Why doesn't it mater if you check oil hot or cold?

driven2services

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Why is it that automatic transmissions and power steering systems allways have a hot and cold mark, but not the engine dipstick? They're all basically the same fluids aren't they? And the engine gets the hottest out of all of them. Just wondering... [Smile]
 
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23,591
The difference is simply negligible. I think it amounts to a difference of 1/8" on the dipstick on my car. More important is to wait at least 10 minutes or so before checking the hot oil, in order to allow the oil to drain back into the pan.
 
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palm beach
on my engine, the oil level is the same hot or cold. even if i shut the engine off and take the dipstick out and check without waiting at all it is the same level. perhaps some engines just shed oil quicker than others.
 

driven2services

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I know it's the same...but why? Why isn't it the same in a tranny? ATF is just basically a 20 weight oil isn't it?
 
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Central Valley, CA
Sometimes the oil will splash back up onto the dipstick and cause an erradic reading. It expands as it warms too. But i haven't a clue as to why there is such a difference between atf and oil, for instance.
 

Patman

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Guelph, Ontario
On my LT1 Firebird there is a big difference if I check the level cold, especially in the winter. If I check it cold, in cold weather, it will often show it being down 1/4 quart, but if I check it 10min after a hot shutdown, the level will show full. So to be consistent and to avoid overfilling, I always check it after a hot shutdown now.
 
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23,591
Hmm, the bigger the sump, the more obvious the difference between hot compared to cold oil level on the dipstick, I guess.
 
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Lake Anna, VA
My Mother's 98 3.8L Monte Carlo is the same way. It will show half quart low when checking cold. 10 mins after a hot shutdown it will be right on the full line. My 2.5L Wrangler @ LS1 will be the same whether hot or cold and this also goes for my brother's 4.0 Wrnagler.
 
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canada
I'm just sorta guessin' here. The motor oil sump has vapour space above it and the surface area of the entire oil pan size. Once the engine is shut off and the oil settled back to the sump, the volume difference due to temperature rise will cause the level to rise. This will be insignificant due to the area (V=LxWxH). In the tranny, the increase in volume has nowhere to go but up the dipstick.
 
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Daytona Beach
Actually, it DOES matter. The difference is the capacity of the sump. An automatic transmission can contain more than 15 quarts of oil, but an engine only 5 or so. If the oil expand due to temperature at about the same rate (which they do) you can easily see that the difference in a tranny would be three times that of an engine. Now squeeze the difference into a tranny sump and it become significant. Engines with five quarts or less capacity may have minor variations...usually many other factors affect the level more. BUT take that capacity to 7.5 quarts and design an engine that IS SENSITIVE to oil levels and once again it becomes significant. Follow the directions in the owners manual TO THE LETTER!
 
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Nothern USA
quote:
Originally posted by JohnnyG: snip.... Follow the directions in the owners manual TO THE LETTER!
The mark on the dipstick corresponds to the level at temperature listed there. I am not sure why transmissions seem to make more difference than crankcases. It must have to do with the ratio of surface area at the top to the capacity. Normally the oil level is close to the top of the oil pan, its widest part. A little more or less oil is spread over a larger area. I think the fluid level in the transmission is much higher, maybe reaching the point where the round top has less area than lower. 15 quarts of fluid are going to expand more than 5 quarts of oil. More expansion and less area will give more vertical rise.
 
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Daytona Beach
Yes, and don't forget that the bulk of a trannys fluid is contained in a closed space...the tourque converter, all of the expansion of THAT oil must appear as a change in level in the pan.
 
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