NOx is influenced by combustion temps. The higher the combustion temp, the higher the NOx readings. To reduce combustion temps, reduce spark advance, and make sure the EGR is working.
How to pass emissions testing:
1.) Make sure all the emissions gear the car was made with is present and connected up properly. That includes a working smog pump and cats.
2.) Make sure that you have fresh tune up with spark plugs, plug wires, cap, rotor, fuel & air filters. An oil & filter change is a good idea while you are at it.
3.) Dump the codes and see what the computer says is wrong…
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.
4.) Post the codes and get help to fix them. Don’t try to pass with codes not fixed. Clearing the computer just temporarily removes them from memory, it doesn’t fix the problem that caused the code to be set.
5.) Be sure to do the testing on a hot engine. Drive for 15-20 minutes prior to taking the test to get operating temps up into the normal range. Do not shut off the engine while waiting for your turn on the test machine. An engine up to full operating temperature puts out fewer emissions.
High NO - high combustion temps - retard timing, check EGR for operation.
High CO - EGR isn't working - fix it.
High HC - fuel mixture too rich - check O2 sensors, smog pump, and cats. Clean MAF element. Do the ethanol thing as suggested.
Thanks for the tips jrichker! I did most of the tune-up stuff like plugs, wires, cap, rotor, all filters, oil change and my engine was warmed up (10 minutes of stop and go driving) when I went for the test.
Should I clean my EGR? Never done it before but it doesn't look like to much trouble pulling it off. What/how should I clean it and is there anything I need to watch out for? Can't really tell but is there a gasket used to mount the egr to the throttle body?