How to read the Temperature/Oil Level/Oil Pressure Gauges on an Air-Cooled 993/964

 

The 993/964 has dry-sump lubrication where: 

• The volume of oil in reserve is scavenged from the engine bottom and stored in an oil tank; 
• Where it is pumped and distributed to the engine; 
• The system holds about 12 quarts of oil in total and; 
• At oil changes just short of 10 quarts is drained and replaced if you follow the procedure to change your oil listed on this page.

Calling the engine air-cooled is a bit misleading as it is in large part oil-cooled by the rather large amount of oil circulating in the system. It even has a fan-driven oil radiator called an oil cooler much like the coolant radiator in a water-cooled engine. Being a dry-sump system the gauges related to oil are not just read directly at face value they need to be read under specific running conditions and a little interpretation is needed to arrive at what is going on with the oil system.

First of all, the system is dynamic: 

• Pumping oil at various rates dependant on engine speed and; 
• Fluctuating oil viscosity based on oil temperature. 

For these reasons the amount of reserve oil volume in the oil tank varies quite a bit by engine speed and oil temperature, 

Also both: 

• The oil volume gauge in the dash and; 
• The dip-stick; 

give a measurement of the oil volume in reserve in the tank, not in an engine oil pan as is the case in most other vehicles featuring wet-sump lubrication systems.

The dynamic nature of the volume of oil in reserve in the tank is why the oil level is measured under the following specific conditions:

• The engine fully warmed as on a 70F to 80F day after a twenty minutes drive, where the oil pressure gauge that was pinned to the top when cold has fallen down into its normal operating range between 2 and 2.5 Bar at idle.
• With the car on level ground;
• With the engine running at idle;
  
  
  
• Measured by way of the dip-stick, not the gauge. The dipstick is more accurate in absolute terms and the gauge only approximately correlates to its measurement. 

Once the behavior of the gauge is understood for a specific car it can be a leading indicator that one needs to check their oil level with the dip-stick. 

For example, in my specific 993 the gauge floats in in the red zone when the oil is properly filled via dipstick measurement.

Also, the entire scale of the dipstick and the oil gauge is a lot less than one quart in oil volume.

To measure the oil under the above conditions it is properly filled if:

• Dip-stick fill level measures at 1/4 of its measuring scale from the bottom;
• Overfilling is more of a problem than underfilling in a 993 as the system holds so much oil. Overfilling will result in excess oil being sucked into the air intake gumming up the works and possibly contributing to problems such as a clogged air injector system. Overfilling also can lead some to the conclusion that their car is burning oil requiring unnecessary engine work where no such condition exists. 
• Filling the oil into the proper measure fill range using the dipstick with a cold or not-running engine will result in grossly overfilling the oil;
• The fill level even under the proper conditions should never be "topped off" to the top of the dipstick measuring range. This will overfill the oil.

When properly filled under the above conditions the gauges should show the following approximate conditions;

 

How the gauges should approximately look when
 the engine is fully warmed on level ground at idle.

• Oil temperature gauge at approximately between 7:30 and 8:00 o'clock position;
• The oil level gauge at about 1/4 of its scale from the bottom, if properly calibrated;
• And the oil pressure gauge is between 2 Bar to 2.5 Bar depending on the viscosity rating of the oil and its exact temperature.

Even if the oil level is a little low so when fully warm and idling it just moves slightly off the dead bottom of the gauge's red section there really should be little concern about oil starviation due to the large amount of oil in the system overall and the less than a quart mesuring range of the gauge.

 Here is a Do -It-Yourself  How-To for changing the oil on a 993 without overfilling the oil tank and information about other 993 body fluids.

At speed above idle the gauges will show: 

• The oil pressure gauge does little more than tell you if the engine is oil starved if the pressure is not above 3 Bar above idle, it should be between 4 Bar and pinned at the top, most of the time;
• The oil level gauge is of no value and will likely just sit at the bottom above idle; 
• The temperature gauge if at or above 9 o'clock tells you your engine is running hotter than it usually does. This can be normal in hot weather (90F+) especially sitting in traffic or when extreme demands are being made on the cooling system such as in an autocross event just coming off a timed run or sitting in the run queue.
• When the oil temperature gauge is slightly above 9:00 the low-speed blower for the oil cooler in the front of the car should kick in and one should be able to hear it. If you are not sure if your oil cooler blower is working at low speed, a known issue with the 993,  see the bottom of this page to learn how to test its operation.

As one drives the car hard and it heats up further in warm weather:

•  The warmer the oil the greater the amount of oil retained in the oil tank and the oil level gauge reads higher at idle;
• The warmer the oil the lower its viscosity and the lower the oil pressure that will be read on the oil pressure gauge at idle.

So as the temperature gauge rises to 9 o'clock or beyond the oil pressure gauge can fall just below 2 Bar at idle and the oil tank fill gauge can rise to the halfway point or above. 

Oil viscosity will affect what the gauges read. If one uses a 40-weight multi-grade oil, say Mobil 1 brand FS 0W-40 in warmer climates (85F+), and finds their oil pressure regularly dips below 2 bar with a fully warmed engine one can consider a heavier-weight oil such as Mobil 1 FS X2 5W-50 at their next oil change. Both of these oils also have slightly higher zinc levels, of 1100 parts per million (ppm) versus. 900 ppm in other Mobil 1 oils. This higher level is recommended for our cars to protect the valve train, but not so high, as up to 1850ppm found in motorcycle or racing oils that may shorten the life of the catalytic converter.

That blue plume of smoke occasionally at startup:
In the normally aspirated - non-turbo car an occasional plume of blue smoke can be observed at startup. Some say it is due to the boxer layout of the engine where some crankcase oil seeps past the rings as the car sits idle and burns at startup. Others suggest it is the result of oil fumes vented from the case/oil tank to the intake that has condensed on the inside of the intake manifold and is pulled into the engine at startup and burns. In any event, an occasional plume of blue smoke is normal for these cars. If it is a constant issue the oil fill level should be investigated also a higher viscosity oil can be considered in the absence of other gross running symptoms, I would not worry about it too much.

Special Considerations for the Turbo model varient: 

If your 993 is the Turbo variant there are special considerations to avoid oil smoking at startup. One theory is when at rest some oil may backflow into the turbo lubricating system and can cause a small quantity of oil to leak internally to the turbos. This oil will burn at startup making for a blue plume of smoke.  Some argue that this is not plausible because at startup the turbos are cold and any oil present will not be hot enough to initially burn and cause immediate smoking. Another theory is much like in the normally asperated Carrera oil vapor is vented to the intake, condenses on the inside of the intake plumbing and is immediatly ingested and burns at start-up  and causes the plume. In any event here is what some have done to minimize smoking at startup.

• Switching to a heavier weight (20W-50W Multigrade) oil, preferably one with the higher zinc levels, > = 1000 PPM such as Mobil 1 FS X2 5W-50. I have come to understand this elevated zinc content reduces the scuffing of components in the valve train.
• Verifying the oil filters used have the internal valving to limit the back-flow such as the  Porsche branded Turbo specific filters, the valve can usually be observed through the oil passageway holes in the bottom of the filter.;
  
• Making sure the oil is not overfilled (lower 1/4 of dipstick), probably having the most influence.

•  There are also aftermarket oil line check-valves available claiming to address this problem systematically.

In Conclusion:

By filling one's oil to the proper level and observing the gauges one can infer an adequate oil volume is in the system and how hot it is.

Hope this helps,

Andy

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