Proof of Inaccurate Oil Level Gauge

Had an oil/filter change yesterday. Discussed with the mechanic the fact that my gauge showed "overfull" all last year. With the filter change, my manual calls for 7.75 liters, which converts to 8.2 quarts (8 quarts + 6 oz ). The mechanic started yesterday by putting in 7 quarts and letting it sit the rest of the day until I picked it up late afternoon. Upon pickup, the gauge showed half full, so I know the gauge at least functions. We then put another quart in because I didn't feel comfortable driving being more than a quart low. My gauge shows overfull again this morning even though it is actually 6 ounces less than 100% full. I added the last 6 ounces to be on the safe side.

I only give my tech 8 quarts to put in with a filter than top with 1/2 qt. after the car sits overnight. 2007 CS. Brings it to full line.

John, how long did the mechanic let the old oil drain out of the engine (while still warm?)? Is it safe to assume that the car was level while the old oil was draining? Could the engine have retained some old oil when the mechanic filled it with the fresh oil?

I don't trust the gauge any further than I can throw it. Intuition says that the most accurate reading ought to be when engine is cold and car is level. My gauge was showing below the bars (ie flashing). I drove the car until it was warm, waited about 10 minutes and - presto! middle of the gauge. Dealer says car must be hot - they drive it a half hour first before they test the level. Seems a bit bogus to me to have to do that. Wastes gas and adds kilometers.

1) Unknown how long it drained because I wasn't there this time, but last year the same shop I did watch and they let it drain a good half-hour or 45 minutes, and they weren't very busy yesterday. They are a pretty reputable shop. 2) Yes, safe to assume level. They use a lift. 3) Anything is possible, but I have watched them do the procedure in the past and by the time they put the plug back in, there's nothing -- nothing -- coming out. If there's any more oil left in the engine, I don't know how you'd get it out.

I suspect these systems are "accurate enough" to prevent problems, not unlike my tire pressure sensors. Those read 2 pounds less than every manual guage I've tried, including both expensive ones and cheap ones. Although I'm sure the information isn't available, you've got to believe there is a +/- variance on any of these systems that Porsche and all other manufacturers find acceptable. And to make them 100% accurate would be cost prohibitive.

I totally agree, but part of the reason I chose to post this was because of another thread in which another person kind of chided me and others for driving with the gauge showing overfull. Yet here I had 8.0 qts in it versus 8.2 qts capacity and the gauge was still showing overfull. There is no explanation other than the variance +/- as you stated. If I followed that poster's advice and was a slave to the gauge, I'd have to drain off probably another half-quart. Running my car at 7.5 qts when the capacity is 8.2 cannot be a good thing, and the gauge be damned.

Last time I did it I was wondering abut the accuracy also, so I warmed the car, then dumped the oil, removed the filter, then let it additionally drain for a day and half absolutely level (I put a level on the oil pan both directions). Kind of overkill, but I was sure it was absolutely empty. Then I correctly filled it and I can say that my gauge anyway - is accurate. I'm pretty sure that previous quicker, and less exacting oil changes, might of left as much as 1/2 quart residual oil in my car.

John, I too have chosen to drive although all of the segments (5) are lit up on my gauge after the mechanic changed the oil. I was told by the dealer Service Manager that this is not a problem. If I got a CHECK ENGINE OIL LEVEL message, however, I would immediately get some oil out. I don't know if it is an inaccurate oil level gauge/sensor, or there is a margin of safety in the level. By the way, my '10 CS will not display an oil level reading until the engine reaches operating temperature. I get a message that the engine needs to warm up if I try to get a reading before it reaches temp. I have read others who report taking their readings with the engine cold. Maybe the earlier models allowed that.

The most knowledgeable Porsche technician in our area takes the warm car and let's it drain overnight. Then refills the car. He agrees the gauge could be more accurtate but when it shows one small block empty at the top when we start the car, he is not concerned. That viewpoint has worked for 10 track days and 40K miles with no problems. So I don't worry about it. IMHO. Ed

I don't have the oil change steps for the 08 Cayman but for the models I do have the oil changes steps for the engine (oil) wants to be 'hot' 90C IIRC and the oil allowed to drain 20 minutes. Since the reference is the factory manuals the assumption is the car is level on a service lift. However, I have changed the oil in my Boxster , Cayman S and Turbo while the rear is in the air (on ramps) the slight out of level makes no difference to the amount of oil that drains. The level can't be accurately checked until I drive the car off the ramps and onto level ground though. Then the specified amount of oil is added to the engine -- in your car's case 7.75l -- and the oil level checked however it is checked. One car I check with the engine off and after having sat unused for some time. The other the engine must be warmed up and running at idle. After adding the correct amount of oil the oil level reading should show the 7 lowest segments lit. The oil level should be at the max line but not above it, or below it. If below or above the oil level checking system is suspect and it may need attention. Sincerely, Rockster.

Referring to my 2006 manual, in the oil capacity section it says 'approximately' 7.75l. I also am not sure how the gauge indicates overfill. As far as I know the top bar indicates 'full' but there is not an 'overfill' bar to my knowledge. Again, when reading the manual it indicates that one should essentially fill the engine so the gauge indicates near full and then fill using 0.4l per bar as an indicator of how much more to put in (or, 1.2l for the range of bars). With variations in castings, exact positioning of the level sensor, etc I can easily see how one may see at least a couple of per cent variation in fill levels, therefore the 'approximately' wording in the capacities. I would read and follow the manual and utilize the gauge as the last word. These gauges tend to be very accurate and reliable from previous research I've done.

What year is your car? 987.1 - Oil level is checked with the motor dead cold. 987.2 - Oil level is checked with the motor at full operating temperature.

Ahhh ... I guess I should start looking at post dates from now on