RPM Fluctuates While Driving: 4 Most Common Reasons (+ Fix)

RPMs or revolutions per minute are essential for every driver since they provide information on the best time to shift gears. Also, it gives drivers an idea of how much work their engine is doing. For this reason, it can be problematic to experience fluctuations in your RPM, especially while driving. So, you may ask:

If my RPM fluctuates while driving, what is the reason behind it? In general, you may experience RPM fluctuations due to four common reasons. Among these are bad crankshafts, worn-out spark plugs, defective air control valves, and vacuum leakage. Since you must stop those fluctuations, you’ll want to check each one and fix whatever the issue is.

In this article, we’ll walk you through all the four common reasons why your RPM may fluctuate as you drive. This way, you’ll know where you’ll check and how you can solve the issue so you can have normal RPM.

Without further ado, let’s get into it!

RPM Fluctuates While Driving

Old and blown-out spark plugs

Spark plugs that have blown out might cause your car’s RPM to vary. It is a pretty frequent problem with older vehicles. The RPM will decrease or spike if the spark plug is unclean or worn out.

Excessive vibration, weak acceleration, terrible fuel efficiency, difficulty starting, engine misfires, and a harsh gasoline smell from the exhaust pipe indicate a broken spark plug.

Aside from variable RPM, you should replace the spark plugs as early as possible if you observe one of these indications. 

Vehicle spark plugs are essential because they generate electric sparks that burn the fuel-air combination in the combustion chamber to generate electricity. The average spark plug lasts 100,000 kilometers.


If the spark plugs are the problem, they will need replacement soon. The process for removing and replacing the faulty spark plug in your car is outlined below.

  1. The first thing you’ll want to do is to search for the blown-out spark plug. The cylinder head usually houses the spark plugs, and you can find it directly above the cylinder combustion chamber. It’s worth noting, though, that cylinder heads are positioned differently in different engines. Thus, you may find your spark plugs in other places.
  2. After you’ve found the cylinder, remove the ignition coils or wires that are protecting your engine
  3. The spark plugs will be located on the cylinder’s upper surface. Take a photo of them and then detach them so that you can adequately restore the new ones.
  4. Remove the worn-out spark plugs and replace them with new ones. Double-check that the replacement spark plugs must be the same size as the worn-out ones.
  5. Lastly, lubricate the spark plugs and threads before fastening them. The ignition coils and wires that you removed previously can now be replaced.

Failing Crankshaft Position Sensor

The crankshaft position sensor is another component to look at if your car’s RPM fluctuates while you drive. This sensor is attached to the engine block and faces the engine crankshaft’s synchronization rotor. It collects data that the motor ECU uses to compute the motor’s crankshaft position and flow rate.

In a nutshell, it monitors the engine crankshaft’s rotational speed, or RPM, as well as its precise location. As a result, if the crankshaft position sensor is destroyed, the RPM may vary.

It may also result in automobile stalling, a long crank duration before the engine runs, noises in the motor, or difficulty igniting the engine. Nevertheless, a faulty crankshaft sensor is easy to repair.

However, it would be best to inspect it to see whether it has any fractures, rusted connection pins, or other issues.


It’s critical to repair your vehicle’s broken crankshaft position sensor since it sends the correct data to the engine management unit (ECU). 

You must bring your automobile to a professional if you’ve never diagnosed a car before. If you want to do it on your own, though, take these steps:

  1. It would be best to unplug your automobile battery before performing any repairs to the crankshaft position sensor. This way, you can protect the electronic panel in your automobile from harm.
  2. To determine the crankshaft position, see your car’s handbook. This changes from one vehicle to the next. The crankshaft position sensor may be near the primary pulley in certain cars, but it may also be near the gearbox ring case or in the motor cylinder head in the others.
  3. Remove the wire connector after finding the crankshaft position sensor. It will let you control and remove the crankshaft position sensor. Snap a photo of the sensor before uninstalling it so that you can put it back.
  4. Update the sensor by disconnecting it and replacing it with a new one. Then, in the same place as previously, reconnect the electrical harness.
  5. You may reattach the power and check your crankshaft position sensor after you’re sure it’s solid and in place. If the sensor were the issue, your RPM wouldn’t fluctuate back and forth as much as it used to when driving.

Engine Vacuum Leakage

Your car’s RPM may potentially vary as a result of an engine vacuum leak. This type of leak generally happens in the engine compartment. 

A loud hissing, loss of power, low fuel economy, check engine light coming on, car halting, and rough idle are all indications of a vacuum leak, in addition to your RPM fluctuating fast.


A vacuum leak, unlike the previous situations, is impossible to detect on your own. Take your automobile to a competent technician to get a vacuum leak fixed.

If a vacuum leak isn’t fixed right away, it might cause significant automobile difficulties. It might also be dangerous to drive and cause an accident.

Failing Idle Air Control Valve

A faulty idle air control valve is another typical reason for your car’s RPM to fluctuate. The control valve is required to keep the engine’s idle speed at a reasonable level. If the control valve is malfunctioning, it will fail to perform its function correctly.

As a result, the RPM might change. Rough idling, engine halting, the check engine light, and load causes slowing are all symptoms of a faulty idle air control valve.


The process for replacing a faulty control valve is outlined below. You can employ a professional technician if you don’t have the expertise or conviction to examine the automobile.

  1. To begin, put your vehicle and switch off the motor. Additionally, the space and illumination in the area should be sufficient. Garages are the most excellent place to get automobile repairs and replacements done. Let the vehicle rest after disconnecting the battery.
  2. Discover what’s wrong with the idle air control valve. Using an OBDII scanner or examining the RPM, you may check the IAC valve. The best way is to examine the RPM, which will help you determine if the control valve is defective and the problem.
  3. After determining the problem, the following step is to identify and repair the control valve. The control valve is usually found adjacent to the throttle and in the intake manifold of most automobiles. If you’re unsure, see your car’s handbook. Remove the wire harness if you discover the valve.
  4. Unplug the control valve and carefully wash it. Debris and carbon may accumulate in some cases, preventing the valve from functioning correctly. To remove all dirt and carbon, use a spray, cleanser, and a clean towel. You may use the cloth to absorb any remaining liquid.
  5. Disconnect the idle air control valve and install a new unit if it doesn’t work after cleanup. It indicates that the IAC valve is faulty and has to be changed. Replace the serpentine belt and the battery after installing a new control valve. You may now check to see if the RPM has returned to normal.


In a nutshell, RPM fluctuates due to four common reasons. However, it happens at specific points, which is why you won’t need to worry if you experience it while driving. The first thing you’ll need to do is to understand what caused the fluctuation.

The four common reasons would be old and blown-out spark plugs, failing crankshaft position sensor, engine vacuum leakage, and failing idle air control valve. If you have seen the problem, you can properly follow the abovementioned steps to deal with the issue.

In the end, note that some issues regarding this matter are fixable in a few simple steps. However, you may encounter complicated problems that need assistance from a professional. If ever you have such an issue, don’t hesitate to take your car to a service center for it.

Also Read:


Image credits – Canva

Share on:

My name is Hank, and I've been in the automotive industry for 27 years. I've been working in my own auto repair shop for the last 13 years, and now I want to help you here, on my blog. Let me know if you have any questions. Read more