Why do you check the engine oil when it is hot?

[wemorgan]

I feel I should know this........shame on me I don't.

Why do you check the engine oil when it is hot?

My Smart ForTwo recommends warming the engine to full normal operating temperature, waiting a few minutes, then using the dip stick to measure the oil.

But why if you have to wait a few minutes for the oil to settle back to the sump does it need to be hot?

How is this different to when the oil is cold?

Does the oil volume change with temperature/viscosity? My gut feeling is that this seems unlikely.

Any information much appreciated.

Thanks.

[10:45PM]

Re: Why do you check the engine oil when it is hot?

The volume does change when hot.
It's a more repeatable measurement when it's warm and ensures it isn't affected by ambient temps (You could be in the bahamas or russia checking your oil level but if the engines warm the engines warm.)

[wemorgan]

Re: Why do you check the engine oil when it is hot?

I like the explanation of it being a repeatable measurement.
Thanks.

[wemorgan]

Re: Why do you check the engine oil when it is hot?

I did some research and found:

change in volume=(coefficient of volume expansion)(initial volume)(change in temperature)

(coefficient of volume expansion) of water = 0.00021
(coefficient of volume expansion) of oil = 0.00063

an online calculator gives the expansion of oil between 10C and 80C as 4%. I'll do some hand calcs to check.

[BBE]

Re: Why do you check the engine oil when it is hot?

I have done the measurement many times both warm and cold, and never found any difference. The only reason I can think of for measuring warm, is that the oil level on the dipstick levels much quicker (considering that the measurement is in fact done inside the dipstick tube).
Bjorn

[wemorgan]

Re: Why do you check the engine oil when it is hot?

A 4% increase on the Smart car's oil volume is only ~0.1L, so yes, hardly noticeable.

[donnaj8887]

Re: Why do you check the engine oil when it is hot?

I feel I should know this........shame on me I don't.

Why do you check the engine oil when it is hot?

My Smart ForTwo recommends warming the engine to full normal operating temperature, waiting a few minutes, then using the dip stick to measure the oil.

But why if you have to wait a few minutes for the oil to settle back to the sump does it need to be hot?

How is this different to when the oil is cold?

Does the oil volume change with temperature/viscosity? My gut feeling is that this seems unlikely.

Any information much appreciated.

Thanks.

Thanks you for the post.


__________________
Admin edit.....Spam link removed, don't do it again

[Thrumbleux]

Re: Why do you check the engine oil when it is hot?

Foaming.

[MrPoo]

Re: Why do you check the engine oil when it is hot?

Foaming.

Now, I've been wrong on this before, so to clarify...

Technically temperature has little to do with it because the expansion of the substance with heat is negligible, however the motion of the crankshaft causes foaming, which obviously causes the substance to expand, therefore you let the engine get to temperature to allow the oil to foam to it's greatest mass, ergo giving you an consistent/accurate representation of the volume of the oil whilst the engine is under load?

Sorry, I've only got to chapter one of my new book so I've got a lot to learn :redface:

[Thrumbleux]

Re: Why do you check the engine oil when it is hot?

Now, I've been wrong on this before, so to clarify...

Technically temperature has little to do with it because the expansion of the substance with heat is negligible, however the motion of the crankshaft causes foaming, which obviously causes the substance to expand, therefore you let the engine get to temperature to allow the oil to foam to it's greatest mass, ergo giving you an consistent/accurate representation of the volume of the oil whilst the engine is under load?

Sorry, I've only got to chapter one of my new book so I've got a lot to learn :redface:

Spot on. And if filled to top with cold oil, by the time the engine is running at temp the oil will have foamed to the point of overflowing. (Not just under load - when running at all)

Foaming made worse by overfill as running closer to crank and the turbulence created by a spinning crank can be enough to lift the oil and drag it around with it (windage).
So oils have an anti-foam additive package - which is dirt. Silicons and silicates (IIRC) which are physically sharp enough to burst the bubbles of foam.

[mikiew]

Re: Why do you check the engine oil when it is hot?

Surely it's so the oil isn't just hanging around the sump?

[Thrumbleux]

Re: Why do you check the engine oil when it is hot?

When it could hang out with Burt Lancaster?

Anyone else for joining the circus?