Air hoses for Volkswagen Combi Bay Window

Two systems, two constraints

• A naturally aspirated engine operates under negative pressure: the engine draws in air. The hose must, above all, remain airtight and must not be crushed. It connects the air filter housing to the throttle body or the fuel injection system.
• A forced-induction system operates under pressure and at high temperatures: the turbocharger forces hot air towards the intercooler, then towards the intake. The hose must withstand pressure, heat and oil vapours, which are always present to some extent in this circuit.

An intake hose fitted to a forced-induction circuit will swell and then burst. The reverse is mechanically possible but is costly and offers no benefit. Identify the circuit before looking for the part.

The intake hose, an unseen wear part

The flexible sleeve between the air filter housing and the throttle body does not age well: heat, vibrations, vapours from the breather. It hardens and eventually cracks, almost always at the base of a fold – in other words, where you wouldn’t normally look.

The result is unmeasured air intake: on a fuel-injected engine, air enters after the air flow sensor and the ECU does not register it. The fuel-air mixture becomes leaner, the idle becomes unstable, the engine stalls when decelerating and acceleration feels sluggish. On a carburettor-equipped engine, the symptoms are similar, with the idle speed rising for no apparent reason. It’s a fault that’s misleading because it points to an electronic issue, when the cause is actually a piece of rubber costing just a few euros.

Check for an air leak

• With the engine idling, bend each flexible hose by hand. A crack that opens up will cause the revs to fluctuate immediately.
• Inspect the undersides of the bends and areas close to a heat source: that’s where they break, and never on the visible part.
• Check the clamps before ruling out the hose. A loose or incorrectly positioned clamp on a bulge produces exactly the same symptom.
• Check the small vacuum hoses connected to the system, as these can harden and split at the ends.

On a supercharged engine, the symptom is different: a detached or punctured pressure hose does not affect the idle speed; it simply removes the boost pressure. The car will run, but will lack power beyond a certain revs, and a whistling sound will occur when the engine is under load.

What to note before ordering

• The internal diameters of both ends, which are rarely identical on an intake hose.
• The shape, as these parts are moulded: bends, spigots, retaining ridges and reinforcements cannot be replicated with a straight hose.
• The secondary connections, which accommodate small vacuum hoses or the breather pipe. A hose without the correct connection will require drilling, which is not a reliable solution.
• The system – intake or supercharging – which determines the material and pressure rating.

Air, petrol, brakes: hoses that cannot be interchanged

The term ‘hose’ covers components with very different requirements. Afuel hose must withstand hydrocarbons and modern fuels, which attack older types of rubber. A brake hose operates at very high pressure and is a safety-critical component. A power-assisted steering hose withstands pressure and hot oil.

None of these can be replaced by an air hose, even if it is the correct diameter. If you are looking for a hose for one of these systems, choose from the relevant range: the material, reinforcements and maximum pressure ratings are completely different.