Does Engine Oil Expand When Hot? Informative Guide


If you have tried sourcing for information on how to check oil levels, you may have seen diverse opinions on what engine conditions to do it. For example, while some say you do it when the engine is cold, others opine doing it when it is hot. These opinions were orchestrated for specific reasons, with one being an accurate reading.

Many believe a hot engine might give a false reading because hot engines make oil thinner. Thinner oil will usually increase in quantity. So chances are you may have heard people say oil expands when hot. So does engine oil expand when hot? Let’s find out.

does engine oil expand when hot

Does engine oil expand when hot?

Yes, like most liquids, engine oil expands when exposed to high temperatures. However, while engine oil will expand when hot, it is limited.

It may start contracting instead of expanding if exposed to an extremely high temperature and becomes too hot. So for those asking, does oil expand when hot? Now you know. But what are the basics behind the expansion of engine oil when hot?

What causes motor oil to expand when hot?

We have stated above that engine oil does expand when hot. But what causes motor oil to expand when heated? Liquids have molecules. So when exposed to heat, it takes up more between these molecules. As for engine oil, its molecular bonds are weak, so they always give in to heat, resulting in expansion.

And the theory behind this is thermal expansion caused by kinetic energy. While thermal expansion is an increment in the volume of liquid due to heat, kinetic energy is the energy generated during motion. So let’s see how thermal expansion and kinetic energy expand engine oil when exposed to heat.

While our cars are driven, the engine gets hot. And at this point, the oil in the oil pan goes through the oil filter and circulates to all moving engine parts that need it. However, while these parts keep moving, they get hot, invariably heating the engine oil. Sometimes you will notice more smoke from car due to engine overheating.

The energy generated by these moving parts (kinetic energy) gets converted into heat energy and increases the engine’s temperature. This motion and increase in temperature allow the oil to flow effortlessly. Hence, creating a kinetic energy space between the molecules of the engine oil causes the oil to increase.

Does engine oil level increase when hot?

Engine oil has a weaker molecular bond than solid, making them melt faster and thinner when heated. The lighter they are, the more the quantity. So yes, as engine oil gets hot, its level increases though the difference isn’t that much. This is why most people often opine that you check your oil level when the engine is cold.

Cold oil will tell you the exact level and help you avoid burns a hot engine might impact. But when the oil is hot, it increases a little, giving a false reading. However, some people still recommend checking the oil when it is hot. So should you check car oil when hot or cold? It depends.

While cold Castrol Oil will give you an accurate reading since it’s in its natural state, checking it when it’s warm might be ideal, especially when checking oil in cold weather. This is true because a freezing oil contract makes it lesser than its supposed state and can also give a false reading.

And if you top the oil to its max in this condition, it might get past the max line.

when the engine becomes warm and pours down the oil pan.

Is it normal for oil dipstick to be hot?

As you drive your car, the engine oil runs through the engine parts to lubricate and cool them. Since the oil collects heat, it becomes hot.  Because the dipstick also sits in the oil, it is normal to be hot. However, it should also cool down as your engine cools down.

Does oil expand when cold?

Oil does not expand when cold. In cold temperatures, oil gets denser or thicker.  This is one reason you experience a slow flow of oil across engine parts when it’s cold, which often limits car performance. Oil contains the coefficient of expansion and can only expand when it’s hot, not cold.

How to check your engine oil level?

check engine oil when hot or cold

Checking oil levels is the best way to ensure you’re running with your oil at the right level. This ensures all engine parts get the correct quantity of oil when needed, which aids optimal performance. So here is an easy way to check your oil level.

  • Park your car on leveled ground. An uneven surface might lead to a false reading.
  • Locate your dipstick; this is usually at your engine bay.
  • Take out the oil dipstick and wipe off the oil with a clean cloth.
  • Return the oil dipstick and pull it out again. The oil level should be between the maximum and minimum mark but very close to the maximum mark as possible.

While this might be quite an easy job, many are often confused about the appropriate time to check oil levels for accurate results. The question is, should I check engine oil when hot or cold?

You can check it when it is cold; you will get a good result since it’s in its original state and the oil has had time to settle in the oil pan. Also, check it when cold for safety reasons, especially if you’re not too inclined with cars. This is to avoid burning yourself from hot engine components.

However, you can also check it when the engine is warm, especially when it’s freezing outside. If you are doing it during extremely cold weather, let the car run for about 2 – 3 minutes. This will give you a good reading and won’t be hot enough to cause burns. In cases where you just drove the car, allow it to sit for 20 – 30 minutes to avoid burns. If you change engine oil regularly, you will get excellent performance of your car and can avoid unwanted engine noise.

Conclusion

This article has answered the question, does engine oil expand when hot? A recap! Yes, it does expand because engine oil has weak molecular bonds that easily give in to heat. Thus, ultimately increasing the oil level a bit. One must be careful when checking engine oil levels to avoid a false reading.

You will get a good result when the oil is cold. Also, check when it’s cold, especially if you’re not experienced with cars, to avoid burns. However, you can also check it when it’s warm. So why check engine oil when warm? You will get a better result, especially when it’s freezing outside. Note that a warm engine is entirely different from an extremely hot engine.

Solomon Osuagwu

Osuagwu Solomon is a vetted auto mechanic with over ten years of experience in the garage and five years of experience as a service writer. He prides himself in writing accurate information on professional repair guides, DIY repair guides, buyer’s guides, comparisons, and car reviews. If he’s not in his repair garage, he’s writing automotive blogs to help car owners and fellow mechanics to troubleshoot and proffer solutions to several car problems.

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