Propane furnace not working right. Exhaust pipe out of house is black at the end.
Does this mean a problem?

Running rich. Call for service before it gets worse.

Is this a 90% efficient furnace? What brand? Has your tank ever ran very low on fuel? Soot in an LP furnace is usually caused by improper output fuel pressure, or not enough fresh air for combustion. If it is 90% secondary heat exchanger may be getting plugged with soot causing a huge problem that will only get worse.

I agree with Hvac1000 and Northern Heat. If you have a lot of black on the vent pipe, that means you have a lot inside the heat exchanger and it will get worse and cause more damage. It may not be safe to use. Be safe call a service Tech.

I have a similar problem with the black exhaust pipe, two weeks ago I called the service tech out to check this the problem, the rollout switch was tripping and we notice that the flame through the pilot hole was not burning correctly. To make a long story short, the service tech needed to replace the heat exchanger. The furnace ran great for a week, however it is back to the same problem that it had three weeks ago. Also, I have notice a real bad odor throughout the house and now the service tech says we should replace the furnace. This furnace is 8 yrs old and is 100,000 BTU, Nordyne. Do you have any suggestions? Thanks

If they replaced your heat X to correct the problem they should at least give you credit for money already spent. Something seems to be wrong with this picture. I would get a second opinion then persue the issue.