Pilot won't stay lit - thermocouple replaced

[arobb]

The pilot light on my furnace continues to go out, usually overnight. The furance is extremely old, but it has been serviced regularly and was last serviced and cleaned in July 2003. I've already replaced the thermocouple, but this has not yet fixed the problem. Any suggestions?

[a46geo]

A down draft from the chimney, crack in the heat exchanger, burner concussion or a weak pilot safety are the things that I would look at first.

With the pilot lit and the burners off, energize the blower and see if it moves the flame. (Heat exchanger problem)

Down drafts are tough to catch in the act. Pay close attention to pilot movement over a period of time. Especially on a windy day.

Observe the pilot when the burners light and then again when they shut down. If the concussion of the ignition or shut down blows or sucks the pilot out, that is the problem. If this happens, it is probably be a gas pressure problem.

Next blow out the pilot and listen for the "clunk" of the safety valve closing. If it is less than 15 seconds, either the thermocouple or pilot safety is weak. Since you have just changed the thermocouple, it may be the safety. The pilot safety is what ever the other end of the thermocouple screws into. Often it is a combination gas valve and to replace the safety, you need to change the entire gas valve.