axl911

Black stuff on the rear bumper is NOT normal. I have a white car and I don't have any black stuff on my bumper, and the car hasn't been washed for weeks at a time (no rain here in Texas).

Several times in the past, I have black stuff on my car but THERE ARE CAUSES. It depends on what the black stuff is:

1. Dry fine dust like back stuff on your finger. This is your car running rich. Like when I purposely richen up the mix at part throttle (takes a voltage divider). 2nd time is when I accidentally unplugged the tank vent valve. Doing so, the car will richen up in anticipation of the tank valve opening and extra air.

2. Oily black stuff that smears on your finger. Oil!!! Or burned oil!!!

3. Other black stuff....tire or brake pads....never seen this. The only exception is when I wiped down my rear bumper down with WD 40 to remove some residue. Afterward, it attracted dust like crazy.

Greg H.

Way worse with a turbo wing and a white car. It isn't brake dust or tires, because I get it just driving around the block.

Greg H.

993Brendan

I have the cat bypasses and get the dust as well. Wipes right off!

goofballdeluxe

Black stuff on your rear bumper is normal. It's tire dust.

IXLR8

FYI, I removed the complete exhaust system on my 993 the winter of 2009/2010 just after buying it. Prior to installation, the exhaust tips were cleaned and polished.

Two summers later, all I see is clean stainless steel on the insides of the exhaust tips...no soot at all.

csertich

I noticed the same thing with my car the firs six months of ownership. Then I did the suspension and got a good alighnment. Dust went away. I concluded (right or wrong) that it must have been tire dust-I was scrubbing off a little rubber due to my car being a little out of alignment in the rear.

chuck

MarkD

+1 absolutely!

tire dust shouldn't build up quickly... a little over time is normal but it shouldn't be much.
If the alignment is off you will get rubber dust behind the rear wheels. Tire life is effected, of course. How much depends on how far the alignment is off

daltvater

I had this on my 87, the culprit was old dry tires. New tires fixed the issue for me.

flyingchappy

Always on my car. I run the car for about 20 miles and there is tire dust. Not oil, not brake. It's a normal occurrence. And I don't see any abnormal tire wear.

Ed Hughes

I run R compounds on the 911 full time, and I've run different brands-several types of street tires too, back in the day. I've never accumulated any "tire dust". No dust on the 993 either.

I'm with the guys above suggesting you having an alignment issue.

csmab

I had oily dust which would smear when wiping prior to my top end rebuild. I get no dust build up on the back of the car after the rebuild.

CalvinC4S

You guys that don't get the tire dust are not driving flat out.

DanL993

I get some after longer drives, and haven't noticed it on the bumper as much as the side panels--back and front. I presumed it was brake dust since it looks just like the powdery stuff I clean off the wheels.

I guess it coud be tire dust--never thought of that.

cabrio993

+993