'95 Geo with black exhaust pipe
 

I'm hoping someone can help more than the Chilton's manual has.

1995 Geo Metro 1.0L / 5speed with 160k miles.
Starts and runs great whether hot or cold.
Used mostly used on rural roads 35-50 mph.
Compression is 180 on all cylinders.
No oil consumption.
Exhaust pipe is black with a little soot ring around the outside as well.
I get about 42-45 mpg.
Ignition timing/valve timing are ok.

Everything I've tried has not helped with leaning out the mixture.

O2 sensor replaced no change.

I stuck my ocilloscope on the O2 lead when engine was idling(warm)
The O2 voltage changes, but the waveform is a whole lot slower than the book calls for, although it's inside the parameters. When I injected a little unlit propane from my torch into the air cleaner horn, the voltage went all the way to the end of the parameter. When I turned off the torch valve, the voltage and waveform returned to previous.

Any suggestions would be greatly appreciated.

Advance the timing all the way to the stops.

You might check in over at geometroforum dot com. The guys over there specialize in these cars.

How far is that? I'm already 5 degrees farther than the book.
My hearing isn't what it used to be, so I'm a little worried about detonation.
There's a hill close by without any traffic noise so I can do some experimenting.
Thanks also for the geometroforum link

I'm assuming you changed the air filter. What does the condition of the spark plugs look like?

At 160K miles, finding carbon buildup in an engine would not be surprising.

Is it fuel injected or carburetor? If it's a carb, that's what needs attention, as the mixture is off. Either way, throwing a can of fuel injector cleaner into the gas tank wouldn't hurt, and it might help.

Air filter is clean and so is our air (I live on an island in the Pacific Northwest).
The car has throttle body injection.
I got the car a couple of years ago, it had a burned valve.
I took the head off, cleaned out any carbon in the EGR passages, head was in great shape. Plugs look fine mixturewise (tan).
Idle is fine, fast idle is perfect when started, starts in hot and cold weather.
Most driving is in light traffic 35 to 50 mph.
I work at home, so the car has only had about 5k miles on it in 2 years.

Drivability is great, just a BLACK FURRY tailpipe.
I've seen others of the same year with RUSTY pipes.

I just got back from draining the coolant, checking the vacuum hoses and EGR valve and throttlebody coolant passages. All are OK.

Thanks.

If it ain't broke...

I just think it ought to get better than 40+ mpg with the kind of driving I'm doing.

It would also be nice not to have to clean off black from its backside when I wash.

Posts here and on the geometroforum state people are easily getting 50+ MPG.

Idle is fine both hot and cold, but the tailpipe looks like it's stuck in warmup mode.

take a look at the o2 sensor again and maybe try taking it out and testing it, I've had bad new o2 sensors.

My gut feeling is that is the case. :)

However, short of that, a good diagnostician (I repair things other than cars for a living) looks for any clues mentioned, slight as they may be:

Maybe it's time to check that again. A previously seen and noted "burned valve" on a car that, down the road timewise, now has 160K might need a bit of attention to that detail. ;)