Why does the oil level change at temp?

amatow · 02-11-2002, 10:36 AM

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?

Jack Olsen · 02-11-2002, 11:14 AM

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

Rod Walter · 11:37 AM

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.

Kahuna · 02-12-2002, 08:51 AM

Think Hg. thermometer

Tyson Schmidt · 02-12-2002, 06:08 PM

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.

Doug Zielke · 02-12-2002, 06:22 PM

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!)

Tyson Schmidt · 02-12-2002, 06:32 PM

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?

Doug Zielke · 02-12-2002, 06:40 PM

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.

Tyson Schmidt · 02-12-2002, 06:51 PM

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.

Doug Zielke · 02-12-2002, 06:57 PM

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

Tyson Schmidt · 02-12-2002, 07:01 PM

Sweet.

masraum · 02-12-2002, 09:09 PM

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.

Jack Olsen · 11:06 PM

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.

Paul Franssen · Paul Franssen's Garage

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

cstreit · cstreit's Garage

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)

critter · 02-13-2002, 05:23 AM

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.

02-13-2002, 08:10 AM

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

Kahuna · 08:44 AM

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.

Joe Bob · Joe Bob's Garage

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

Kahuna · 02-13-2002, 10:26 AM

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