bring it to a shop and have the codes read, you will have a stored fault
code telling you what it is.
I had an intermittent  check engine light and it was one of my O2 sensors

It's not hard. But if you don't know then I'd suggest you purchase a
service manual such as Bentley's.

Which light exactly?

There should be 4 spring clips that join the top and bottom of the air box
together.  Pry those apart with a screwdriver and remove the air filter.  Go
to this link, see part #3.  That's the clip.
http://www.realoem.com/bmw/showparts.do?model=1413&mospid=47322&btnr=13_0323&hg=
13&fg=20

You need to find out the code.  On OBDI cars like your's, follow the
instructions here...
http://www.pelicanparts.com/bmw/techarticles/JF-Tech/E30_E36_On_Board_Diagnostic
s%20.htm

or this one more towards the bottom of the article...
http://www.pelicanparts.com/bmw/techarticles/Mult-Code_Reading/Mult-Code_Reading.htm

Kyle.
98 740iL
97 M3

Sounds great, enjoy your car.  Be sure to keep the timing belt changed
at the prescribed intervals.  That is one failure that would put a
damper on your fun for a while.

The Check Engine light after a long decent is probably a sign that the
engine is running slightly lean at idle, as measured by the Oxygen
sensor (assuming here that you have one).  This is accentuated by the
engine running at higher rpms as occurs with engine braking.  If you
extract the check engine diagnostic codes this should come back with
something like "engine mixture too rich/lean".  As I recall, older
models were not able to report which condition was occuring, just that
there was a fault with that particular parameter.

I don't think it is a big problem if that is all that you have, but keep
an eye on things and see if it gets worse and happens at other times.

-Fred W

On my 89 325is, I always had a problem with the check engine light
coming on while engine braking on a long downhill. My foot was off the
accelerator, the RPMs maybe in the 3000 range. It's running leaner and
would trigger the O2 sensor "out of range".

This happened with every O2 sensor I had on the car. I've heard of it
on other cars as well. Was never a problem for me.

The air filter is contained in the big black box behind the left
headlight.  There are several clips around the perimeter of
this box that have to be unsnapped, and i can't remember
if you have to remove the airbox to get it separated or if
you can leave it in place and remove the upper portion.
Whichever it is, it's real easy and obvious once you get
in there to do it.

The check engine light is telling you there's something wrong
(probably minor - don't worry).   Usually it's a vacuum leak or
a small rip in the rubber air intake boot or maybe even just a
bad seal on the dipstick or oil filler cap.  You don't need to take
it in to get the fault codes checked - with the E30 325i you can
do it yourself with the ignition key and your foot on the gas pedal:

http://www.geocities.com/calvintan_e30/tech_5.htm

Randy
89 325is