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Engine Oil Level


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I am having a heck of a time checking the oil level in my 1990 C4. The car is an ROW and did not come with an owners manual. I do have the 964 "bible" that is very helpful but it does not answer my question. I had an oil change done at a reputable shop when I first bought the car. I noticed a small leak from the area of the oil tank. When I use the dipstick it does not show any oil. The guage on the dash showed a level but as I have no experience I am not sure what is normal.

I added a litre of oil thinking that it would register on the dipstick. It didn't. I ran the car for a day or so and the level on the guage read fairly high. I added another litre of oil and actually thought I saw oil on the dipstick but I must have been dreaming since it has never registered again. Now I know I have too much oil since the guage on the dash reads high all the time. I suspect that the only way to get it back to normal will be to drain the tank down and add back the 6 liters (??) that are supposed to be in the tank.

My real question is why I can't see oil on the dipstick. Is there an elbow in the tube inside the tank that directs the stick down into the oil? If so do you think it could break off so the stick would not touch the oil. I tried to unscrew the tube at the tank but it is seized and I did not want to break it off.

Any ideas???

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The crucial factor here is WHEN you check the oil with the dipstick. As I sure you appreciate, the 964 has a dry sump oil system which means that only the barest minimum of oil is in the engine at any one time, the main oil supply being in the tank.

When the engine is started from cold the oil path is kept short to ciculate a relatively small amount of oil from the engine through the filter and back to the engine. Only when that oil is sufficiently heated to open the thermostat (in front of the right rear wheel) does the supply path open up to include the oil cooler in the front right wing and the supply tank. The dipstick and, indeed, the oil level gauge, reads the level of the oil in the return part of the tank and thus will not register the level until the thermostat is open and the engine is idling for a minute or so.

The whole system capacity is around 11.5 litres but the dipstick will only register the last 1.5 litres (from bottom mark to full) and this is true of a correctly operating gauge as well. It is generally recommended to NOT fill the oil to the full mark since 11.5 litres of oil can expand quite considerably once it gets hot and any amount of enthusiastic driving (or sitting in a traffic jam) will likely cause the oil to expand over the full mark and start to cause other problems in the inlet tract.

If the gauge is working correctly (and it's not unknown for the sender to go faulty) it would ideally sit in the red during normal driving and rise to roughly half way up the scale (or, perhaps a little higher) once the thermostat is open and the car has been idling for a while on level ground. Any higher is not a major problem but you should be wary of the oil expanding too much under those conditions mentioned above.

If the gauge is pegged to the top of the scale with just the ignition on and the engine not running it would indicate a faulty dender or gauge. Rising to the top mark when idling is (just about) OK, but rising to much more than 1/3 to 1/2 while driving is a good sign that it's over filled.



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