Why check oil at 'operating temperature'?

[Flup]

This might be a bit of a numpty question -- I know next-to-nothing about all that stuff under the bonnet.

On p298 of the handbook it says to check the oil when the engine is at operating temperature. Does this really make a difference? Does oil expand when hot, or is there another reason?

 

[Flup]

I posted this elsewhere and got this as part of the response:

In asking you to check when the engine is hot, the oil will not be pooled in the oil pan. Instead, it will have been fully circulated to all the moving parts that badly need that oil to function. As a result, you'll be checking the oil reserve that remains in the pan.

so if that's correct, it would suggest that checking the oil when the car cold will result in seeing a higher dipstick reading than you should.

 

[GTD184]

The answer to your question is contained in both your posts, although to gain an accurate reading the car needs to be left for 10 minutes or so after warming up and on level ground prior to an oil level check, this is due to oil capacity losses within the oil galleries, valve gear and turbo charger.

If you check the capacities section within the service handbook they will indicate an oil capacity with and without oil filter, with an allowance of half litre for the filter.

 

[joema2]

...On p298 of the handbook it says to check the oil when the engine is at operating temperature. Does this really make a difference? Does oil expand when hot, or is there another reason?

Yes oil expands when it is hot. A typical coefficient of thermal expansion for automotive engine oil is around 0.0007 per degree C: http://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html

If the oil capacity is 4.5 quarts (4.25 liters) and the cold to warm oil temp range is 50 F to 180 F (10 C to 82 C), the volumetric expansion would be 4.5 qts * 72 deg. C change * 0.0007 = 0.223 quarts (0.21 liters).

If the dipstick is calibrated to take 1 quart between min and max, checking oil levels hot vs cold could make a small but measurable difference due to thermal expansion of the oil.

Whether that is the reason for the "check when warm" advice, I don't know. They could obviously calibrate the dipstick to read correctly on a cold engine. However that could limit ability to accurately check the engine oil level when at operating temp.

They could put both hot/cold min/max marks on the dipstick, but that would probably confuse some people.

In general it seems higher performance engines have the "check when warm" procedure. If it's a family car there's obviously no problem checking it when cold -- it will never be on the race track. But if it's a sports-oriented vehicle, checking the oil level at operating temperature is more important.

 

[joshgreensauce]

Yes oil expands when it is hot. A typical coefficient of thermal expansion for automotive engine oil is around 0.0007 per degree C: http://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html

If the oil capacity is 4.5 quarts (4.25 liters) and the cold to warm oil temp range is 50 F to 180 F (10 C to 82 C), the volumetric expansion would be 4.5 qts * 72 deg. C change * 0.0007 = 0.223 quarts (0.21 liters).

If the dipstick is calibrated to take 1 quart between min and max, checking oil levels hot vs cold could make a small but measurable difference due to thermal expansion of the oil.

Whether that is the reason for the "check when warm" advice, I don't know. They could obviously calibrate the dipstick to read correctly on a cold engine. However that could limit ability to accurately check the engine oil level when at operating temp.

They could put both hot/cold min/max marks on the dipstick, but that would probably confuse some people.

In general it seems higher performance engines have the "check when warm" procedure. If it's a family car there's obviously no problem checking it when cold -- it will never be on the race track. But if it's a sports-oriented vehicle, checking the oil level at operating temperature is more important.

I agree, this makes sense.

BUT WHY THE HELL IS MY OIL LEVEL LOWER when it's at operating temperature?? It's at the top of the hash mark when cold, and in the middle when warmed up...don't want to overfill - what should I do?

 

[jimlloyd40]

I agree, this makes sense.

BUT WHY THE HELL IS MY OIL LEVEL LOWER when it's at operating temperature?? It's at the top of the hash mark when cold, and in the middle when warmed up...don't want to overfill - what should I do?

You responded to a guy who hasn't been in the forum for 7 years.

The answer to your question is when you check the oil level with a cold engine all the oil has drained back to the oil pan. When you check it at operating temperature the oil hasn't all drained back to the oil pan.

 

[joshgreensauce]

You responded to a guy who hasn't been in the forum for 7 years.

The answer to your question is when you check the oil level with a cold engine all the oil has drained back to the oil pan. When you check it at operating temperature the oil hasn't all drained back to the oil pan.

Alright that makes sense to me. I have seen a bunch of posts on forums where people were experiencing the opposite with the justification that with the higher temps, the oil expands.

With that then, I guess I'll top off the oil a little more if I'm in the middle of the hash marks currently when warm and on the top of the hash marks when cold.

 

[Strange Mud]

nice use of search.

I did some experimenting with this and checking 5 minutes after warm shut down and the next AM (car parked in garage) there was no realistic difference in the two readings. If you are in middle of marks no need to top off unless you want to.

Mud

 

[jimlloyd40]

Alright that makes sense to me. I have seen a bunch of posts on forums where people were experiencing the opposite with the justification that with the higher temps, the oil expands.

With that then, I guess I'll top off the oil a little more if I'm in the middle of the hash marks currently when warm and on the top of the hash marks when cold.

The oil doesn't expand.

 

[Strange Mud]

it will expand (slightly) but that is offset by the thorough drain/return to pan. Truth it's moot and within accuracy of reading.

 

[aaronc7]

My experience with wet sump is same.... cold oil reading/sitting overnight is virtually identical to 5-10 minutes after engine shutdown when hot. If anything, maybe a tiny bit higher on the cold engine sitting overnight as it has more time to all drain back into the sump.

My standard measure is when it's cold / sitting overnight as I find that is most consistent. May not be the case for certain engines or setups, dry sumps which have specific ways to correctly measure etc. If you had a big engine oil cooler without any sort of anti drainback valve then the hot engine, 5 mins after shutdown might be more accurate.

Sorry, long answer to a simple question probably

 

[ChrisMk77]

Alright that makes sense to me. I have seen a bunch of posts on forums where people were experiencing the opposite with the justification that with the higher temps, the oil expands.

With that then, I guess I'll top off the oil a little more if I'm in the middle of the hash marks currently when warm and on the top of the hash marks when cold.

Middle of the hash mark is good, better than full when cold.