Flexible exhaust manifold

An exhaust flex pipe accommodates relative movement between the engine and chassis, thermal expansion and contraction, and vibration in the exhaust pipework. This flexible connection reduces mechanical stress on turbo outlets, exhaust pipes and exhaust aftertreatment modules. If leaks or recurring cracks occur, replacement alone is not enough: also check the engine mounts, exhaust hangers and alignment. Otherwise, the new component will again be forced beyond its range of movement.

Always identify exhaust flex pipes using the vehicle’s chassis number. Visual comparison or selection based solely on the model name is never sufficient. On MAN TGS and TGX models with D20 or D26 engines and Mercedes Actros models with OM 471, for example, connections and pipe routing vary according to year of manufacture, emissions class and chassis configuration. The exact exhaust configuration must also be checked on buses such as Lion’s City and Citaro and vans such as TGE and Sprinter.

Available designs include strip-wound flexible metal hoses and metal bellows expansion joints, which may feature external braiding and an internal liner. These designs are not directly interchangeable: gas-tightness, sensitivity to torsion and permissible axial, lateral and angular movement differ. External braiding alone does not provide a gas-tight seal. The stainless steel grade, wall thickness and bellows geometry also help determine resistance to thermal fatigue and corrosion.

Check the connection diameter, free installation length, flange profile, clamp type and any specified flow direction. The permissible operating pressure, exhaust gas temperature and range of movement must match the original specification, including loads during regeneration of the DPF (Diesel Particulate Filter). Follow the dimensional tolerances and installation position specified in the vehicle or component documentation; a flex pipe is not a solution for incorrectly fitting exhaust pipes.

Soot deposits around the bellows convolutions, ticking or blowing noises under load and an exhaust gas smell indicate possible leakage. Broken braiding, worn-through windings and cracks at the end fittings are consistent with excessive movement or installation preload. A loose internal liner can rattle or restrict flow. Exhaust gas leaks can also cause heat damage to nearby wiring harnesses and heat shields.

1. Allow the exhaust system to cool and inspect the entire component using a light and mirror. Check for soot trails, contact marks, clamp seats and weld transitions. Assess the engine mounts and rubber exhaust hangers for cracks, sagging and excessive play.
2. Measure the connection diameters with a vernier calliper and the distance between the mating faces with a length-measuring tool. Check lateral offset and angular alignment with a straightedge and angle gauge. Compare the results with the specified installation dimensions.
3. Locate leaks with the engine off using a suitable smoke tester at a regulated low test pressure, following the manufacturer’s procedure. Do not use unregulated workshop compressed air; it can damage the bellows and connected components.
4. If there is a loss of power, also investigate possible obstruction. Measure exhaust backpressure with a suitable pressure gauge at the specified test point and compare the readings with the manufacturer’s limits at the stated engine speeds and loads. If the internal liner is suspected, inspect it with a borescope. Measuring the pressure differential across the Particulate Filter alone does not rule out a restricted flex pipe.

Support adjacent exhaust components before removal. Replace damaged seals, clamps and fasteners as specified and clean the connection seats. Install without twisting, forced extension or compression, unless a specific cold installation position is prescribed. Only tighten the connections to the specified torque once alignment is complete.

Do not weld a cracked bellows shut: this will not restore the required flexibility. Address worn hangers and missing heat shields at the same time. After installation, check clearance and gas-tightness, first when cold and then at operating temperature using suitable exhaust extraction. Braem can provide technical Support in selecting the correct design based on the chassis number.