Silicone is an elastomer, and every elastomer loses stiffness as it warms. A hose that feels firm at ambient temperature and noticeably softer after a hard run is behaving as the material does. The question is not whether it softens, but whether it still holds its shape under the pressure and vacuum it meets.
So judge hot softness by behaviour rather than by feel. Does the hose balloon under load and collapse when the throttle closes? Does it seep at a clamp once hot? Squeeze it cold first and remember that; the cold state is the reference, and a hose that is soft cold has a different problem.
Hot stiffness comes mostly from the fabric layers, not from the rubber. A hose with a light reinforcement chosen for pressure only will feel soft once hot, because the rubber is carrying the load. A hose with a heavier woven layer, or a wire helix, keeps its bore at the same temperature.
That is the real difference between a general-purpose hose and one built with a heavier reinforcement, which is what heavy duty silicone tubes are for. Ageing also matters: heat cycles degrade the reinforcement over time, so a hose that was firm when new and is soft now has lost fabric strength, not just rubber hardness. Both look the same in the hand and behave very differently under boost.
Silicone swells and softens when it absorbs hydrocarbon. A hose on a crankcase breather, a turbo inlet or anywhere oil mist collects can go spongy, grow slightly longer and start weeping at its joints. The inside surface is usually tacky, and the hose does not return to size after it cools.
This is the distinction that matters against heat softening. Heat softening recovers on cooling and leaves the hose the same size. Fluid swelling does not recover, and the hose stays oversize and soft. Fluorocarbon lining silicone hoses exist for exactly this duty, because the lining resists the oil and fuel vapour that the silicone behind it does not.
A hose that is firm along its length and floppy at one end has usually been crushed. An over-tightened clamp compresses the wall, cuts into the reinforcement and leaves a band where the structure has failed. The rubber still looks intact, so the fault is easy to miss.
Look for a deep groove under the clamp, a cut line at the edge of the band, or a bulge immediately beside it. A hose trimmed short, so the clamp sits off the beading instead of behind it, produces the same local damage and the same leak. That end will always be the first to weep, hot or cold.
Once a suspect hose is off, cut the old one lengthwise. Check the wall thickness around the circumference, since a crushed section shows immediately. Look at the inner liner for cracks, blistering or a tacky film, and at the reinforcement for frayed or separated fibres.
Smell the inside. A hose that smells of fuel or oil was the wrong material for the position, whatever the outside looks like. A hose that smells of coolant and shows staining along the fabric has been wet internally, which points at a system fault rather than at the hose itself.
For a boost or coolant run that cycles hot and cold and sees vacuum as well as pressure, the wall has to resist collapsing when the throttle shuts and hold when it opens. That is the case for a hose with a reinforcement chosen for both directions of load.
For a position where oil or fuel vapour is present, material choice comes before thickness. And for any hose, the clamp has to be tight enough to seal and loose enough not to cut, which in practice means a properly beaded end and a clamp seated behind the bead rather than on it.
