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Why does my RPM go up and down while driving?

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What does RPM exactly mean?

RPM simply stands for revolutions per minute. It is a measuring unit for how fast a machine is working. In the case of cars, it means the number of revolutions the crankshaft in the engine makes per minute and how many times a valve goes up and down a cylinder.

The RPM increases as we press the accelerator in the car and so does the power. But this happens only until a certain point. It must be noted that it is not obligatory for an engine to produce its maximum power when it is at its highest RPM. It depends on a variety of factors both internal and external.

One of the instruments in the dash of the car is a tachometer. It does nothing but indicate the current RPM of the car while it is in drive or idling. At the top of the tachometer is a red line which indicates that revving the engine post can cause serious damage to the vehicle itself. This is a concern related only to manual transmission cars. Auto transmissions or CVT transmissions do not really face this problem but it also depends on how hard the accelerator in the car is pressed.

Flooring might lead to different results. Automatic transmission or CVTs have increasingly made driving a much better experience and help in pulling out the best from the engine and increase fuel efficiency as well. In these cases, the tachometer is nothing but a fun thing to have on your dash.

Manual transmission car drivers have to master the skills themselves so that they do not cross the redline and damage the engine considerably. Automatic transmissions prevent such things from happening but it is always tempting to go faster and again it depends on how the driver wants to drive their vehicle.

Abnormal RPM activity and what they mean

What I have learned over time is that small issues that we tend to ignore are finally responsible for something big. This happens in every aspect of life, and as well as in the case of cars. People tend to look over small issues and might not even notice them but they are the predecessors of big issues. A car’s tachometer might fluctuate when they are in idle mode or drive. Here are some of the issues which might be a reason for this to happen:

  • Spark Plugs: It might be possible that the spark plug needs a replacement. This happens mostly in older cars. Tuning up the engine might also boost the performance in a great way. Good maintenance of components like spark plugs or distributor plugs will make a big difference as time goes by. They might not seem to be so important but they are, no matter what you believe.
  • Vacuum Leaks: If the fluctuation in RPM seems to be abnormal it will be wise on your behalf to check for vacuum leaks in the car. This happens in the engine bay and you might not have the expertise to handle this yourself. Take it to the mechanic. If the leak is big enough, then it is quite possible for you to hear it.
  • Issues regarding the engine: If your car is behaving abnormally while in drive couple with rough engine idles and abnormal vibrations, getting the engine a proper diagnosis is what you shall be looking at. This is not a small issue so get it checked as soon as possible.
  • Multiple misfires might also be the reason for the engine to surge. The tachometer just shows what the engine does.

Some of the things you can do to identify the problem in the car

The engine surge which makes the RPM go up and down might be caused by a defective or stuck idle air control valve or the IAC. You will have to check if the IAC is working or not.

The vacuum line to the valve must be removed. If the engine runs smoothly after this, the IAC is defective and must be replaced. If there is no difference you must unplug the harness and follow up to see if there is any power in the IAC. Check the resistance from the plug to the IAC to the pins of the computer if it lacks power. If the IAC is in proper working condition, then the problem must be with the throttle pedal sensor position. This either needs to be calibrated or replaced.

To calibrate the sensor:

  • Turn the key on with the engine off and floor the gas pedal slowly.
  • While holding the gas pedal on the floor, turn off the key.
  • Again, turn the key on and put the gas pedal to the rest position.
  • If this does not work, turn to professional help.

Things to do if your RPM drops to zero:

  • Check the fuel pump.
  • Clean the throttle body.
  • Clean your injector.
  • Check the RPM sensor.
  • Check your car emulator.
  • Check for gas leakage.

Final Thoughts

Well, there are quite a lot of reasons for your car’s RPM going gaga or falling down without reason as we can see from the article.

The cause might be a small one such as a spark plug or something as big as an engine misfire. Taking it to the mechanic is the best thing you can do but before you do that you can try the things mentioned above. The faster you can identify the problem, the faster you can fix it and get the smooth ride back that you have been craving for!

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