Why does the oil level change at temp?
 

As a new owner of an 87, I wondered why the oil level would change so much with temperature. I know that things expand when they are heated but I cannot believe that the oil expands so much as to make the gauge change. I speculate that once the care warms up, oil starts flowing to the remote oil cooler and this may make the level in the tank increase. This still seems strange.

Anyone know for sure why there is so much variation in the oil level engine cold to engine warm?

Because it's expanding within a closed -- and pressurized -- system, and because there's a total of about 13 quarts of oil in there.

It may not even be that the oil has to expand very much to show a diffrerence on the gauge. A sending unit could be constructed to indicate a full oil tank after only 15 degrees - or any other amount - of float travel (I don't know how much travel the oil tank sender really does have). Just pointing out another variable for which you must account.

Another reason for waiting until the engine is warm to check the oil level is to be sure that the sump is dry as intended.

Think Hg. thermometer

It isn't oil expansion with heat that is the issue here. It is oil flow, or a severe lack thereof, when cold.

With a warm engine, the oil quickly drains back down into the bottom of the case where it can be picked up and pumped back into the tank quickly.

With a cold engine, it takes longer for the oil to drain back from the top-end into the case.

This simply means that when the oil is cold, there is more oil in the engine, and less in the tank.

And, as alluded to in earlier posts, the gauge only registers a small percentage of oil. So if the engine to tank level differential is, say, 2 quarts between hot and cold, this could be the difference between the gauge reading half, and the gauge reading low in the red.

:confused:

Why would there be more oil in the case when the motor is cold?

Doesn't the scavenge pump keep most of it out of the case and in the tank when the motor is running?

When the thermostat for the external oil cooler opens up, the oil in that circuit then goes to the tank, increasing the level.

(OK....tell me I'm FOS!)

Because Doug, in order for the scavenge pump to pump the oul back into the tank, it first has to drain from the heads back into the case.

Also, the cold oil has to flow down through the strainers in the tank before it can be read by the level float. When it is cold, it takes much longer to get to the bottom of the tank, so effectively you have a lot of cold oil being well distributed throughout the system.

Think about it this way. Stand in a big bucket that has a level float in it. Fill it half way up with honey. Now have someone use a pail to scoop the honey out and pour it all over you constantly.

Now do the same thing with water.

Which one do you think will register the highest level in the bucket?

This has the makin's of a good 'un, Tyson! :)

My Mobil 1 oil is *not* like honey when the motor is cold. I would respectfully suggest the flow is not that sluggish, especially when the motor has been running for a few minutes (but not yet opening the external t-stat).

But what about my contention of the oil in the external cooler circuit. There must be 2 or more quarts there. When the t-stat opens it must add to the volume in the tank. It is only when the t-stat opens that the gauge begins to read.

Well then how do you explain why my '72 does the same thing? It doesn't have an external cooler.

The deal here is that the level reads increasingly higher as the oil heats up. The reason the level reads lower when cold is the same reason it reads lower when revved up. (Which is why you take the reading warm at idle)

The pump is positive displacement and is forcing it up into the engine.

However, the oil trying to make it's way back to the scavenge pumps' pick-up tube, is relying entirely on gravity, which is at the mercy of viscosity. The higher you rev the engine, and the colder the oil is, the higher this differential is.

Now that makes sense, Herr Doktor Schmidt.
I will now go and wash off all this honey! ;)

Sweet. ;)

Tyson, you're scaring me. I'm sure it was a lack of sleep that caused you to omit the part about Catherine Theta-Jones standing in the bucket with the honey bath instead of Doug. We'll let you slide this time.

Doug, as for the external thermostat, I believe there is always oil in the external thermostat, especially when it is open. So when you shut the car down some of the oil may flow out, but probably not much. When the oil gets hot enough to open the thermostat the oil that is in the cooler circulates into the engine, but then some of the hot oil circulates into the cooling circuit. From that point of view there should always be the same amount of oil in that circuit or possibly even a tad less when the oil cools and the 'stat closes.

Otherwise, I have to say I learned some things from this post as I always thought it was more of a heat induced expansion that caused the oil level to go up when hot.

I stand corrected as well. Brainiac earns his moniker again.

Now I just need to shake that image of Doug standing naked in a barrel of honey. :(

Now, Gentlemans, Gentlemans...who said "naked"???
:D

Well it's fairly apparent that Jack couldn't imagine standing in a bucket of honey any other way...

...then the initial question is, why did Tyson say "honey" rather than Motor oil?

(tongue planted firmly in cheek) :D

Its 6:00 am and now I have to start my day thinking about two guys pouring honey on each other.Bad visual but really funny post.:eek:

Oil expands about 10% over the normal Porsche temperature operating range.

When the oil is cold the tank registers about 1.2 quarts low.

Thats is why you check the oil "at temperature" with the dip stick.

Ken

I agree with the expansionists. Part of the confusion is that there are 2 influencing variables operating at the same time, 1 is the expansion 2 is the draw down due to the pumps increasing demand as revs build. M1 is not thick at any summer temp and flows as well at 70 as at 200. If you compare levels at the same rpm only changing the temp then the cold will be lower than the hot solely due to expansion.

The effect is magnified by the narrowness of the tank relative to its vol.

Then there is the issue of the oil thermostat.....

One would presume that the t-stat would be in the same state at the same temp. Just MO

OK. I'll try to explain this a little better this time. Here goes.

The oil level is a measurement of how much oil is in the tank ONLY.
So what determines how much oil is in the tank? Simple. The ratio between how much oil is being sucked out to how much is being pumped back in.

Now, we know both pumps are positive displacement pumps right? OK. But they are different sizes on the '77 and later engines. The pressure pump is bigger than the scavenge pump. '76 and earlier are equal size. But let's assume for now that the amount being pumped out is the same as that being pumped back in.

Well, thermal expansion contributes to this somewhat, but at 5-8 quarts of oil in the tank, we're only talking about .5-.8 quarts difference at 10% expansion from cold to hot. So where does the rest of the difference come from? (The level reading on these cars drops an indicated 3 quarts or so between cold and hot.)

The lag in flow back to the bottom of the tank where it can be measured by the sending units float.

Where does this lag come from? From the multiple layers of oil strainers in the top of the tank that are there to remove air bubbles from the oil.

The oil is being drawn out at a rate consistent with RPM. It is being pumped back into the tank at a rate consistent with RPM, provided there is enough oil drained back into the case for the scavenge pump to pick up.

So, the oil level should read consistent with RPM, right? But it doesn't. Why? Because the oil being pumped back into the tank is dumped at the very top, and relies on gravity to pull it down through many layers of strainers before it makes it to the bottom of the tank, where it can be measured. The thicker the oil, the longer it takes, and the lower the oil level reading.

And yes, even Mobil one flows better when hot than when cold. Trust me.

Anyone who would like to witness this phenomenon first hand can come and look directly into my '72's oil tank with the cap off, and watch the pool of oil on top of the strainer get smaller as the engine warms up. ('72 is the only year you can do this with, due to the oil tank location, and it's filler tube orientation.)

Tyson.... how would you like to be a teacher with a class full of dummies? ;)

Hmmm.... My front mounted tank has no strainers, and I currently have no thermostat in oiling system, but my oil level goes up when warm also.

That's partly because of the drain back from the heads to the crankcase. The oil still has to drain back down the heads, through the oil return tubes and back to the scavenge pump. The scavenge pumps oil supply is inconsistent, whereas the pressure pump has a guaranteed supply, due to the large reserve of the oil tank. Same situation. Advantage: pressure pump. I'll bet your level is a bit more consistent than ours.

But why doesn't your tank have the strainers? It's pretty important to remove the aeration from the oil.

Not sure why you mentioned the T-stat. I'm not in on the T-stat theory. My '72 doesn't have an external cooler or an external T-stat either, and does the same thing.

Just to point out that opening the stat to cooler doesn't change level. Its a BIG tank, system holds more than 16 qts. There is a strainer type filter in line after the cooler, before the tank, hopefully that removes the aeration. I would guess that the volume of oil helps also.

OK, wow! That's a lot of oil.

Yes I totally agree on the thermostat thing. When the t-stat opens, you are simply exchanging hot oil for cold oil, so it's a wash. No change in volume.

With that volume of oil in the oil tank, yours may be 90% due to oil expansion alone. How much does your reading change from cold to hot?

I'm going to make a preemptive argument here.

Ok, let's say your engine is at 180 degrees and idling. The level on the gauge is at half. If you rev the engine, the oil level drops. The temperature of the oil hasn't gotten cooler, but the level dropped.

If the oil level change was due to thermal expansion alone, this shouldn't be the case.

We know that the level drops with RPM, and we know it drops when cold. The only thing these two have in common, is that they both have an effect on oil flow. The ratio between oil being forced at a constant rate, and oil dripping back into the case or tank. (And then there's all the cold oil clinging to things inside the engine.)