Black Soot Deposits on Rear End

[Speed Yellow]

I recently purchased a used 2003 Boxster with 40K miles back in February, 2007. I have had it for a four months with no real issues, though I only get to put a few hundred miles per month on the odometer. I recently took it to the dealer for some last minute warranty work (warranty expires in July). Issues included a slightly leaking Main Seal (they replaced it) and my complaint about a fine, gritty black soot deposit that accumulates on the rear of the car. I told them I had already replaced the stock air filter (it didn't look too bad) but I still suspected too rich of a fuel mixture.

They discovered a bad Mass Air Filter and replaced it. The idle seems slightly smoother (it was never very bad) and there seems to be just slightly less power at the top end (but this could be entirely my imagination). Also, the water temp dial indicates that the engine now runs about 5 degrees hotter than before, but rock steady even in the recent summer heat with A/C load.

The dealer is two hours' drive away. By the time I got home, the soot buildup was just as bad as it was before. No discernable problems, other than that.

Is there the possibility of the soot buildup being burned oil in the exhaust, such as coming from a bad Air-Oil separator? I used Mobile One engine oil, as directed, on the previous oil change. Would the techs have checked the condition of the spark plugs as a natural diagnostic step for complaint of too rich a fuel mixture?

Should I take it back to have them check the source of the soot buildup before the warranty runs out? The dealer tech people are always telling me that the Posche engine uses up some oil over time. Perhaps some fine, gritty soot buildup is just a normal thing for aging Porsche engines?

Any advice would be greatly appreciated. Thanks!

[FrayAdjacent986]

MAF is the Mass Air Flow (sensor). I also suspect with a leaner mix, the engine will run a little bit hotter.

You should take it back and have them check the AOS, and other things. Your car should NOT have a sooty buildup on the bumper.

[Speed Yellow]

Thanks for your reply, Fray.

Yes, I meant Mass Air Flow (MAF) sensor. I typed up that note near midnight last night, sorry.

I really didn't think that much soot is a normal case of an aging engine, but I have no prior experience or knowledge of Porsche engine technology and characteristics. I am planning to return it to the dealership. However, finding people willing to ferry me back and forth for a four hour round trip each time is getting to be a little difficult!

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Any additional advice would be appreciated, such as what specifically to tell the techs to check on while they investigate. I get the impression at this dealership that the techs don't hunt for things too readily unless they are prodded to do so. For example, I would have thought that they would run some sort of tail pipe emissions sensor test to check for abnormal particulate levels to verify that the bad MAF was indeed the source of the soot. Obviously, they didn't do that.

Thanks.