How important is the Secondary Air Pump ?

It helps the cats run at a higher efficency while they warm up.

Yes, but that will throw a code and turn on your check engine light.

Yes.

Kyle.

Ok, after I went out and pulled the intake hose and cap/filter off the top to see what it looked like, I then put it back together and wrote this post.

This morning, no noise ... and no CEL.

What's up.

Ok, after I went out and pulled the intake hose and cap/filter off the top to see what it looked like, I then put it back together and wrote this post.

This morning, no noise ... and no CEL.

What's up.

You probably had some corrosion or other crap preventing it from working. By removing the top, you dislodged it causing it to work, at least temporarily. If it starts acting up again, consider replacing it.

Kyle.

I had a similar problem with my 328IS last year- the computer was lighting the check engine indicator, and the code was for the secondary air injection system. Upon close inspection, I found that the vacuum hoses that control the diverter valve were rotted and leaking, thus keeping the diverter valve closed all the time. This valve allows the pumped air into the exhaust manifold when the engine is cold, then closes when it's warm to prevent exhaust gasses from entering the pump.

Anyway, 2 50-cent vacuum hoses later, no problem! It's remained working just fine since then, so I recommend you check your engine for a similar problem...

Sometimes it takes more than one drive cycle to set the CEL. It will probably turn itself on in a few days.

Yes that's true, (that it takes more than 1 cycle sometimes to set a visible code), but this repair was a little over a year and 20000 miles ago, and it's been fine since (no CEL or 'pending' codes with code reader). It really was the vacuum hoses that actuate the valve- I checked everything else thoroughly at the time, thinking that it couldn't possibly be that simple, but in this case it was...