Emission Control Problems: High HC, CO or NOx. Below are some generic diagnosis notes to help you begin pinpointing why you failed the smog/emissions/MoT etc. tests.
[Excerpted from "Exhaust Emissions Diagnosis: The Precursor to Finding Engine Performance Problems", Larry Carley, ImportCar, June 2000.

GENERAL DIAGNOSIS. Elevated hydrocarbon (HC) emissions usually indicate ignition misfire due to fouled spark plugs or a bad plug. But high HC emissions can also be caused by burned exhaust valves (check compression), lean misfire (check for vacuum leaks, low fuel pressure or dirty injectors), or rich fuel conditions (excessive fuel pressure, leaky injectors or a dead O2 sensor).

High carbon monoxide (CO) emissions are a telltale sign of a rich fuel mixture. On newer vehicles with feedback fuel controls and fuel injection, leaky injectors, excessive fuel pressure and sluggish or contaminated O2 sensors are all possibilities to investigate.

Harder to diagnose are elevated oxides of nitrogen (NOX) emissions. Causes here may include a defective EGR valve, EGR vacuum solenoid or motor, plugged EGR ports in the manifold, over-advanced ignition timing or engine overheating.

IDLE EMISSIONS. A vehicle that has sharply elevated HC or CO emissions at idle will usually have a noticeable misfire and/or rough idle. The most likely causes here would be:
• Fouled spark plug(s);
• Shorted spark plug wire(s) or defective plug boot(s);
• Vacuum leak;
• EGR valve stuck open;
• Burned exhaust valve;

An extremely rich fuel condition can also cause elevated HC and CO at idle, while an extremely lean condition will only cause HC to rise abnormally. A leaky EGR valve can act like a vacuum leak and cause a lean misfire at idle. HC and CO will be somewhat higher as a cold engine warms up because the fuel system may still be running in open loop. Until the engine reaches a predetermined temperature and/or the oxygen sensor gets hot enough to produce a good signal, the ECU will supply a relatively rich mixture while the system is in open loop. A faulty thermostat that is stuck open or a defective coolant sensor may prevent the system from going into closed loop.

NOX emissions are always lowest during idle and decel because that’s when engine load and combustion temperatures are lowest.

ACCELERATION EMISSIONS. During acceleration, the engine momentarily drops out of closed loop and receives a richer fuel mixture for more power. During this time (depending on the system), the Airflow Sensor and the TPS sensor play critical roles in controlling the fuel mixture.
Most fuel-injected engines have either a throttle position sensor or switch that indicates when the engine is at idle. When this device indicates that the engine is no longer at idle, the on time of the injectors is increased to temporarily richen the fuel mixture. The same thing happens any time the engine comes under load and manifold vacuum drops. The AMM sensor tells the computer the engine is under load, and the computer responds by adding more fuel.
It is normal to see some spikes in CO during acceleration, but unusually high CO readings indicates that the fuel mixture is too rich. Possible causes might include:
• Flooded charcoal canister or a leaky purge valve;
• Defective mass airflow (MAF) sensor, or
• Defective throttle position sensor.

If the feedback fuel control system is working properly and there are no apparent sensor or purge valve problems, the catalytic converter may be contaminated or not functioning.
Elevated HC readings during acceleration indicate ignition misfire under load. The causes could be:
• Defective knock sensor;
• Weak ignition coil(s);
• Excessive resistance in spark plug wires;
• Arcing inside the distributor cap;
• Worn, fouled or incorrectly gapped spark plugs;
• Over-advanced ignition timing; or
• Lean air/fuel mixture.

NOX readings will rise sharply during acceleration and will peak a few seconds after the cruising speed is reached. If the EGR system fails to recirculate exhaust back into the intake manifold, combustion temperatures will rise causing an increase in NOX. The higher temperatures may also cause some detonation (spark knock) to occur, which may be audible when the engine is under load. Causes of elevated NOX emissions during acceleration include:
• Defective EGR valve;
• Leaky EGR valve plumbing or control solenoid;
• Carbon deposits in EGR manifold passageways;
• Carbon buildup on pistons and in combustion chamber;
• Over-advanced ignition timing;
• Defective knock sensor or too low octane fuel;
• Engine overheating (check thermostat, fan, coolant level);
• Exhaust restrictions

[Chris Herbst] High NOX is caused by high combustion temperatures. In addition to the items above, check for low fuel pressure due to bad pump, clogged filter, or a bad (low pressure) regulator on fuel supply system.
CRUISE EMISSIONS. At cruise, the engine is lightly loaded and running at high rpm. Under these conditions, HC and CO should be low if the oxygen sensor and feed back control system are working properly, and the catalytic converter is in good condition.

High CO readings during cruise indicate a rich fuel condition. Causes here may include:
• Defective O2 sensor;
• Exhaust leaks upstream of the O2 sensor (check manifold gaskets and air plumbing
connections);
• Defective AMM sensor;
High HC during cruise would indicate a steady misfire or loss of compression (leaky exhaust valve).

DECEL EMISSIONS. When decelerating, the engine will typically either lean out the fuel mixture or shut the fuel off completely (some fuel-injected engines). The computer typically uses inputs from the Vehicle Speed Sensor, TPS, Airflow sensor, and engine rpm to determine when this occurs. When the throttle closes and manifold vacuum shoots up, the computer cuts back on the fuel. Normally, HC, CO and NOX emissions drop during deceleration because the engine is no longer under load and is receiving little or no fuel.

If CO emissions remain high during deceleration, the engine is receiving too much fuel.
Causes may include:
• Leaky fuel injectors; and
• Faulty VSS, TPS, or airflow sensor

Appending:

The Mike J. AWD With Stand Alone Engine Management E Test Pointers


Aftermarket cams
Aftermarket cams introduce a lot of overlap between the intake and exaust. This will make it tougher to pass idle emissions. The larger the overlap the more you will have to compensate (I have HKS 272s).

Catalytic Converters
All cats were not created equal. There are cats out there that work well and there are cats that don't. If you buy the $100 3" hi-flow model (like me) you are probably getting one that doesn't work well. Again this will lead you to over compensate with other tuning.

It is best to get your cat hot before the test. If you can do a couple boosted sprints then drive it directly into the test bay that would be best case.

Air Fuel Ratio
Todays catalytic converters work best when the ECU cycles the air fuel ratio around 14.7:1. However, 14.7:1 is a compromise between having low HC and CO emissions and having low NOx emissions. HC and CO actually got down as you go leaner, but much over 14.7 and the NOx emissions go through the roof. Go any richer then 14.7:1 and the COs go through the roof (and HCs go up). But since AWDs just do an idle and 2500 idle test NOx is not measured. So going leaner then 14.7 can help you.

You can go too lean though, try to keep the car from missfiring much.

For my car to pass I had to run 15.2:1 at 2500 rpm and 15.5:1 at idle (1075 rpm).

Timing
For the 2500 rpm test I ran 16 degrees advance and that worked fine.

For idle I actually seemed to find that retarding the timing helped reduce HCs. I have read a few places online that this is true only for idle. I had to reduce my idle timing advance down to 4 degrees. Don't ask me to explain this one I'm just not sure. It might have something to do with the cams... I just don't know. I do know that retarding the timing helps reduce NOx emissions (but AWDers don't care about those). Retarding the timing did not seem to affect CO emissions.

You do have to make sure you keep the burn good though. DOn't retard or advance the timing to the point it starts missing.

Idle rpm
It can be an advantage to up the rpm at idle for emissions. Higher idle gets better air velocity and so a better mixture leading to better and more consistant burns. It also reduces the effects somewhat of aftermarket cams. There will be a limit though on the test machine. For my 98 that limit was 1250 rpm (has to stay under this value or you fail). The machine was also reading about 80 rpm high so stay a little under. I set mine for 1075 rpm for the test I passed.

Small Note: All Mitsu's like the DSM's and C53A Colt Turbo's received a 3 WAY CATALTYIC CONVERTER with no air provision. You can use a CAT with an air provision but just leave it plugged as they come that way. Do not purchase a 2 WAY catalytic, so this will help you to know what to look for. This is what my car needed all along, including after converting it back to stock in every way and flushing the gas!

I guess I should have read this before my post in General Tech. That'll teach me to search, sorry for my douche-baggery.

Jerry, thanks for the info.