Which oil level reading is accurate??
 

I took my wife out on a date this afternoon and we drove the 997.1. The restaurant was ~30 miles away and the car was running for around 45 minutes (i.e. it was fully up to temp). The car sat for around 1hr and 15 mins while we ate. Prior to starting the car for our return trip home, I allowed the oil measurement countdown to complete (it took around 5 seconds). The oil reading showed about 1/3 full which equated to the 1st mark above the minimum level line. This caught me off guard because all my previous oil readings showed completely full.

The car has been parked in the garage for the last 6 hours so I went out to check the oil level again. The countdown was short (around 5 seconds) and the oil level is now showing at the full level mark. The engine has cooled down, but is still mildly warm.

So which one is the true reading? I've read you should take your oil level measurements when the engine is fully up to temp. However during this time, oil is still draining to the sump as was the case when we were at lunch. Since the car has been sitting for 6 hours now, I would assume all the oil has returned to the sump. Wouldn't this be the more accurate measurement? I don't want to add any more oil to the car if it means I'm overfilling the thing.

Sean

Happens to all of us, I find that just being consistent with checking helps. For me, doing it first thing in the morning works best- the oil is cold but it's all down there. I also will check it twice generally.

As long as I'm between the arrows I don't sweat it too much, but if I'm planning to add oil, I will try and wait until my next morning's reading.

I've had several occasions where I check during the day and it's very low, drive to get some oil wait the obligatory 10 (or more) and like you it registers full again.

I chock it up to owning a lovable beast with idiosyncratic tendencies.

I think that the lower reading is the correct reading. You stated that you got that reading after a reasonably long drive with the engine at full op temp. It is likely that that drive expelled (evaporated) moisture in the oil, moisture which made up the previous higher reading. Moisture in the oil occurs when the car is often driven short runs.

The measurement indicator really only displays the final quart of oil. It's not the entire oil capacity. Best practice for me is to check it often and if I get a lower than expected reading I make a note to check it again later and see. It varies a bit. I usually wait until it's half way down the indicator for two consecutive readings before adding anything. It's better to run a half a quart lean than overfilling.

I find the 997.1 oil measurement system to be annoyingly inconsistent. I check and record my oil level before I start the car in the morning. I find it generally will cycle between two ticks on a random basis. I've on occasion had it go from 3/4 full to 1/4 full to full back to 3/4 full over a 4 day period. Same time of day were recorded (first thing in the morning). So, I typically try to get it to give me repeated readings at a level before I decide to add any oil. I'd say a single 1/4 full reading is nothing to sweat. If you get it more often than not, consider adding some oil.

If what you are saying is true, that would be a heck of a lot of moisture in the oil (3 tick marks on the oil level reader = 1.2 L). I doubt my engine would be running at all if that were the case.

The bottomline is my car reads full after sitting for long periods of time, but reads low (but still within range) after recent drives. I just wish the owners manual was more clear about this subject. I thought the whole purpose of that annoying oil countdown (I've seen as high as 30 minutes) was to allow sufficient time for the oil to drain back into the sump. So I would have to assume the Porsche engineers want the oil readings to be taken after the car has been sitting . . . the longer the better. So that would be consistent with what Comanche_699 said in post #2.

Sean

My observation is consistent with yours. The reading is always one notch higher if my car has been sitting for few days than if it has been sitting only several hours. This must be the result of a long process for all of the oil to make its way to the lowest point after the engine is turned off. Therefore, the higher reading is more accurate in my opinion. Also, I always drive my car for more than one hour so I am not buying the fuel dilution/water accumulation explanation.

With 10 quarts or liters of oil in our cars, that equals about 2.5 US gallons of oil in a 997.


10 liters of oil is heated from 80oC to 180oC. The volumetric expansion of the oil can be calculated by using equation 2:

dV = (10 liters) (0.00070) ((180 oC) - (80 oC))

= 10.7 liter

http://www.engineeringtoolbox.com/vo...ion-d_315.html

If I've done my math right. Got a C+ in grade nine math.

Just a quick view comes up 10.7 for me as well. You are talking a 7% growth which would have major implications in chemistry! I'm laying in bed with my Note 2 and minimal resources, but this smells wrong to me.

Does the oil really get to 180C? And does it really start at 80C? F vs C in this case. Metric vs SAE. Kind of how we crashed a probe into Mars many years ago!

The equation is for change in volume, but the answer is stated as total volume.

I think your temperatures and oil volume may be off too. My 997.2 takes about 8.1 quarts or 7.7 liters, and my oil temperature difference from hot to cold is about 90 deg C.

It's a digital dip stick measuring the top quart. Add oil when you get no marks and monitor the oil level as it moves up and down within the readable marks. Don't overfill.

All this math is fascinating stuff, but it doesn't answer the question of which oil reading is accurate.

As I said in my first post, when my car has been sitting for an extended period of time (i.e. overnight), it shows completely full on the oil gauge. However when the engine is fully up to temp (and oil is presumably still in the top end of the engine), the gauge reads 1 line above the minimum level. That's a difference of .8 Liters. Now if I were to add oil to a hot engine in order to make the gauge read completely full, I'd most certainly have an overfill indication the next time I check the oil level on a cold car.

So here's my question: Is it OK to show an overfill on a cold engine but within limits on a hot engine? This doesn't make sense to me which is why I'm asking.

Sean

Not trying to be too accurate, nor do I plan to take my 997 to Mars. Plus if my insurance company did find out, I'm sure they would not cover me. I was purely trying to show that there will be a difference in oil volume between hot and cold.
Your car takes 8-10 liters, depending on years. The gauge shows to add a liter when fully down. Adding that liter represents one eight or tenth of that capacity, or 10-12%. I add when the bar gauge gets near the bottom, I don't want it too full.

A car can be driven for long periods and never get rid of accumulated moisture if the oil temp never reaches 212F/100C. And it should be at that temp long enough to evaporate it all. Moisture accumulation is of course dependent on local climate conditions.