where's the pressure?

mr.flat6 · 12-31-2001, 06:42 PM

where is the the oil pressure sensor located in relation to the entire oil circuit? When I start up cold the pressure is great but the oil tank level gauge is at red bottom. Eventually it gets to full top. I'm trying to understand how the pressure is made at startup if the oil tank is emtpy.
Anyone with insight into this?
Thanks!john

rstoll · 12-31-2001, 06:52 PM

There is 13.5 qts in there somewhere.

Early_S_Man · 12-31-2001, 07:39 PM

John,

You don't say what year or model your car is, but the '73 oil tank style was used in the rest of production years thru '89. Without a front cooler, total system oil capacity is 10.5 liters, and the upper and lower marks on the dipstick represent approx. 8.5 and 10.5 liters.

With an external front cooler, total system capacity is 13 liters, and the upper/lower marks on the dipstick represent approx. 11 and 13 liters.

For Sportomatic models, an additional 1.5-2.5 liters is added to the above numbers. For all models, '73 thru '89, the oil change quantity is approx. 10 liters.

The diagram below is for an engine without an external front oil cooler, 1974, to be specific ... and about 1.5-2.0 liters are considered to reside in the engine-mounted cooler, engine oil galleries, and plumbing. That means about 8.5 to 9.0 liters of oil are contained in the filter and tank when the oil level on the dipstick and gauge are at the top mark. When the engine sits overnight or longer, some oil ... 2-4 liters drains back into the sump, but there is still plenty of oil in the tank to supply the pressure pump when the engine is started!

mr.flat6 · 01-01-2002, 07:40 AM

thanks for the info. I have a 1981 911sc.
So, are you saying that even though the oil tank level gauge is at red bottom there still is oil in the tank?
John

john walker's workshop · 01-01-2002, 07:51 AM

pull the drainplug if you have any doubts. get a big pan ready.

Doug Zielke · 01-01-2002, 08:02 AM

The gauge only reads (approx) the top 2 liters in the tank. And is only accurate when the oil is at operating temp, and therfore, expanded in volume.
The dipstick shows the *actual* anount of oil in the tank at any given time. And as Warren suggests with the diagram, there is a lot of plumbing for the total oil fill to "hide" in.

masraum · 01-01-2002, 12:39 PM

As already mentioned the oil level gauge is only accurate once the oil is at operating temp, and idling on level ground. This much oil expands a lot as it warms, so until it heats up you won't see much on the gauge. Also if the engine is not running or is not idling (being revved) the oil is sucked out of the tank and pumped through the engine so the level goes down. Even if you get your car warm/hot and still see the gauge indicate in the red you may still have plenty of oil.

mr.flat6 · 01-01-2002, 12:44 PM

Ok guys! thanks for all the input! but.....I understand the oil tank supplies oil to be pressurized to the top end. Correct? If the tank is empty how can it supply anything?

Early_S_Man · 01-01-2002, 01:19 PM

flat-6 ... I think you are missing the point! If the gauge is reading at the bottom of its' range, and the dipstick isn't showing anything, there still can be 6-7 liters/quarts of oil in the tank! And THAT amount of oil can serve all of the needs of the engine while it is warming up! As soon as the engine starts ... the scavenge pump starts putting the oil in the sump ... back in the tank!!! The 'cold' oil flows a bit slower that oil at 190° F, but it does still flow, and it doesn't usually take more than five minutes for the gauge to show the level rising back in the monitoring range.

BTW, the last thing you want to do is fill up your oil tank so it reads near the top of the gauge or near the top dipstick mark! That is a SURE recipe for oil leaks and oil into the air box! The engine performs just fine when a fully warmed-up egine reads in the lower third between the di[pstick marks.