So you've failed smog, now what?
From a thread on TechSHO on 12/13 Jul 07:
Dave Sasak, Typical story
My son just bought a rust free, 1995 MTX, just over 100K. I've driven it - runs smooth and strong. He took it for the Ohio emissions test today and failed - HC was just a little high, and NO was about 1.5X the limit. Any thoughts on where to start? Appreciate any help anyone could offer. ... The HC was 121, against a 120 limit, CO was .30 against a .70 limit, and NO was 2080, against 850 limit. Car has lived all it's life here, so I don't think it's an EGR model (not here at the moment). No chip, MAF, etc. There is no CE light showing - will it still throw codes? Checked for oil in S/P holes - not!
In part it depends on where the CO reading was. Was it normal, or did it happen to be almost zero? Do you happen to have a Calif model with an EGR valve in back? Run the ECM codes too and get back to the group, then maybe we can help some more. Does it have a "chip" or any other non-stock looking parts?
There is nothing much to adjust, so mostly it boils down to normal maintenance items. It could be anything from oil in the plug wells, old plugs, tired O2 sensors, weak fuel pump, tired cats, or just carbon on the pistons(yeah maybe if granny drove it!!).
Let's analyze your condition:
1) Hydrocarbons - This is a measure of unburned fuel, and is caused by being too rich and / or not getting enough spark off the plug. Since you are only a bit high, you can probably pass this element with minimal maintenance.
Gap/Clean/Replace plugs (as required).
Are your wires in good condition? Make sure resistance is not too high. Clean contacts between cables and plugs / Coilpack. Use dielectric to keep out moisture.
Is your charging system in good condition? You want to see about 14.6V in charging mode at 1000rpm or higher. Make sure DIS screws are tight. Turn off all power robbers during voltage check, and during emissions test (a/c, lights, etc).
Clean the MAF!
What gas mileage do you get? If it is abnormally low, you need new oxygen sensors. Dying O2 sensors almost always cause a rich condition.
What condition are your injectors? Cleaning services are available for $50-100. Injector cleaner is cheaper, but not real effective for injectors that have had less than great fuel, and hardly even seen injector cleaner.
Vacuum leaks will cause a rich condition. Check for cracked hoses. Or just replace all the lines.
2) NOx - Nitrogen Oxides are created when the combustion chamber is too hot. You are way over, so this is the area you need to concentrate on most.
Have you decarboned lately? Carbon has insulating properties, and will cause the combustion chamber to retain heat instead of passing it through the cylinder walls. SeaFoam works well and does not disturb the catalytics.
Is your temp gauge indicating a hot motor? The closer to proper operating temp, the lower your NOx readings. A little coolant flush each year is a good thing. Fresh coolant is good as well.
Bad timing. Check it. If it is off, NOx can suffer.
Do you have the proper spark plugs? An incorrect heat range will contribute to NOx.
Finally, there are the catalytics. They do go bad. Along with O2 sensors and fuel injector service, I'd save looking at these until all the other items have been addressed, and the car has been re-tested.
For the test day, try to go when it is raining, or at least very humid. Water in the air reduces combustions temps, and will reduce your NOx.
Warm the car up with a 15-20 mile highway run, and keep the engine idling at 1500-2000rpm if you must wait in line for the test.
Change your oil - dirty oil increases emissions (more friction at chamber wall surface, higher concentration of gases in crank-case).
If you don't use premium gas, or even good gas, do. Higher octane gas will permit lower NOx.
Go with 1/4 to 1/2 tank of gas. A full tank can let liquid gas into the purge system, which makes its way to the intake.
Dave, NOx is usually caused by excessive heat. Suspect the EGR system if present, thermostat, base timing (may be defaulting to 10 deg. BTDC, especially if there's a code 213 or 219). Can be reduced by running the test with the heater on and the shorting plug near the Self Test connector removed per Gary's recommendation (doesn't throw a CE light). I tried removing the plug once when I failed on Nox: reduced o/p by a factor of ten.
HC is caused by a misfire/ raw gas. Suspect plugs and wires, leaking injector.
Codes may be present even though the CE light has not come on.
For the record, CO is caused by running rich: suspect O2 sensors, fuel system.
To lower the head temp flush the cooling system and run pure water with a surfactant like Redline's Water Wetter. Their tests show almost 20 degree reduction in head temp, much less though if you use any anti-freeze with it. Go back to at least 20% anti-freeze after the test if you plan on using the A/C.
Try a cycle of Auto-Rx (www.auto-rx.com). My 200K car had recently developed smoking / exhaust smell issues and the auto-rx has already stopped it after only 1K miles.