RE: Do I have a Cat?

["Bob Sutterfield" <Bob@xxxxxx>]

John asked:
> When I bought my '89 325i years ago, I was told it had a cat. 
> Over the years various garages/shops etc have told me the car 
> does/does not have a cat. There are two silencer type things 
> (mufflers) in the exhaust system. One up front (pic on site 
> below), and one at the rear.

My cars have three fat lumpy things in the exhaust line: cat, resonator,
and muffler (in that order).  The setup may be different on your side of
the Pond, or if your exhaust has ever been replaced or changed.

> I also think the car has an oxygen sensor and I also thought 
> that if there IS an O2 sensor then there is *always* a cat as 
> well (but was never really sure about that). Another thing I 
> probably have confused is that I thought the O2 sensor 
> monitors unburnt oxygen *after* the cat - but the suspected 
> O2 sensor in my car is fitted before the cat ie just after 
> the exhaust manifold.

Newer cars have two O2 sensors as you described, one ahead and one
behind the cat.  The difference between their readings tells the
efficiency and efficacy of the catalytic process.  Our older cars have
only one O2 sensor, just downstream of the exhaust manifold.  It lets
the engine computer monitor only the engine's combustion processes, not
the cat's.

> The suspected O2 sensor is at www.telmek.net/pics/o2.jpg

Yes, that's the O2 sensor, beneath a heat shield.

> and the suspected cat is at www.telmek.net/pics/cat.jpg

Yes, that's the cat, wrapped up in thermal/acoustic insulation.  The
shield also keeps water from splashing on the hot cat itself, which
would contribute to metal fatigue and cracking.  However, if water gets
in to soak the insulation, you're worse off than if there were no shield
at all.
- --
Bob Sutterfield
'87 E30 325iS http://bmwe30.net #1129 DAS KAR
'88 E28 535iS http://m535i.org   #154 IHR FUNF
BMWCCA #169277 Rocky Mountain Chapter
See you at http://www.bmwofest2002.com

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