E30 318i (M42) NOx doubled since last inspection (also a failure)

Hi, This is the same situation I had the last time I went for inspection... Nox on my 91 BMW 318i failed. Only this time, it has doubled from the accepted level! My car an E30 318i with the M42
1.8L engine was designed without an EGR or any other sort of NOx control, except, i suppose for the catalyst in the cat converter.
Background: For reference, in 2003, my emissions (dyno test) were as follows: NOx 1114 (standard is 1134) HC 97 (standard is 146) CO% 0.33 (standard is 0.82) CO2% 14.1 O2% 0.2
Failure in 2005: NOx 1167 HC 96 CO% 0.35 CO2% 15.3 O2% 0.3
Replaced cat converter, reading that passed in 2005: NOx 1074 HC 133 CO% 0.34 CO% 14.8 O% 0.2
My failure today: NOx: 2126 (DOUBLED!!!!) HC:161 CO%:0.54 CO2%:14.1
As a reference, my 98 Chevy truck (owned since new) had these results on its last dyno test (currently exempted because its an OBDII vehicle, so these are a little old, but done on the same system): NOx 16 HC 7 CO% 0.01 CO2% 15.2 O2% 0
Spark plugs are very clean and correctly gapped, etc., I even ran the new Amsoil PI through the tank just before to ensure that CCs, etc. were cleaned out. No dice.
I do have a dinan chip, Ill be removing it as of right now, to hopefully be able to pass... The car runs great and gets me good fuel economy, Id hate to have to spend $500 on senseless repairs to get a waiver, just because this car never came with an EGR from the factory (by design).
UGH!
Any thoughts or comments would be most appreciated. I have heard that ingnition coils (this car has a distributorless system) going bad can cause funny problems like this. Any thoughts???
Thanks,
JMH
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JMH wrote:

Hi JMH:
I'm not an absolute expert on car automotive emissions but I do know about industrial gas emissions so maybe this will help.
Nox is formed by high gas temperatures during the burn cycle. Most manufacturers handled this by introducing the EGR vale which allows inert gas to be recycled back through the air intake to the engine. The inert gas helps to lower the reaction temperature and thereby lower the NOx output.
Since your car doesn't have an EGR valve the problem is most likely a lean fuel condition. As the gas/air mixture goes lean it raises the combustion reaction temperature and thereby the NOx levels. There is some indication of this in the emissions numbers you gave us. I'd be most suspicious of faulty injectors or old o2 sensors, either can cause a lean condition. Injectors can be cleaned and O2 sensors replaced.
Don't know if this will help but its my best guess.
RJD
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I would say your dinan chip was retarding ignition leading to incomplete combustion (high HC and CO), and in some way to the high NO figure.

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lean mixture. check to make sure you have no vacuum leaks and then check the fuel pressure. The regulator may not be working.
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