When you do an APU performance check on a 85-129 APU, or whenever inspecting it for leaks when its running, pay special attention to the combustor case. It can happen that this housing is cracked at the welded area. This causes a big air leak and even the flames are visible through the crack! But this same crack is hardly visible when inspecting it when the APU is not running. See picture below of such an event where you can see the flames in the combustion chamber.