Car RPM Fluctuation While Driving

Causes of Car RPM Fluctuation While Driving

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If you’re driving a manual vehicle, the tachometer is a very essential tool that keeps track of your engine RPM when driving. Now, if you happen to inspect this gauge keenly, you’ll notice that there’s a zone highlighted with a red line. This is usually the shifting zone. Now, for you to enjoy smooth shifting, the engine’s RPM must stay consistent so long as you don’t increase or decrease the speed. But, even so, there are times when you might notice car RPM fluctuation while driving.

Now, why is this a major issue? You see, RPM is a very critical component as it tells you when to shift to a higher or lower gear. In case it’s faulty, the engine’s speed can get to the red zone of the tachometer range which can potentially damage the engine.

Since this problem is a potential cause for concern, it must be addressed as soon as it’s detected to avoid any further damage. So, in this post, we’ll be looking at what exactly causes car RPM to fluctuate and how the problem can be fixed.


But First, What is RPM?


Now, before we discuss why your car’s RPM keeps fluctuating when driving, it’s very important that you understand what it really means. This way, it will be easier for you to understand why it’s important to diagnose this problem as soon as it appears.

Simply put, RPM stands for revolutions per minute. It’s usually a measure of how many times the engine pistons go up and down as well as the number of rotations the crankshaft makes in the engine per minute. So, based on these parameters, it’s easy to understand why your car’s RPM shifts whenever you accelerate or decelerate your vehicle.

Now, for you to see how the RPM is shifting, carmakers usually add an RPM gauge on the dashboard. This gauge is known as a tachometer or a rev counter. While modern cars have reduced the number of gauges from the dashboard, the tachometer has remained to be an important gauge alongside the speedometer as it keeps track of the engine’s RPM while still serving as a crucial diagnostic device.


Why Does it Matter?


RPM matters a lot when driving. A tachometer is used to keep track of the RPM when driving. In most cases, this gauge has calibrations that allow it to measure engine RPM in thousands. When driving, you must keep track of the RPM to know when to shift to a higher gear.

You see, as you accelerate the car, the engine spins faster, which results in a higher torque or rather more power. But, the engine is programmed to have a maximum designed speed beyond which its power production starts to drop. So, by mastering these peak power revs, with the help of the RPM gauge, you’ll know when to shift to a higher gear to keep the engine smooth and efficient.

Lastly, you need to understand your vehicle’s normal RPM when driving. Although the range differs from one vehicle to another, the normal RPM of any vehicle when driving should range between 1,500—2,000 RPM. So, in case your tachometer is showing that your RPM is bouncing up and down when driving, then it’s clear there’s a problem.


So, What Causes Car RPM to Fluctuate?


Now that you’re aware of what RPM really is, our next focus is to discuss a few factors that cause your car’s RPM to fluctuate. Although your mechanic is responsible for inspecting and troubleshooting your vehicle, it doesn’t hurt to know some of these causes.


1. Faulty Idle Air Control Valve


The first possible cause of RPM fluctuation in your car is an issue with the idle air control valve or the IAC. Now, the IAC is tasked to control the amount of air getting past the throttle body when the throttle plate is closed. As a result, this controls the engine’s idle speed to ensure the engine stays within its normal rate even when more load is put on the engine.

In most cases, the IAC is controlled by the car’s onboard computer. Now, in case the IAC is faulty, this can cause miscommunication between the engine and the onboard computer. This can lead to rough idling, engine stalling, and the “Check Engine” light turning on unexpectedly. In extreme conditions, the IAC can cause your car’s RPM to fluctuate massively when driving.

How to Fix it

Now that you’re certain the problem is caused by a faulty IAC, there are various ways you can remedy the situation. First, the IAC might have accumulated carbon buildup since it rests near the throttle body. So, here, you’ll need to clean it thoroughly to remove any debris and carbon buildup.

Two, the IAC might be cracked or severely damaged in which case you’ll have no other choice but to replace it with a new component.


2. Faulty Crankshaft Position Sensor


Another possible cause of RPM fluctuation is a faulty crankshaft position sensor or the throttle pedal position sensor (TPS). Now, the TPS is connected to the engine crankshaft facing the timing rotor. It has two major functions, which are keeping an eye on the throttle pedal position and keeping track of the engine’s rotation speed by controlling the amount of air getting inside the engine.

Since the TPS is a very crucial component of your car, any form of damage can lead to severe consequences to your car’s drivability. For instance, you might notice acceleration issues, car stalling, RPM fluctuation, troubles starting the engine and engine vibrating when driving.

How to Fix it

Now, fixing the TPS is not as easy as it sounds. That’s because the TPS is located in a delicate position that will force you to disassemble various parts to get to it. Depending on your car’s model, the TPS might be located close to the main pulley, the transmission bell housing, or the engine cylinder block.

So, to fix the TPS, you’ll have to turn off the engine, disconnect the battery then disconnect the wire harness that connects it. Inspect the component for any signs of damage then repair or replace it depending on the situation.


3. Vacuum Leaks


Another reason that can point to sudden shifting of your car’s RPM is leakage on various hoses inside the combustion chamber. You see, for you to drive safely, several parts must work hand-in-hand to guarantee the efficient and smooth performance of the engine.

One of those components is the engine vacuum that’s responsible for maintaining proper fuel/air ratio for proper combustion. Now, in case the engine vacuum is loose, cracked, or worn out, you’re likely to face a lot of issues as the fuel/air ratio is likely to be compromised.

This can result in engine misfires, which can lead to rough idling, hissing sound, loss of power, car stalling, and possibly rapid RPM fluctuations.

How to Fix it

A vacuum leak is a very critical problem that demands professional inspection. So, in case that’s the cause of the problem, you need to take your car to a licensed mechanic to diagnose the problem and repair the component.


4. Worn Out Spark Plugs


Spark plugs are considered some of the tiniest components on your vehicle. While the engine vacuum is responsible for maintaining a precise fuel/air ratio, the spark plug is responsible for igniting this fuel/air mixture to create combustion.

Since the spark plug is among those parts that wear out over time, there are times when it might produce a lower electrical current that will be insufficient for ignition. When this happens, there will be inconsistent fuel/air mixture that will cause misfires.

Misfires will then lead to excess engine vibration, which will then lead to RPM going up and down when driving.

How to Fix it

To fix this problem, you’ll have to remove bad and worn-out spark plugs to replace them with new ones. In most cases, spark plugs are located in the cylinder head just above the cylinder combustion chamber. So, once you’ve located the spark plugs, simply take them off and replace them with new ones.


5. More Serious Engine Issues


Other than what we’ve discussed, there are other serious engine issues that might cause your car’s RPM to fluctuate. Here are some of the issues you need to be aware of.

  • Faulty Ignition Coils: Ignition coils and spark plug wires perform the task of transmitting high voltage to the spark plug to ignite the fuel/air mixture. If these coils/wires are worn out, then there will be an inconsistent voltage output, which will lead to misfires hence RPM fluctuations.
  • Dirty Fuel Injectors: Fuel injectors are used in modern vehicles to deliver fuel to the combustion chamber. In case they’re clogged or dirty, this can lead to rough idling of the engine hence sudden rise and fall of engine RPM.
  • Incorrect Cam Timing: This is when the timing belt is loose or off the gears by a tooth. Here, you’re likely to experience rough idle when the engine is running.



Although we’ve discussed some of the possible causes of RPM fluctuation, you don’t have to take matters into your own hands before you fully verify the problem. Other than RPM issues, you need to identify other signs in your car to guarantee the exact cause of the problem.

When it comes to fixing the problem, some of these causes can be fixed at home by any enthusiast DIYer while others should be assigned professional mechanics to solve them. But, regardless of the cause of the problem, this guide has discussed at least five major culprits and a few ways you can fix them.

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