due to short or insulation in the winding: the most recommendable thing in this case is to replace the valve with a new replacement.

Do not overlook the possibility that the problem of the engine speeding up and slowing down for no apparent reason is due to a fault in the sensor that indicates the throttle position.

If this component is not in good condition and is unable to correctly detect the position of the accelerator pedal, it will send erroneous information to the car’s control unit, altering the behavior of idling.

In the event that the fault is located in the accelerator pedal position sensor, it is best to replace this part. Depending on the type of vehicle, the manufacturer of the product and its technical specifications, this new part can be obtained over the Internet for between 50 and 130 euros, approximately.