Smog FAQ

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Courtesy of Mike Wassall


Passing Smog (Hopefully)












Retarded timing Decrease Same or increase No change No change Large decrease
Very retarded timing Some increase No change Some to large decrease No change Large decrease
Advanced timing Increase Same or less No change No change Large increase
Rich mixture Some to large increase Large increase Some increase Decrease Decrease
Lean mixture Increase Large decrease Decrease Increase Some to large increase
Very lean mixture Large increase Large decrease Decrease Large increase Some to large decrease
Ignition Miss Large increase Some decrease Some decrease Some to large increase Some to large decrease
Compression loss Some to large increase Decrease Decrease Increase Some to large decrease
Worn engine Increase Increase Decrease Decrease Same or less
Air injection Large decrease Large decrease Some to large decrease Large increase No change
EGR working No change No change Some decrease No change Large decrease
EGR leaking Increase No change Same or less No change Same or less
Cat working Decrease Decrease Increase Increase Decrease with 3-way cat






With the coming of Smog II, aka treadmill smog testing to my area soon, I’ve been doing some reading up on smog testing/controls etc.  As luck would have it, a friend who is in the trade and studying up for his end of Smog II, meaning his testing license, was kind enough to lend me some of his books.  So the table above is a compilation from the prep books for the state test.  I am not a smog tech, or automotive professional, it is just a hobby.  While the above chart should hopefully prove useful, I do have a few more comments to pass on from my readings and personal experience.


If your car has an O2 sensor it has to be working to have a chance of passing.  Test it with a decent quality analog meter or any digital multimeter.  With the engine running and warmed up, the O2 sensor should be putting out about a half a volt, DC.  Actually the books say averaging .45 volts.  And it should be bouncing back and forth, always swinging up and down, from .15 up to .7, at least, more likely from less than .1 up to over .8.  But it should be up and down.  And the testing should be done with it hooked up to the harness, not disconnected.  If it is zero, it is likely dead, they do die and replacement is recommended every 30k, test it disconnected from the harness to verify, and replace it if necessary.  I have had them last much longer than that and not much over 20k, so you never know.  A high reading indicates rich, low a lean condition.  If it runs low, it will probably pass as long as it isn’t misfiring, as long as you don’t have to pass Nox.  If it is rich, it probably won’t pass.  At idle, adjusting the bypass screw in the AFM can compensate for some excessive richness, but really isn’t the cure but may work as long as you can get it to cycle up and down.  A tight exhaust system at least to the cat is necessary, it turns out an exhaust leak is something of an in and out thing and will suck in some air and give a false reading to the O2 sensor which will in turn richen things up since it sees too much oxygen.


If your car is pre oxygen sensor, adjusting the AFM bypass screw will make a definite change in your idle readings, back it out to lean it out, but will make little if any difference in high rpm emissions. 


High CO indicates a rich condition according to the state books.  But it can also be caused by gas in the oil if it has been running too rich, so change the oil if CO is high or you suspect it might be.  It tends to be more of a problem with carbureted cars, but sticky or dirty injectors could also do it on injected cars.


My friend the smog tech says often, not always, but often using the “Guaranteed to Pass” stuff from the auto parts store will in fact make it pass.  I have never used it, but he says it frequently works.  Fuel injector cleaner can also help, I have had some success with Techron, and I have heard very good reports about something called I think it is BK44.  I haven’t used it, I don’t even know who makes it, but those are the only two that seem to get recommended.


If you are fighting a high Nox problem, or even if you have to pass it, the state test books say running premium gas can help.  I’d always thought premium was just for octane, but apparently it can do a little more in some circumstances.


A warmed up cat can make a huge difference, I recently smogged my 87 E24 project car that has all of 180 miles on it in the past two and a half years.  At first on the machine it was reading about 600 PPM on HC, which is way too high.  After a good high RPM burn in (70F degree day and the car was already warmed up to normal temp with an 80C thermostat) the idle reading dropped to 44PPM, and the 2500 reading was 6PPM.  Now I have a problem, I know that, I need to put some miles on it and figure out what is going on, but that shows what a good working cat can do.  But it shouldn’t be necessary, a 528i with a bad cat, but running good is likely to be about 200PPM.  Borderline for passing, but nothing like my 6er was.


Passing smog, the number one thing is make sure the timing isn’t too far advanced, that will make it very hard to pass.  Even try retarding the timing a bit as long as they don’t check it like they are supposed to in California.  But it shouldn’t really be necessary on 528i’s and the 530i already had very retarded timing.  And make sure the vacuum retard is working also or else that will create related problems.  Make sure it is running right and in good tune, plugs OK or better, fresh points if it has them, reasonably clean oil and air filter.  One thing that might make a little bit of difference if you’re are right on the borderline is adjust the valves a little loose, that is the same as a milder cam and could make the difference if you are close, loose like .014, or .016, or even .018, but it will make noise at that point but shouldn’t hurt anything besides your ears.


Like I said, I’m no expert on this subject, but I do know (or have read) a little bit.  I hope this will help you pass what is always a very anxious time for me, smogging a car.


Mike Wassall

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