Narrowing down problems .....

[richie_s999]

K here it is.

1993 325is

motor have knocking sound at idle from rear of engine bay but runs smooth, and gas milage is horriable, funny smell of melting plastic but nothing is melting???

First I thought the noise was a crank bearing but it's been a few
months and has not failed and motor sound fine at anything above idle

so my thoughts are

1). Bad coil or bad ground on coil

or

2) cats getting plugged and not flowing at low rpm

thought or sugestions from your experiences.

[Ceeker]

Richie, those are not uncommon for knocking. My buddies 97 328is did the same thing. Had a slight knock at the back but the motor ran strong and no sign of power loss after months of driving.
You can live with it or throw some sawdust into the crankcase..LOL fill up the slack between the bearing and crank journals..LOL.. just to make sure that you know I'm kidding right??

[blk3]

remove the idea of the cat from your list. if it was plugged you would have flow issues at a higher rpm level and not at idle. smell could be leaking rocker cover gasket. maybe.

[richie_s999]

It's the compination of things that had me
thinking of the cat. Car is only driven about 20km a day by the fiancé to work and home. Haven't had any spare time latley to really look into it.