Oil Question - but not another oil thread

[motogman]

I know/understand that the oil level in the tank of a 911 must be measured at idle after the engine is warm.

If I understand correctly this is to get the oil which drains into the sump area pumped into the tank so that the level is representative of operation. If measured cold not running, I believe the tank level will be low and one could overfill based on such an observation.

It would seem that it does not take long to pump the oil that drains into the bottom of the engine out and get the circulation flowing properly.

The question is how long do you need to run the motor before you can have confidence in the level measurement? Does the oil have to get warm? How warm? Warm enough to open the circulation to a front oil cooler? If so why?

[Flieger]

Check it after a drive. No sense wasting fuel idling to get it warm. It could be a long time. You want it at operating temperature- thermostats open, etc. The oil expands when warm just like most everything else and there will be a significant difference between level if you measure cold vs. warm. I would say 180F is warm enough, since that is about where my car runs when the outside temp is comfortable.

[motogman]

Flieger, you partially hit upon my problem.

The car right now is up on jack stands and I am doing some work on it. I let it run (mostly idle with some throttle blips and partial throttle operation) for about 10 minutes but I don't think it built much heat. Certainly enough to dry out the exhaust but I am not sure it is enough to open the thermostats. When I checked the level in this condition the oil was low.

If it is only the thermal expansion difference in volume that is of concern I am not too worried because I don't think that is very big delta in volume. I should be able to find a representative coefficient of thermal expansion to do the calculation.

Hence my questions...

Does the oil get pumped out of the sump pretty quickly and then the oil level should be representative (neglecting thermal expansion)? Or is it necessary to get the thermostats open for a good reading?

Which brings up the fundamental question about what is necessary and sufficient for a good reading?

[RSTarga]

The thermostat to the front needs to open to get a good reading. After an oil and filter change put 8 quarts in and drive until the thermostat opens 180 degrees. You really can't and shouldn't warm the car up stopped.

[motogman]

You really can't and shouldn't warm the car up stopped.

From experience I think I understand the can't part. It is hard to build enough heat idling to get the engine to operating temperature.

But I am interested in the shouldn't part. Is there some harm done with idle at cool temperatures? Perhaps carbon build up but this should be rectified with a brisk run but what other issue is there that would suggest that letting a car idle is a shouldn't do thing?

[M_deJong]

In my experience it varies by car. My 71T with its small oil tank will give stable oil level readings when only part warm, say 120-150 whereas my 74S 3.2 with its big oil tank and smaller scavenge pump wants 180+ before it stabilizes.

[motogman]

M_deJong,

That is an interesting observation. It raises the questions; what is the history of oil tank size and scavenge pump capacity?

Do you know if the sensors between the 71 and 74 are the same?

I suppose that oil viscosity, wear/clearances, hoses, operating conditions and other things are factors as well. A complicated system.

Which leads to your first comment that it varies by car.

[Jay Laifman]

This is something that should be checked on a regular basis while driving (by the gauge) and the dipstick too. So, if for some reason you do not know how much oil is in your car, and there is ANY doubt that it's not at least close to correct, I would strongly suggest just draining it and filling it with the right amount. Otherwise, you're just wandering in the dark, with danger at hand.

[motogman]

While I agree that draining and refilling is an option, it might be difficult to determine the right amount of oil when the car has a different motor and coolers etc. from an OE spec car.

I started this thread because I had put in the recommended amount of oil but it was not registering on the gage and since I had the car on jack stands I was not able to warm it sufficiently to have a good hot idle check condition. I now have it adjusted properly but it took about 2 more quarts than was originally recommended. Since I knew what I had put in it, draining it again would have made no sense.

One of the reasons these cars have a lot of oil is because a substantial portion of the cooling comes from the oil circulation. So a useful question is with low oil volume when does it become dangerous to the engine (not that you really want to try it but more from an overall understanding of the engine and oil system).

When the engine is not running some oil will drain back into the sump area which is clear by the oil that drains from the engine case when doing an oil change. The oil pump has two stages a pressure stage and a scavenge stage. The pressure stage takes oil from the tank and feeds pressurized oil to the engine. The scavenge stage sucks the oil from the sump and runs it through the coolers and the filter and back to the tank.

Given this understanding, it would seem that engine damage would only occur (1) when there is so little oil in the tank that there is not sufficient oil to feed the engine or (2) when the engine/oil temp gets too high due to insufficient cooling (either by too little oil or too hard of usage/temperatures. If there is sufficient oil in the system to continuously feed the high pressure pump and the engine is not run to high temperatures, I would think that there would not be any engine damage.

I don't recommend or want to run low, but again, this description is more to help understand the operating conditions and when harm can really be done.

[Jay Laifman]

Hmm. Good point. Well then all I can add is that my stock 2.4 with trombone cooler really doesn't register on the gauge and dipstick until at least 160-180.