The brief
The 320i kept bouncing after bumps instead of settling. The rear was visibly sitting lower than the front. And one rear absorber had oil weeping down its body.
Three signs of rear shocks past the end of their life. The bounce-and-keep-bouncing is the damping gone; the sagging rear ride height is the shock no longer holding the body up the way it should; and oil on the shock body is the seal that has let go and let the damping fluid out. Once a shock is weeping oil, it has stopped doing its job.
The diagnosis
Visual inspection confirmed the oil seepage on one rear shock body, which means the internal seal has failed and the damping fluid is escaping. A bounce test on both rear corners showed neither was damping properly: push down on each and the body kept oscillating rather than settling in one cycle.
So the call was a pair replacement, not just the one weeping side. A new shock on one corner and an old, worn one on the other leaves the rear axle loaded unevenly, one side controlled, the other floating, which makes the car drive worse, not better. Both rear shocks together, with fresh top mounts and bump stops since those get disturbed for the swap anyway and an old top mount on a new shock is a weak link.
The work
Lifted the car, disconnected the lower mounts and the top mount nuts, and dropped both rear absorbers. Fitted new BMW-spec replacements with fresh top mount bushings, bump stops and dust boots, torqued to spec.
Then ran a bounce-and-settle test on each rear corner before the road test: push down hard, and the body should return once to ride height and stop, not bounce three or four times. Both corners passed cleanly.
The outcome
The rear sitting level with the front again. No rebound bounce after bumps. The ride compliant without floating.
The 320i went home with the rear suspension tight. For the owner, that means a car that feels composed and planted at the back again rather than loose, and rear tyres that will now wear evenly now that the axle stays settled.
With fresh shocks and mounts, the rear is good for years, and the difference is obvious from the first drive.