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 pursue the issue.

Black soot will be high if gas furnace has not been converted to propane, most or all furnaces are equipped to operate with natural gas. In order for a gas furnace to properly work without creating soot, a conversion kit to propane has to be bought seperatly and installed. The conversion kit consist of a small spring and orfices that are installed at burners to regulate gas flow. By doing this the air and gas pressure will mix properly giving you a nice blue flame and leaving the soot out. If a gas furnace has been working with out the proper LP kit for a long period it will cause the heat exchangeer to get plugged and will possibly need new heat exchanger or even new furnace, would recommend asking your dealer or a/c and heat company for proper conversion kit. Installation will have to be done by qulified personnel.