Petrol Exhaust And Cats

Layout

Work only when the system is cold. Twin exhaust manifolds feed a downpipe and catalytic converter(s), then a rear section with two or three silencers. Service parts usually arrive as three or four sections. The whole run hangs on rubber mounts. Cats stay dangerously hot long after shutdown; even a brief brush burns.

On 2.0 the cats sit at the manifolds, a common front silencer and Y follow, then the rear box. On 2.5 and 3.0 the cats again sit at the manifolds, a Y and front silencer lead into a twin rear silencer assembly.

Inspection

Raise the car safely. Look for open seams, holes, crushed pipe, loose clamps, and sooty stains at flanges. Shake the silencers: contact with the floorpan or a suspension arm means renew the rubbers or twist for clearance. Rattles are often a loose heat shield or a collapsed baffle. Oxygen sensors thread in at 40 Nm; do not crush the hex or twist the loom.

Renewal Notes

Work from the back forward. Soak flange nuts in penetrant. Fit new gaskets and rubbers. Anti-seize on the studs helps the next strip. Book torques where the joints open: catalytic converter to manifold 25 Nm, converter to engine 23 Nm, flexi-pipe to converter 55 Nm. Some manifold joints use coil springs and self-locking nuts wound only to the bolt shoulder; overtightening shears the stud and does not cure a leak. Renew gasket and springs if the joint weeps. Keep fresh pipe clear of the floor and the tank, and refit every heat shield.