What Causes RPM Fluctuation During Driving? & How To Fix It

Many people don’t pay attention to the RPMs unless there is something wrong. As soon as there is RPM fluctuation while driving, it’s natural to take notice. Is it possible to have this problem?

Transmission problems can cause many RPM fluctuations. Transmission problems can cause RPM fluctuations if the spark plugs get dirty, or there's a vacuum leak. Sometimes, the crankshaft position sensor might send an incorrect engine speed signal to a tachometer.

This guide will discuss the main causes for your RPM fluctuating when driving. This guide will help you diagnose and repair the issue for smoother driving. 

What exactly are RPMs?

RPM means Revolutions Per Minute. When the car accelerates or decelerates, the engine’s crankshaft rotates. RPMs, which measure the rotations of the crankshaft and how fast they are moving up/down in the cylinders, is used to calculate these rotations. You can view the RPMs on the car’s tachometer, usually sitting next to the speedometer. 

Naturally, RPMs increase when you accelerate. The RPMs will decrease when the accelerator is released. The majority of engines rev up to 4,000 to 6,000 RPMs before shifting when operating normally, but this value depends on what type of vehicle and the motor that’s installed.

RPM Fluctuation During Driving

1. Defective Transmission

All gear shifting is handled by the transmission. If there’s a problem with the automatic transmission, the shifting could become irregular or delayed. The clutch may also slip if you have a manual transmission car.

It’s also possible that there’s a transmission leak. The engine may rev higher if the fluid level is below the normal range, as the torque converter won't work properly. 

2. Malfunctioning Crankshaft Position Sensor

The engine timing can be negatively affected by a faulty crankshaft sensor. The RPMs can fluctuate because it controls timing. The crankshaft position sensor is also used by the engine control module (RPM), and dashboard tachometer (RPM), to determine the engine's speed. A fluctuating RPM can indicate that something is wrong with your wiring or crankshaft position sensor.

The bad crankshaft sensor could also be causing stalling and trouble starting, as well as added vibration. This problem should be fixed before you become stuck. Some car models use the camshaft position sensor, so it shouldn’t be overlooked. Look for related codes using an OBD2 scanner.

3. Fouled Spark Plugs Oder Coils

Spark plugs ignite air and fuel mixtures entering the combustion chamber. The plugs may become worn and cause engine vibration.

The vibration could cause fluctuations in RPM and make the engine feel as if it has its own mind. Also, you may notice poor acceleration or jerking. You may have to replace your spark plugs as soon as you reach 30,000 miles depending on the type. 

Also, you should inspect the ignition coils. These ignite the spark plugs but are subject to failure due to engine heat. You should check your distributor cap and spark plug wires if you drive an older vehicle.

4. Vacuum Leak

It’s always possible that the fluctuation has to do with an engine vacuum leak, especially if it occurs while accelerating. When there’s a vacuum leak, the engine’s air supply gets interrupted. 

RPM fluctuations, reduced fuel efficiency and rough idle are all possible. A vacuum leak can cause the vehicle to stall, leaving you stranded, so don’t wait to get it looked at. 

The intake manifold is usually affected by vacuum leaks. These are typically caused by cracks in the vacuum hoses. You may also notice a leaky intake manifold gasket, which is located between the cylinder head & the intake manifold.

RELATED: How to find a Vacuum Leak & How to Fix It (8 Easy Steps)

5. Dirty Fuel Injectors

The fuel injectors are what allow your engine to get the right amount of fuel to keep it running. The injectors can become clogged over time due to dirt, dust, and other debris. If this occurs, fuel delivery can be obstructed and the engine will become starved.

A clogged injector can lead to fluctuating RPMs that cause the car to become jerky. You may not see the car accelerate in the manner you expected. Bad spark plugs, vacuum leaks, and other symptoms can cause the same symptoms making it hard to diagnose. 

6. Bad Throttle Position Sensor

An electric throttle body can be used to assist the car in accelerating if it has a low TPS. It does not matter if you push on the accelerator or not. The RPM fluctuations can be noticeable. 

Other symptoms are misfiring or difficulty starting the engine. As the problem gets worse, you might not be able to get the car moving, so don’t overlook problems.

The throttle body can be damaged or dirty. However, it is often possible to clean it without having it replaced. The pedal position sensor is another reason. It sends an electrical signal to the throttle control indicating how much acceleration should be applied. It isn't common but it should be noted.

RELATED: Cleaning a throttle body (10 Easy DIY Steps)

7. Dirty Air Filter

In order to produce power, fuel in the combustion chamber needs to be combined with sufficient air. Air filters trap contaminants and ensure that clean air enters the engine. 

If the filter isn’t changed, it can become clogged, which stops air from entering the engine. This can cause fluctuations in engine RPM and may also result in an acceleration delay. Thankfully, it’s one of the easiest fixes to perform. 

8. Bad tachometer

While it's rare for the car's tachometer to become damaged, some models do experience it so be careful. Modern cars have the tachometer integrated in the instrument cluster. If it becomes damaged you might need to replace the whole cluster.

However, this is a rare problem and it can be costly to replace the cluster. You should first check for wiring problems and use a scanner to scan the code.

How to fix RPM Fluctuation

You should seek immediate attention if you notice RPM fluctuations. If you are mechanically inclined, there’s no need to visit a shop unless you can’t complete the repair. You should check the Diagnostic Trouble Codes to determine the cause of the Check Engine Light being on. 

You might need to perform a simple tune-up in order for the vehicle to start up again. To see if the car runs again, you can replace the spark plugs and the filter. Some other issues are more complicated to fix and difficult to diagnose. To diagnose and fix a vacuum leak or transmission problem, you will require special tools. You might want to take these situations to your nearest auto shop.

What are the normal idle RPM values?

When your car is idling, there shouldn’t be any surging or fluctuating either. The idle speed should be between 500-1000. If you are used to driving your car, you will get accustomed to what’s normal for your particular model. 

One of these problems could cause a rough idle. You may also notice a defective idle control valve (IAC), that regulates the airflow at idle. 

How can you get lower RPMs?

Low RPMs and fluctuations in the engine could indicate a problem with your throttle body, spark plugs or air filter. It’s also possible that there’s a vacuum leak, which would create misfiring situations too.

Follow the steps above to diagnose. You don’t want to ignore the low RPMs because it could quickly cause engine stalling. 

How can you get higher RPMs?

There could be many issues if the RPMs rise above the normal level or to the red limit. The most obvious thought is that there is a transmission problem, but you don’t want to overlook simpler issues. 

First, perform a tuneup to determine if it resolves your problem. You will need to perform more detailed diagnostic procedures after that. 

Estimated cost to fix engine RPM fluctuations

Costs for repairs vary depending on the problem. A DIY repair that may resolve the issue could cost you $50-100. 

You might also consider more extensive engine repair options. A replacement of the throttle position sensor might cost between $150 and $250. The crankshaft sensor replacement may be around $175 to $255. 

Repairing a leak in the vacuum or changing the transmission could cost you a lot of money. If your car isn’t worth a lot of money, it might be time to get something newer.

Related: What Does A Tune-Up Cost?