Why Check Transmission Fluid When Engine Is Running? (5 Reasons Why)

If you know the basics of maintaining a car, you will know that you need to check various fluids within the engine, such as the transmission fluid. However, checking each fluid may take specific methods to check them efficiently.

You may be curious to learn why you should check your transmission fluid when your engine is running. We did the research, and here is what we found!

Why Check Transmission Fluid When Engine Is Running?

Transmission fluid should be checked while the engine runs because transmission fluid expands the hotter it gets. Therefore, you will be able to see how much transmission fluid you should add. More importantly, checking and adding transmission fluid to a warm engine will prevent you from overfilling the reservoir and creating foam.

Do you want to find out more reasons why you have to check transmission fluid while the engine runs? We made a list of facts below, so read on!

1. Avoid Burning Yourself

Firstly, you need to ensure your engine is running to make the engine slightly warm when you check the transmission fluid to avoid burning yourself.

Usually, most cars get as hot as 195-220 degrees Fahrenheit, but any hotter than that will lead to an overheating engine.

However, you should not try to check the transmission fluid at this temperature and only check it one to two minutes after you start your car.

Alternatively, you may wait five to ten minutes after shutting off your car to let it cool off and then check your transmission fluid.

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That way, your engine will be slightly warm, which is the perfect temperature to effectively check your transmission fluid without burning yourself.

2. Torque Converter

Generally, you would have to check the transmission fluid while the engine runs because of the torque converter, a compartment found within the transmission.

Typically, the fluid level in the torque converter will be higher than the fluid level in the pan when the torque converter gets full.

This is because the torque converter is the primary area in your transmission where the fluid is stored.

The fluid will drain into the pan to leave the torque converter when you stop your engine.

Therefore, you will not know how much transmission fluid is in your car unless you check the transmission fluid while the engine runs.

3. Measuring Total Amount Of Fluid

Measuring Total Amount Of Fluid

Usually, when people check their transmission fluid levels, they will also check the amount of other fluids present.

Any fluids that are around your transmission will be more present when the engine runs, including the torque converter.

For instance, transmission fluid and engine oil respond differently depending on how hot they are, so the engine will have to be running to measure them effectively.

Normally, transmission fluid is a bit sensitive to pressure compared to motor oil, so you will be able to see how much transmission fluid there is when the engine makes it move.

4. Transmission Fluid Expands

One of the essential reasons that transmission fluid gets checked while the engine is running is that transmission fluid expands.

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Generally, transmission fluid will continue to expand the hotter the transmission fluid gets.

How much the transmission fluid will expand will depend on the type of transmission fluid you have and how hot the transmission fluid is.

For instance, transmission fluid can expand 50% at 250 degrees F compared to the transmission fluid volume at 100 degrees F.

5. Prevent Foam

Another primary reason you need to check your transmission fluid while your engine runs is that it will help you from overfilling your car with it.

If you add too much transmission fluid to your gearbox, foam will begin to form because the gears in the gearbox will splash into the full reservoir.

Also, when the gears splash the transmission fluid around, the temperature in the gearbox will get strangely high and create a chemical reaction.

When there is foam present in the gearbox, the foam will affect the oil’s viscosity, which will then affect the efficiency of the transmission fluid.

Generally, this will make it more difficult for you to shift gears because the transmission fluid will not properly flow through them.

As such, the gearbox’s parts will start overheating from the lack of lubrication and excess friction, which can lead to transmission failure.

To know more, you can also read our posts on why cars depreciate so fast, why cars need oil changes, and why cars stall.


Generally, it is recommended that you check the transmission fluid while the engine runs because it is the best way to measure the total amount of fluid around the transmission.

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Moreover, transmission fluid expands the hotter it gets, so seeing how much transmission fluid there is while the engine is hot will keep you from adding too much transmission fluid.

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