Checking Engine Oil Level

maintenance
engines
mercury

#1

Must the engine/oil be hot in order to obtain a accurate reading of the engine oil level? My youngest son, who is studying to become a certified auto mechanic, insists that the engine oil must be hot. I contend that is a bunch of horse puckey…that the engine oil can either be hot or cold, as long as the vehicle is on level ground and, if hot, you allow sufficient time for the hot oil to drain from the engine into the oil pan.


#2

I agree. Hot or cold, In any case it is most important to actually check it and top it off if needed.


#3

I agree with you. Hot or cold, nomatter, as long as it has a few minutes to drain down. I’ve been doing it this way for over 40 years.

But let your son “strut his stuff”. Some faculty member probably erroneously told him this and he’s trying to show how much he’s learning. Encourage this. It’s a positive thing. He’ll learn soon enough.


#4

You are both wrong.
Engine oil should always be checked cold. Oil, like every other liquid, expands when heated. The dipstick in every single car measures the oil level when the oil is cold. I’m probably wrong about every single car, but what I mean is that a cold measurement is the standard, and it is universal, and there is no reason why anyone would ever make a car with a dipstick that measures the engine oil using any other standard.

Some cars have dual level indicators (sounds fancy but it means two lines on the dipstick) for some fluid level readers, simply indicating the correct level for both cold and hot cars. My Toyota provides this dual measurement for the automatic transmission fluid. It has a “hot” full line on the dipstick and a “cold” full line. There is a very good reason for this, and it is that most owners once assumed that the transmission fluid should be checked cold, like the engine oil. But that wasn’t the case, as the automatic transmission fluid is checked when the car is hot.
Anyway, trust me, check your engine oil when the car is cold. And unless you have two lines on your tranny fluid dipstick, check that fluid when the car is hot.


#5

I’m also on your side and agree that the instructor is veering off a bit.


#6

Joe and Mountainbike have “Top 20” ratings for good reason. Trust their advice…


#7

To the accuracy of the dipstick, it doesn’t matter. Much more important to check it on a level surface, a few minutes after shut down if hot.


#8

It’s not as simple as that.

Yes, oil expands when heated. However, so does the oil pan it’s sitting in. These effects tend to counter each other as far as dipstick level is concerned.

To know what effect heating the oil+oil pan would have on measurement level, you’d have to what the difference is between the thermal coefficient of expansion of the pan (which can, AFAIK, be either Fe or Al) and the oil.

As a practical matter, however, a typical car might have an oil capacity of 5Qt, and oil is typically not added until -1Qt, and a car can typically survive +1Qt overfill without damage. It then seems quite unlikely that a hot fill would be off by 20% relative to a cold one.


#9

Every Saturday morning, I perform “The Ritual” on my '98 Chevy S-10 4WD PU and '10 Equinox LT2 AWD. Everything is cold. I make a cup of coffee and head for the garage. Check oil, fluid levels, serpentine belt, tire pressures, brake-, turn signal-, backup-, head- and fog lights, battery voltage, wipers, hoses and look for anything out of the ordinary. Takes about 30 minutes (I’m an old retired guy so I go slow)


#10

If the oil had to be checked at a certain temperature, the owner’s manual would say so, as it does with transmission fluid when it has to be checked a certain way.

Young people who are studying will come up with all sorts of things. Experienced mechanics will make things up. If you can check it cold, you won’t ever get burned or even feel the heat. Don’t sweat every time.


#11

The most important piece of advice you got in these replies was from MountainBike, when he said:

But let your son “strut his stuff”. Some faculty member probably
erroneously told him this and he’s trying to show how much he’s
learning. Encourage this. It’s a positive thing. He’ll learn soon enough.

You will never notice any difference in the life of your engine whether you check your oil hot or cold. You will, however, see huge dividends if you show your son confidence that you believe in him.


#12

If the oil had to be checked at a certain temperature, the owner’s manual would say so, as it does with transmission fluid when it has to be checked a certain way.

Agreed.

Some owner’s manuals say to check the oil with a warm engine and some say to check the oil with a cold engine.


#13

Thanks to all of you who took the time to respond to my posting. I just wanted to make certain that there was no accepted norm that the engine oil must be hot prior to checking the level. The owners manual was of no assistance in addressing the issue. From a common sense perspective, it just did not seem to “click and clack” correctly with me.

My son is doing very well in the training/educating he is receiving at Ivy Tech in South Bend, i.e., he is a consistent honors student, and as been very helpful to me and some of my neighbors by doing good quality, low cost maintenance on our cars and trucks. If he runs into a problem, he has the Ivy Tech instructors to fall back on for advice, assistance, etc. He truly enjoys doing auto repairs and hopefully, this training will lead to him obtaining a life long hard skill and a reliable job when he graduates from Ivy Tech. He has been out of work for 2+ years, so this may be his last chance at “getting it right”. Again, thank you for your comments. They are truly appeciated.


#14

For the record, attached is the Coefficient of Expansion for engine oil. If the car takes 4 quarts and we assume that there’s a 170 degree F difference in temperature between ambient and full operating temp, the amount becomes the coefficient x 170 x 4, for a grand total of .2652 quarts (just over 1/4 quart).

IMHO 1/4 quart either way is insufficient to be conerned about. The important things is that the oil is checked regularly.

http://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html


#15

The change in hot vs cold engine oil is too small to worry about. On the other hand I would worry more about auto transmission fluid levels and temperature. The point about being on level ground is very important for some cars.