the [reputed] problem isn't while the engine is running but after it's shut off hot, and then cold [ambient] air washes across open valves [hot air draws it up the manifold]
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
