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.

 

[stevesey]

As long as the car has been stood still for longer enough I can't see why it would matter. Obviously if the engine was briefly stared when cold it would take longer for the cold oil to drain back down. Interested in what others have to say.

 

[BTB 500]

I believe the volume increases slightly when hot.

 

[Satch]

Depends on the oil, but a "standard" 5W-40 oil can increase in volume by about 7% going from 15C to 100C.

 

[wemorgan]

On the Smart car forum that I'm a member of one member said that it makes the measurement repeatable. As in, when hot it will not matter whether you live in Africa or Norway. I quite like that explanation.

 

[lynall]

I cant see it making much difference but doesnt the smart only hold 3 litres? so probably more crucial than most.



Lynall

 

[BlackC55]

It is because of the amount it expands when hot.

Always check oil at full engine temp.

 

[wemorgan]

I cant see it making much difference but doesnt the smart only hold 3 litres? so probably more crucial than most.

Yes I believe it's about 3.2L, with 0.7L being the difference between min and man on the dip stick.

So my next question to all is: how much does oil volume change, for example between 10C and 80C? I'll have a search myself and see what I find.

 

[BlackC55]

Its quite alot really. Mobil1 seems to expand more too.

 

[wemorgan]

If water expands when frozen, I'd expect oil to contract when heated.

edit: I'm getting closer to an answer

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.

Just checked the numbers and 4% stacks up.....this rises to 6% if the oil is 100C.

 

[Will]

If water expands when frozen, I'd expect oil to contract when heated.

Huh?

Water expands in volume when it freezes (because it crystalises), but also in volume when it is warm (as does oil)!

Will

 

[wemorgan]

Huh?

Water expands in volume when it freezes (because it crystalises), but also in volume when it is warm (as does oil)!

Will

Yep I agree........oh the shame of online mistakes

The above equation shows volume change is proportional to temperature.

 

[stewarthp]

Hi Chaps,
The easiest way to explain why your oil level should be checked when the oil is hot is that this will give a more accurate indication of your oil level on the dipstick as the oil will be at normal operating temperature and therefore not liable to expand any further. It will also reduce the danger of over filling the sump assuming you don't add too much even when the oil is hot.

 

[lynall]

Meant to say always dip mine when cold and look for a level of about 5/6 mm down from the max mark.
So the smarts still have a dipstick then?


Lynall

 

[wemorgan]

So the smarts still have a dipstick then?

Yes, they could only take cost cutting so far and did at least give owners a dip-stick

BTW on the Smart car the % volume increase from cold to hot only gives a 0.1L increase.....quite marginal I'd say, but the hot measurement is at least repeatable.

 

[lynall]

Cant argue about that but after my leaky injector experience of the oil level rising im quite **** about checking the oil level, car parked in the exact same spot at work left for at least 5 hours and so far its been fine.



Lynall

 

[yachtman]

Does the smart have a side mounted oil filter ?

Perhaps then would be to ensure it was full, and or take into account the volume of oil in the oilways. With a low sump content, the filter etc content would become more important as to normal running level.