Silicone is chosen for its heat tolerance, flexibility and stability, not because it resists every fluid. Its polymer structure lets small molecules of oil, fuel vapour and some additives move into the wall over time. Once inside, those molecules act as a plasticiser and separate the polymer chains, so the material becomes softer and more forgiving to the touch.
That softening is progressive and usually painless, which is what makes it dangerous. The hose still looks correct, still holds pressure when cold, and still passes a visual inspection. What has changed is the wall, and the wall is what resists pressure, holds clamp load and keeps the bore round.
On a charge air system, the fluid is normally oil mist carried through the intake and turbo, plus crankcase vapour. On a coolant system it is glycol, corrosion inhibitor and the by-products of ageing coolant. In both cases the fluid arrives as vapour or as a thin film, and both are absorbed into the elastomer rather than running off the surface.
The inside of the hose is also the harder place to inspect, so the change is invisible until it is advanced. Fittings, clamp positions and the underside of the hose are the areas where fluid sits longest, which is why swelling and softening so often show up at the ends rather than in the middle of a run.
Absorption and diffusion both speed up with temperature. A hose that would tolerate oil mist at moderate temperature for a long time will absorb it much faster in the hot section close to the turbo or the exhaust. Heat cycles also open and close the surface, effectively pumping fluid into the wall and then holding it there.
Pressure adds to the effect by forcing the softened inner surface against the reinforcement. Where a hose runs hot, oily and pressurised at the same time, the three factors compound. That combination is the reason a hose in one part of the engine bay can look new while its neighbour is already soft and swollen.
Textile and wire reinforcement does not absorb fluid and does not soften in the same way, so for a while the reinforcement keeps the hose in shape and preserves the appearance of a healthy part. What it cannot do is replace the wall. As the elastomer around it loses stiffness, the reinforcement carries a larger share of the load.
That is when a hose starts to bulge between reinforcement layers, to deform under clamp load, or to change shape under vacuum. On a fluorocarbon lining silicone hoses build, a fluid-resistant inner layer slows the process by keeping the fluid away from the structural wall underneath, which is why lining choice matters wherever fluids are present.
Bulging between clamps, a section that has grown visibly larger than its neighbours, a hose that has begun to weep at a joint it previously sealed, or one that needs a firmer clamp to stay sealed are all signs the wall has changed. A soft feel when squeezed, or a wall that flexes far more easily than a comparable performance silicone hose, points the same way.
Pressure testing in the car rarely catches the early stages, because a softened hose often still holds at moderate load. Internal swelling is also easy to miss during a visual inspection, since the outer cover can remain intact while the bore has already deformed. The evidence usually appears at the joint or the bulge first.
Where oil mist, fuel vapour or aggressive coolant is part of normal service, the material decision moves from the outer cover to the inner surface. A hose with a fluid-resistant lining puts a barrier between the aggressive medium and the structural silicone, so the wall keeps its stiffness for longer and the reinforcement is not asked to do the whole job.
For clean charge air and plain coolant, a conventional silicone construction remains the simpler answer. The judgement is made from what actually flows through the hose and what it carries with it, not from the bore size or the appearance of the engine bay. Match the lining to the fluid, and choose the wall construction for the pressure and heat duty.
