Is my cat dead?

[Futuramic]

Not the mewling kind but my catalytic converter. The car is a 1995 3 Series, 1.8 Is. It has developed a rev related rattle. When giving the accelerator a prod the car emits a rattling, scraping sound from somewhere around its middle section. The sound is best described thus: small solid pellets bouncing around in a closed metal container.

This seems odd to me as I thought cat failure, as in a full case of monolith collapse, was accompanied by loss of power and a nasty smell. My exhaust produces a petroly scent but not the sulphorous, rotten egg odour usually associated with such things. In actual fact it smells more like the emmissions from my old car; which had no cat.

It may be possible, then, that particles of the substrate are becoming detatched and rattling inside the cat box.

I have discounted the timing chain from my ponderings as the noise seems to come from underneath. It is present whilst stationary too.

I propose doing absolutely nothing to remedy the fault. The car has just been MOTd and passed without issue. Any ideas?

[Si_steve]

Heatshield?


Edit: spelling

[TheEnd]

It's more likely to be a heatshield that surrounds the cat breaking up at the corner mounts and dropping onto the cat, but if that looks solid, than maybe the cat has broken up.

It's not that common for a BMW cat to break up, but it's not impossible too.

[10AE]

Exhaust/heat shield loose?

[Baffled Spoon]

It could be the exhaust baffles that make the car quieter. I had a similar rattle devlop in the back box of one of my old cars and it was attributed to the sound deadening material falling to bits.

[Dan_1981]

Oooooh mine does this too - i thoguht it was a heat shield that was loose at first as it does sound like somethign vibrating away.

But nope heat sheild nice and solid.

I'm thinking cat too.

Hmmmm bypass pipe.

[944fan]

Tap the shell of the catalytic converter with a rubber mallet. If you get the same souund doing that then it is likely the internals of the cat are breaking up.

[mr_tony]

You won't know until you open the box. If you do watch out for radioactivity...

[paranoid airbag]

mr_tony said:

You won't know until you open the box. If you do watch out for radioactivity...

DAAAMN YOUUUUUU beat me to it

[seagrey]

sometimes what happens is the rockwool or fibreglass they use to insulate the monolith from the steel casing comes adrift and leaves a small gap that allows the monolith to rattle,if you catch it early before it breaks up you may get lucky,I have seen one`cured`by my mate on his focus.
He put a small selftapper through the box into each corner and nipped them up until they held the monolith still.
It may be a bodge and the monolith would probably break anyway in the end but it certainly has stopped the rattling noise.