Check fluid hot and cold


#1

I was wondering, why do you check your oil when the engine is cold, but you check you transmission fluid when the engine is runing and hot?


#2

I check motor oil when hot, but with the engine off. The crankshaft is blowing the motor oil around too much to get an accurate measurement when the engine is running. Cold may mean too much oil settled from filters and oil galleries.

The transmission fluid is checked hot with engine running so that the torque converter is full of fluid, thanks to the pump. With the engine off, too much fluid runs out of the torque converter for an accurate measurement.

All fluids should be checked with the car parked on a level surface. I also check the fluids when hot so that everything is fully expanded from heat expansion.


#3

Wow, I was always told to check the oil with the engine cold.


#4

I’ve always checked my oil in the morning except in the winter when I check it after driving. We had a thread on this recently, and I looked up the thermal expansion coefficient of motor oil and calculated that the total volume change between ambient temperature and full engine operating temp (I believe I used 70F and 210F) was just under 1/4 of a qt. Since I keep my oil level pretty close to the “full” line, I figure 1/4 qt is meaningless.


#5

I’ve always checked my oil when it’s cold. After all…you don’t add hot oil to the engine when the oil is changed. The difference between hot and cold is negligible so it really doesn’t matter.


#6

You can check it hot or cold. It matters very little (there was recently a thread about that). The margin for error is pretty darned big since the safe range on a dipstick is typically a quart or so. Nothing about where the oil is or its temp will change things enough to throw it off that much.


#7

These days, many owner’s manuals tell you to check your transimission fluid with the engine OFF but the tranny fluid warmed up.

I check my oil after the car’s been sitting for a long time because it’s easier, just pull the stick and look, you don’t even have to wipe it off.