Thermal expansion

[05foresterXT]

Do different oils have vastly different rates of thermal expansion? How cold does oil need to get to stop contracting? I have been using Rotella T6 in Subaru Forester, and I noticed that if I check the oil level hot, it is above the notch on the dipstick that is above the "full" mark. When I check it cold, it is between the "low" and "full" marks. It didn't seem to do this as much with Mobil 1 Extended Performance 5w30, and it also didn't seem to contract so much in the summer.

As long as the oil is over the "full" mark when hot, it should mean that I haven't lost any, right?

 

[chubbs1]

I am no physicist but I don't think thermal expansion should be that drastic. We would probably hear about it more often if it was an issue. You are probably a little low, I always go by the level of the oil when the engine has been off for 7-8 hrs. That being said, if you have no oil on hand, I would not be rushing out to top off. If you do have oil, put in enough to bring it to just under the full mark on the dipstick (cold). You are fine and it is a very good question. Probably over the heads of most on here and even the manufacturers of the lubes for that matter.

 

[Kiwi_ME]

Perhaps more likely than thermal expansion could be the extra volume is due to air bubbles entrapped in the oil.

 

[05foresterXT]

I'm reluctant to add more, because it hasn't ever read high when hot before until I added some oil when it read low in the morning.

I did a search, and apparently heavier oils do have more thermal expansion than lighter oils. My car's owners manual says to check the level at fuel stops.

 

[Eddie]

My Forester has two full marks. There is a line to show add & full at cold or normal running temp and a hole to show the max HOT temp level. To answer your question, yes, oil does expand & contract depending on the temp. There is no minimum contraction in the absolute sense. When the oil cold it should be between the full & add marks and when hot, the max should not exceed the second hot mark. I think this is explained in the owners manual which is a good read. Ed

 

[GMorg]

The marks on the dipstick are designed to measure the oil at operating temperatures - adjust oil level in response to checking when hot. On the other hand, the dip stick is not a high resolution measuring device and there is room in the crank case for significant error. Just try to keep the oil between the lines when hot.

 

[45ACP]

IIRC, it is not a good idea to check oil level when car is at operating temperature because it is not an accurate reading! Oil is still up in the head and is not fully indicative of total capacity! Now that said, i have noticed about a half-crosshairs (half quart, the crosshair is a quart) difference between hot and cold engine.

Always check engine first thing in the morning cod, unless your car has a wonky procedure (Mercedes Benz) to do it different. Even then id just check it hot or cold if i had an MB. if i cheke it cold, the worst it could be is overfull a little bit - not a big deal up to 1 quart. If hot then id get it closer, but id make sure its full on crosshairs, if not a tough over (full on crosshairs is fine.)

 

[SteveSRT8]

Originally Posted By: GMorg
The marks on the dipstick are designed to measure the oil at operating temperatures - adjust oil level in response to checking when hot. On the other hand, the dip stick is not a high resolution measuring device and there is room in the crank case for significant error. Just try to keep the oil between the lines when hot.


This one is right on. Anywhere between the lines is just fine.

 

[45ACP]

Originally Posted By: SteveSRT8
Originally Posted By: GMorg
The marks on the dipstick are designed to measure the oil at operating temperatures - adjust oil level in response to checking when hot. On the other hand, the dip stick is not a high resolution measuring device and there is room in the crank case for significant error. Just try to keep the oil between the lines when hot.


This one is right on. Anywhere between the lines is just fine.


Thought it depended on the car though universal advice was to check oil level first thing in the morning after car sat all night, that way all the oil is down in the pan/crankcase not up in the head with its heat holding some of it in the walls and the oil not yet dripped down from the head.

 

[IanInCalgary]

Different base oils will expand differently (e.g. conventional oil will expand at a different rate from different types of synthetic). The cubical expansion coefficient for conventional oil is about 0.0005 1/deg-F. If the oil temperature increases 150F the resulting expansion is 0.0005*150 = 0.075, or 7.5%. For a sump capacity of 5 litres (quarts) this amounts to 375 ml (12 fl oz). This would be noticeable on the dipstick.

It is similar to checking the automatic transmission fluid level. There are often 2 sets of level marks on the dipstick - one for cool fluid temperature; and one for hot fluid temperature.

Ian

 

[GMorg]

If someone can find an auto manufacturer that recommends checking oil "first thing in the morning", please share this example. I disagree that cold engine oil checks are "universal". Checking 6-10 minutes after shutoff, on level ground, is closer to universal advice from manufacturers.

 

[05foresterXT]

At this point GMorg's advice seems to go along with my experiences, and what it says in my vehicle owner's manual. I had for a long time thought that I had an oil consumption problem, which has stopped since I stopped adding oil when it read low when cold. It never was a problem in the summer time, but now that it's cold out at night, I believe the low cold level is inaccurate.

 

[cp3]

I check mine cold as well. I don't think the amount of expansion once hot will cause any issues. I keep it topped to the full mark on the dipstick.

 

[mechtech2]

There is a lot of expansion due to heat of the parts in the engine, not just the oil. This takes up volume, and will change the readings cold/hot.

 

[45ACP]

Originally Posted By: GMorg
If someone can find an auto manufacturer that recommends checking oil "first thing in the morning", please share this example. I disagree that cold engine oil checks are "universal". Checking 6-10 minutes after shutoff, on level ground, is closer to universal advice from manufacturers.


Im not going by advice in car manuals... im going by advice of my family that tinkers with their cars for years and likes to take muscle cars to the tracks on weekends (when they have whatever their latest project the like to fool around with is) etc etc has done for a long time. Of course ive checked oil hot. Perhaps the reading would be most inacurate IMMEDIATELY after shut off, when the oil may not even be a consistent line on the dipstick, but rather spots above the crosshairs.

We also dont follow car manual advice about oil recommendation, as to viscosities.

Nonetheless, if i can get to my car's owner manual, ill give it a look-see. It usually says "level ground," for obvious reasons.

Quote:
I check mine cold as well. I don't think the amount of expansion once hot will cause any issues. I keep it topped to the full mark on the dipstick.


+1

 

[Jim Allen]

Originally Posted By: IanInCalgary
Different base oils will expand differently (e.g. conventional oil will expand at a different rate from different types of synthetic). The cubical expansion coefficient for conventional oil is about 0.0005 1/deg-F. If the oil temperature increases 150F the resulting expansion is 0.0005*150 = 0.075, or 7.5%. For a sump capacity of 5 litres (quarts) this amounts to 375 ml (12 fl oz). This would be noticeable on the dipstick.

It is similar to checking the automatic transmission fluid level. There are often 2 sets of level marks on the dipstick - one for cool fluid temperature; and one for hot fluid temperature.

Ian


This is the winner post in this thread. An actual scientific explanation for what happens and how to measure it.

There is a 1/16-1/8 inch difference on the dipstick of my 5.4L hot to cold. For the final drive of my IH tractor(which holds 22 gallons) it's more like 3/4 inch.

I don't believe it's so important to be EITHER hot or cold but to be consistent in the way you check and have a baseline for that temp. E.g., after an oil change when you know you've installed the correct amount of oil, you observe what the cold level is, or vice versa. Personally, I like know what it should be with a full sump hot or cold so I can check it either way and have some idea of the true level. Level ground is a given in any case.

 

[Loobed]

My old 1984 911 would read a quart difference between ht and cold. My 911 sump was 12 quarts.

 

[05foresterXT]

I keep seeing people mention "hot" and "cold" as if they are the only two temperatures that exist. The point of my original post was that I'm interested in whether my car is still consuming oil or not.

In the summer time, the only time I could get a read where the oil sat on the "full" dot was when the engine was "cold." As soon as the fall temperatures started setting in, and we had sub 40 degree nights, I noticed low reads in the morning. I never thought that the oil would shrink to less than it was when I bought it, but at this point, it seems that is the only likely place it could be going. The "hot" dipstick readings have remained consistent from when I first filled the engine with oil.

 

[45ACP]

Originally Posted By: Loobed


My old 1984 911 would read a quart difference between ht and cold. My 911 sump was 12 quarts.





What oil weight did you use in that car.

 

[GMorg]

Since the OP wants to follow oil consumption and since there can be a wide range of ambient temperatures throughout the year and throughout the day, the only time he can see a consistent temperature and thus take the thermal expansion out of the equation is to check the oil near operating temperature. The variation in oil temperature within 10 minutes of shutoff is probably narrower than the within day variation at this time of year - assuming that 180-210F was actually reached during the drive.

My point of checking at a fixed temperature is so that you won't have to guess about consumption. My suggestion is to check near operating temperature, others appear to use a fixed time of day. If you live in NY and see -10F to 100F throughout the year, I still think that you are best off checking the oil at engine shutoff.