Dear Tom and Ray:

What would cause a muffler to explode? We just had work done on our '93 Saab 900S. It was running very sluggish up hills, and diagnostics showed several leaks in the exhaust pipes leading out of the catalytic converter. After having a new converter and pipes installed, the car was better, but still a little sluggish. Within a week, we were accelerating to get on a highway, and the muffler exploded. It literally burst open along the side. What kind of pressure would cause this, and should our mechanic have picked up on a muffler problem in the first place? -- Angela

RAY: Well, here's the key question, Angela: After the muffler exploded, did the car run better?

TOM: If so, the muffler might have been the source of the sluggishness. Often what happens is that when a catalytic converter fails, its innards disintegrate and get pushed farther down the exhaust system, where they can lodge in the muffler baffles and plug it up.

RAY: That would cause exactly the symptoms you describe -- the car would be sluggish when climbing hills or accelerating. If the tailpipe is blocked and the engine's exhaust can't get out, the fresh fuel and air have no room to come in, so the engine bogs down.

TOM: So both your catalytic converter AND muffler might have been clogged. And when your mechanic replaced the converter, the acceleration improved a little bit, but the pressure on the muffler multiplied. At some point, the pressure got to be too much and the muffler burst at its weakest point -- the seam.

RAY: While that makes you unwelcome in most bedroom communities after dark, it also allows the exhaust to escape. So the car should run better. And once you replace the muffler, you should be all set.

TOM: Mufflers sometimes explode due to backfires, caused by too much unburned fuel in the exhaust system. But given all the corroborating symptoms you have, I'm betting yours was just badly clogged. Next time, try Mufflermucil.