E28 idle roughness

[Aaron Lung <alung@xxxxxxxxx>]

I'm due in for a smog check shortly and
I'd like to fix this chronic roughness in
idle I have on my 88 E28 528e.  

I believe it has to do with the almost failing 
idle emissions everytime I send it in for a test.  
It's high on the HC's.  

  last test          HC ppm   CO   CO2  O2
            idle     79      0.04  15   0.3
            2500rpm  1.0     0.02  15   0.3
                 

I've changed out the usual: air filter, plugs,
wires, O2 sensor, and distributer cap/rotor, yet while
idling, it'll do a subtle puff occasionally 
as if it misfired.  I've also adjusted valve
clearances to spec on a cold engine (does it 
make any diff if I do when it's hot?)  

What else can be done?  The last time, the
garage guy suggested I swap out the cat.  Uh, 
that's an expensive way to shotgun the problem...

I use regular unleaded Chevron gas if that
make any difference (I doubt it)

My gas mileage appears to be a consistent and
normal 24-26mpg

I don't seem to see/detect any vacuum leaks.

any suggestions?


thanks
aaron
88 528e 68Kmiles

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