Re: Failed Emissions Test on 1985 e28 m30 3.4l

[C2slayman@xxxxxxx]

Peter,
       There are four things you can test for - cat con operation, burned 
valve, misfire and O2 sensor.
       
       The fact that you fail at idle and not speed might indicate an old cat 
con.  If it's not getting hot enough at idle, it will be less effective.  See 
if you can find a mechanic or friend with an IR thermometer.  Get a temp 
reading at the inlet and outlet of the cat con - the outlet should be hotter 
if the catalytic process is running.  Check the temp at idle and speed to see 
how things heat up.  (Make sure when the emission test is done that the speed 
measurement is done first followed IMMEDIATELY by the idle measurement.  That 
way you get full benefit of a hot cat con.)

       Increased emissions at idle could also be a burned exhaust valve.  Do 
a compression test to check for this one - this might also explain the rough 
idle.

       Low speed ignition misfire might also be a problem - but it has to 
improve at speed in order to explain why you fail at idle and pass at speed.  
See if a mechanic can put it on a scope and get a trace of  plug firing.

       Also check the O2 sensor for proper operation.  It's voltage output 
should vary between ~0.3 to ~0.7V.  If it is dithering back and forth, you're 
OK.  A constant reading or one outside this range indicates something is 
wrong in the fuel injection circuit (either bad O2 sensor or something else). 
 I doubt it is this because I would expect you to fail at idle and speed.  
But it is a possibility.

       Also, there is typically an idle speed HC trim adjustment (not exactly 
sure on your car).  Has this been done?

Good Luck,
Charlie

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