A rotten silencer is the fault your neighbours notice first. The car gets louder, takes on a low rumble under acceleration and starts popping on lift-off. It is not an emergency in itself, but the sound is a sure sign that a hole has appeared in the exhaust system, and the question is worth closing before it turns into a smell of exhaust in the cabin.
Why a silencer rots
The exhaust system lives in hard conditions: outside there is water, road salt and stones; inside, hot gases and condensate. The condensate is the most dangerous of them. On short town trips the system does not warm up enough, moisture collects in the bottom of the box and does not evaporate, and together with combustion products it forms an acidic environment that eats the metal from inside. So silencers almost always rot from the bottom and from the inside out — from outside the casing can look decent right up to the end.
Winter use with salt on the roads speeds it up, as do a poorly chosen cheap silencer made of thin metal and running a rich mixture that puts unburnt fuel into the system. Mechanical damage is its own story: a kerb or a speed hump can start a crack along a seam.
What it sounds like and what else you notice
The set of signs is nearly always the same:
- Growing loudness. The car sounds deeper, especially when cold and under load. The sound comes not from the tailpipe but as if from under the floor.
- Popping on lift-off. Air is drawn in through the hole and the remaining fuel burns in the system, producing characteristic pops and bangs. That picture is covered on the symptom page exhaust backfire.
- Rattling at certain revs. Detached internal baffles resonate.
- A smell of exhaust in the cabin — the most unpleasant sign, requiring an immediate response.
- Soot marks on the box or the pipe underneath: a dry black patch around the hole is visible even without a lift.
You can check by ear and by hand. With the engine running, have a helper block the tailpipe firmly through thick cloth: the pressure in the system rises and hissing becomes clearly audible at the hole, sometimes with a wisp of smoke. Do this on a cold or barely warm engine and only briefly.
Not to be confused with its neighbours
| Sign | Rotten silencer | Another source |
|---|---|---|
| Loudness grew gradually over months | Yes, typical | — |
| A roar appeared suddenly, with vibration at idle | Less often | A torn flex pipe |
| Metallic rattle at 1,500–2,500 rpm | Possible | A heat shield |
| Power dropped along with the noise | No | A blocked catalytic converter |
| A pouring rustle of debris | No | Broken converter ceramic |
The key divider is power. A hole in the exhaust does not take it away — the gases simply escape more easily. If the car has gone both loud and slow, it probably has two separate problems, or one that is not in the silencer. The catalogue of noises from below is on rattling under the car.
How the repair is decided
The diagnosis is simple: an inspection on a lift, usually free when you book the repair. What follows depends on the state of the metal. A local hole in a sound box is welded with a patch; a pipe is replaced in a section. If the casing has rotted through and gives under a finger, a new box is the sensible answer — otherwise a fresh hole appears next to the weld within a couple of months.
Two things worth asking. Whether the rest of the system was tapped along its length rather than only at the visible hole: a box that rotted from condensate rarely did so in one spot, and welding one patch on a system that is thin everywhere buys weeks. And whether the exhaust hangers are sound — a sagging system rubs the body and loads the seams, so a new box on old rubber rots ahead of its time.
If you can hear that the car has got louder but cannot tell where the system has failed, record the sound in the Pro-Stuk app — at idle and while raising the revs smoothly. The app matches the recording with your answers and suggests which parts of the system to inspect first.
