1984 911 Carrera oil issue

[Phinneas]

If my car sits for more than a few days, upon starting the oil tank level shows in the red for a few miles. Eventually it will slowly creep up to a full or nearly full level.
Whilst this happens oil pressure is fine.
My feeling is that the oil is draining from the tank into the sump through the scavenge pump. Is this possible? Normal?
Am I right in thinking that the only cure is to replace the scavenge pump which appears to require engine removal.
Looking in my Bentley manual I do not see any one way valve on the oil return to tank line, is there? Should there be?
Is this an issue I should worry about or just run the car daily to prevent the issue.

Thanks for any and all advice

Peter

[ben parrish]

That oil indication is normal…oil has to heat up to read correctly. It will also drop and rise with revs once warm. Remember, the oil level guage is notoriously incorrect. BTW, you don’t have an oil scavenge pump per say on a Carrera..may show that in a diagram but it is still just the main oil pump and based on you saying the pressures are good..all is good.
If you get a lot of smoke on start up, that would indicate excessive oil in the crankcase…do you have that issue?
I think you may be worrying yourself unnecessarily.
Have you had this car very long or another air cooled 911 in the past?
Look at this link from the late Grady Clay..
question re oil flow engine vs tank

[Schulisco]

The oil level gauge is barely exact. It's more a gauge to show the trend of the oil level change than a precise measuring instrument for the oil level. The one and only method to get to know the precise oil level of the engine is measuring via dipstick with warmed up and idling engine and car standing on an even and flat surface as the users manual says!

The aircooled 911 has a dry sump engine instead of a wet sump on most other cars (no matter if water or aircooled). The oil tank level only shows the range on how much the level may vary depending on car's operation conditions. On a wet sump engine you won't have an oil level gauge though. Some have an oil pressure gauge, but all cars have an oil warning light to warn a too low and dangerous oil pressure. This is mostly less than 0.5 to 0.25bar depending on the car. On aircooled 911 afaik 0.25bars. Refer the users manual of your car. It says that the oil warn light may flicker on hot days with hot engine on idle. But when it lights up and stays on then shut down the engine immediately to prevent serious engine damage! As long as your oil warning light doesn't light up all is fine!

This video compares the oil circulation with human body which is uncommon but it nails the head: On all conditions when more oil pressure is required (to get heat out of the engine and lubricate all bearings) the pressure must increase to increase the circulating volume. That causes the decreased oil level and vice versa. The vascular system in this picture is the oil tank as it is flexible and it decreases the volume to create higher pressure and it expands to lower it and taking up more volume.

https://youtu.be/p_LjKjJEHNM

Fluids aren't compressible. To increase pressure you need more fluid being compressed.
When the engine is cold the oil has a high viscosity. This creates more resistance against dynamic movements and this results in higher pressure too. Additionally you need this higher pressure to get the oil on every bearing. When engine and oil warming up, the oil's viscosity decreases and so the oil pressure decreases too. When idling, less oil is needed and most of oil is stored in the tank up to almost a full tank. When cruising oil gauge shows miedium oil level and revving the engine shows a low oil tank level but this doesn't mean that the oil tank is empty! There's still enough oil in the tank! Refer then to the oil pressure gauge: It climbs in parallel with approx. 1bar/1000rpm up to 4bar. Then oil pressure relief valves open in the engine to prevent damages and spilling oil due to too high pressure. Also don't forget when the thermostat for the front oil cooler opens, there is more room for the oil to expand which additionally lowers the oil level a bit.

You find many videos on YouTube explaining dry sump engines.

Thomas

[Ian Comerford]

I would add a simple piece of advice to Thomas’s detailed reply - always check your oil at the end of a drive, before you put the car away. You will then know that it is correct for when you next use it and don’t have to wait for it to warm up to check it

[Cairo94507]

I pull into my garage, set the brake, let it idle for a couple minutes and with the engine running, check the oil level with the dip stick in the tank. My dip stick is like new and it is difficult to read the oil level as the oil is usually nice and clean. I take a paper towel, lay it on my tool box and then lay the dip stick on the paper towel. When you remove the dip stick it will leave you a nice image of the dip stick and the corresponding oil level that is easy to see. The older I get, the more work-arounds I have to find.

[GH85Carrera]

Just totally ignore the gauge, until the engine is fully warmed up. After that, it is only a guide to oil level, when at idle for 60 seconds, on a flat surface

When the engine is cold, the gauge is useless.

[Phinneas]

Thanks everyone for their input
I have owned the car for 9 years, 8 of which it sat in my barn until I found time to get it running, which it has been doing so for the last 6 months or so.

Yes I get some smoke at start up but it soon disappears.

I do do the usual level check in accordance with the manual and all is ok. My concern was the drain down when not used for a period but it seems that's normal

I do know about dry sump motors I have one on my 1933 Morgan three wheeler and if the scavenge pump stops in the wrong place when you shut the engine down then the sump fills with oil over time. On the JAP engine its a hit or miss issue and is just the way its designed.

Again thank you for all input I can rest easy on this issue and worry about something else.

Peter

[Schulisco]

Originally Posted by Phinneas

Yes I get some smoke at start up but it soon disappears.

My theory on these mostly greyish smoke clouds on startup: They're caused by old detoriated fuel, especially when resting in the thinner injection lines. Today's gas mostly contains a fraction of ethanol which attracts water and let the gas detoriate over time. Since I add a fuel stabilizer on every fillup hardly no more smoke. When the engine burns oil the smoke is more blueish and it takes some more time to burn it than fuel caused smoke.

Thomas

[proporsche]

you can actually set up your oil level sending unit so it is synchronized with your in car gauge.All you have to do is adjust these 2 little arms on the oil level.One is for top and the other is for bottom at your gauge.By connecting the wires to it you can check on your gauge the level so it is the same.
Once you do it you will know the oil level while on idle-and yes it is always good to check manually at the dip-stick.
Ivan