Car engine "running rich"??

Factual Questions
1

My father informed me that my van (fully automatic, computer-run 1999 Honda Odyssey) needs a tune-up because the exhaust pipe is black inside. He says this is a sign the engine is ‘running rich’, that the black deposit is unburned fuel components, and I’m probably getting worse mileage than I ought to.

Now, just a few months ago, I had to take the van for emissions testing in order to get my new tabs. The van passed without a problem. If it were ‘running rich’, would the emissions test not catch that?

I have heard that vehicles with onboard computers don’t need tune-ups like older cars did. Certainly there are no dummy-lights on, on my dashboard. We did have one that we thought might be the catalytic converter, but it turned out to be that the gas cap hadn’t been tightened properly once. We had the light turned off, and it didn’t come back on.

He suggested I take the vehicle to the dealer. We do this only at great need, as they do not provide loaner cars although they’re happy to rent you something, and they rarely can give you an estimate when you can have the car back. So…does this sound like there is actually a great need? The engine seems to run well, has plenty of power, no oil leaks, is not burning oil, no weird noises, no dummy lights, and passed emissions without a problem. But the exhaust pipe is black inside.

Thoughts?

2

I can’t tell you if it is running rich, but i can certainly tell you that it’s possible even with modern ECM systems. A problem with one of several sensors can cause that.

3

My '99 Jeep Cherokee runs a little rich, as evidenced by the soot on the tailpipe.

The 4.0l engine is known to be prone to cracked exhaust manifolds. I took the vehicle into the shop for another problem and the mechanic mentioned the manifold was cracked. Apparently this causes the oxygen sensor to detect a lean condition, so the computer adds more fuel.