Small-bore lines carry signals rather than flow. A vacuum line to a wastegate actuator, a boost reference to a fuel pressure regulator, a breather or a sensor take-off all depend on the tube holding its internal volume and passing a pressure signal without leaking or collapsing. The volume of air in the line is part of the signal path.
Because the duty is a signal, the requirements are different from a charge or coolant hose. The tube must hold vacuum without flattening, resist heat and oil vapour in the area where it runs, and stay attached to small fittings that are often plastic and fragile. Small errors here show up as odd running behaviour rather than as a visible leak.
Under vacuum, a thin-wall tube can flatten over a bend or wherever it is kinked, and the signal then becomes slow or wrong. A soft tube can also collapse progressively as it warms, so the fault appears only when the engine is hot. This is the single most common reason a small line causes strange behaviour.
Resistance to collapse comes from wall thickness and from reinforcement. A thick-wall silicone tube holds its shape far better than a thin one of the same bore, and a reinforced tube resists collapsing over a bend as well. On any line that sees real vacuum, the wall is the specification that matters most.
Bore size controls how quickly a pressure change travels along the line. A larger bore passes a signal faster, but it also adds volume to the system, which slows the response of an actuator that has to fill or empty that volume. Smaller bores respond faster in volume terms but restrict flow and are more prone to blockage.
In practice the original bore is usually correct, and the useful work is in keeping the line short and free of unnecessary volume. Coiling excess length, adding T-pieces or fitting a restrictor all change how the signal behaves, so those changes should be made deliberately rather than as a by-product of routing.
Silicone is the usual choice because it stays flexible over a wide temperature range and does not harden in service the way some rubber compounds do. It is not ideal against fuel and some solvents, so lines that see those fluids need a lining or a different material. The material choice follows what the line is exposed to, not its size.
Wall thickness then sets how much abuse the tube will take. A thick wall tolerates a tight bend, a rough fitting and a little vibration. A thin wall is easier to route in tight spaces and easier to clamp onto a small barb. Straight silicone tube in a range of bores and walls covers most lines, and it is worth stocking one size larger than the original where space allows.
Small fittings are where these lines come off. A smooth plastic stub with no barb will release a tube under vacuum and vibration, and a metal barb pushed into a soft tube can cut it if the bore is slightly small. The tube should slide onto the fitting with light effort, not with force, and it should be pushed fully home.
Retention is usually a small spring clip or a narrow clamp rather than a heavy band, because the fittings are not strong enough for a firm clamp. Where a line has come off more than once, the answer is to inspect the barb and the tube bore, not to tighten harder, since overtightening a small tube simply cuts it.
Route small lines away from exhaust components, turbo housings and sharp edges, and support them between the ends so they do not sag onto something hot. A tube touching a hot surface hardens locally and then cracks, and the crack is usually hidden behind other components until the line fails outright.
Keep bends gentle and avoid pulling the tube tight around corners. Where a line must pass close to heat, a sleeve is a sensible addition, but the first answer is a better route. Straight silicone tubes in the right wall thickness, routed with slack and supported properly, cause far fewer problems than an elegant run that is stretched from fitting to fitting, even if the same performance silicone hose material is used throughout.
