NOx Emissions Mystery
 

I've enjoyed viewing this board for a while. The discussions are often interesting and helpful. Many knowledgeable posters generously share experience, and most of all, appreciation for this great little car.

I have a '91 Protege DX (SOHC), 5-speed, with 249,750 miles. Performance is excellent, and I consistently get between 30 and 34 miles per gallon of gas in DFW area driving. I've never modified the engine, and have maintained it over the last 13 years.

I sure would be grateful for some input on why I may be failing the NOx component of the TX state inspection test.

The standard is 1026-1134 ppm and I keep getting readings of 2757-2819 ppm. Everything else (HC, CO) gets comfortably passing grades.

Despite changing the catalytic converter, it still fails, with the numbers above. Could a failed oxygen sensor make a big enough difference in the reading? There is no EGR valve on this model, purchased new in California in June, 1991.

I'm stumped, and so are what appear to be competent mechanics who have looked at it. Anyone have any ideas?

Again, I sure would be grateful for some suggestions. I hate the thought of having to give up the car over this silly test.

Try adding that octane 105 stuff to your fuel. It could help burn the fuel more efficiently. Im not sure if it would help your car, I know it helped my parent's van that was burning oil, and it passed emissions.

your timming is off.
your engine coolant temp sensor is on it's way out
while you are at it might as well get a new O2,
and run 89 octane.

had a similiar issue when doing emissions, on a stock B8ME

High NOX is the result of high combustion temperatures. Normally the egr system helps with lowering NOX. Ignition that is too far advanced can lead to higher combustion temps. Running lean can also lead to higher combustion temps, so if the O2 sensor is more than 50k miles old, replace it. The catalytic converter also cleans up some of the NOX, but if it is marginal, I would expect other tailpipe emissions to be high also. What were your other readings for hydrocarbons, CO, etc in relation to the limits (it is on the printout)? They can help point to the problem. How much higher than the limit is the NOX? We need to know the magnitude of the problem.

exexx,

As stated above, the standard for NOx is 1026 and 1134 ppm at 25 mph and 15 mph, respectively. I was getting readings of 2757 and 2819 ppm.

The HC standards were 141 and 146, but I passed those comfortably with 71 and 94 ppm. CO also was well within range -- standard of .79 and .82, and I got .04 and .06.

Everything was well below the limits except for NOx. I ended up adjusting the timing way down to pass the test, but then had to turn it up again to have power to drive normally.

I suppose it wouldn't hurt to change the O2 sensor (probably needs it anyway), or check the timing. I've religiously had the timing belt changed every 60K miles.

Thanks, everyone, for the input.

Based on the CO reading, you are probably running a bit lean. You may want to consider replacing the O2 sensor.
Your ignition timing may be advanced well beyond the specification. Be alert for pinging as we get into the warmer temperatures of summer.