Failed Emissions

I have a '91 s-10 4.3 L. I fail the emission test in CA. Below is the read out of the different categories. I only failed in the NO (PPM) at 15 mph. category (2752). With that in mind what should I repair, check, do to past the next test? Thanks Kevin

%CO2 %O2 HC (PPM) CO (%) NO (PPM) mph RPM MEAS MEAS MAX GP MEAS MAX GP MEAS MAX GP MEAS

15 2278 14.7 0.4 129 316 15 0.80 2.10 0.00 1037 1831 2752

25 2197 14.6 0.3 109 266 25 1.01 2.31 0.23 897 1631 678

Is the "Service Engine Soon" light on? If so, what codes are present?

NO or Nitrogen Oxide or Nitric Oxide: It is usually referred to in the automotive industry as Oxides of Nitrogen, or NOx. It is directly related to EGR flow. The slight amount you're over may be due to a partially restricted EGR passage in the intake manifold and it may or may not trigger the light to come on. California emission requirements are lower than the other 49 states. The parameters in the ECM may not be set to trigger the SES light to correspond with California emission laws.

Old Master, My check engine light is not on. The amount I'm over in the NOx 15 mph. category is I think a lot - Max. is 1037 a Gross Polluter is 1831 and my S-10 got a 2752. What is ECM? Thanks for the great info! Kevin

Old Master,
What about the oxygen sensor or the cat? Would that make the NOx really high? Thanks! Kevin

restricted egr, low fuel pressure, too much timing,

ECM or Electronic Control Module, sometimes referred to as PCM or Powertrain Control Module. It gathers and processes data from the various engine sensors and makes adjustments to fuel mixture, EGR, timing, etc.

Rather doubtful that an oxygen sensor or catalytic converter could cause a rise in NOx. Look at your 02%, 14.7, perfect. The oxygen sensor and fuel delivery/control systems are working fine. When a converter fails, hydrocarbons soar sky high, yours are not. Concentrate your efforts on the EGR valve operation. If an EGR valve is totally inoperative on an engine without PCM controlled ignition timing, the engine will spark knock, ping, detonate, what ever you want to call it, terribly. The reason is that EGR lowers combustion temperatures and inherently lowers NOx.

Old Master,
Thanks so much for all the great info! It really explained things well! Does my '91 S-10 4.3 L 5 speed have a PCM/ECM? The truck run good, no knocking or pinging at all. If you were me, would you try to clean the EGR valve or buy a new one. Thanks again! Kevin

Old Master,
One more thing. When I failed the emission test, they said my timing was off 5 degrees. But I check it when I got home and I don't think they disconnected the wire you need to unplug under the dash on the passenger's side. When I check it right, it was at 0 degrees. Would them reading the wrong timing or punching in the wrong timing into their computer give a false reading in the NOx category? Thanks! Kevin

Wherever you go you'll get a different answer on PCM or ECM, basically it's the same thing. You won't have spark knock because the PCM/ECM controls the timing. If/when the knock sensor detects spark knock, the PCM/ECM retards ignition timing until the knock sensor is satisfied. As a result, you lose performance. Depending on how severe the knock is, you may not even notice the difference in performance.

You checked the ignition timing the correct way. When the connector is plugged back in, and the OBD I system is in closed loop, the PCM takes over complete control of ignition timing. The PCM makes changes to ignition timing continuously, and unless they checked AND ADJUSTED, (because you found it to be correct), base ignition timing, they would have no way of knowing what it should be at any given time. They're blowing smoke...or smoking something, one or the other. ;)

After you address the EGR system, you might want to clean combustion chambers with a bit of water injection. Running more ethanol will also slow combustion, lower chamber temperatures, and lower the NOx emission. If you get it to teh point where it is close by repairing the system, the extra couple steps might get you within limits.

After you address the EGR system, you might want to clean combustion chambers with a bit of water injection. Running more ethanol will also slow combustion, lower chamber temperatures, and lower the NOx emission. If you get it to teh point where it is close by repairing the system, the extra couple steps might get you within limits.

Excellent advice!

Old Master and Blue Bow Tie, Thanks so much guys for all the great help! I'm coming here from now on for all my automotive questions! THANKS! Kevin