high nox

[stephen p luce]

95 K1500 5.7l failed smog with high nox 723ppm at 15mph. I checked the timing, it's a zero, checked the egr operation, works just like the book says it should, the o2 sensor is switching. What else is there to check? I did a cat temp test at 2000 rpm inlet 354 outlet 248. I'm thinking cat converter, although the smog ck paper work say's pass, maybe that's the visual inspection.

 

[Schurkey]

95 K1500 5.7l failed smog with high nox 723ppm at 15mph.

What is it supposed to be?

I checked the timing, it's a zero,

Does the electronic advance work? Is the timing being retarded by excessive knock sensor counts?

checked the egr operation, works just like the book says it should,

HOW did you "check" the EGR? WHICH "book"? Does the engine stall when you activate the EGR at idle? A "working" EGR system doesn't have any effect on NOx reduction, if the exhaust passages from exhaust valve(s) to EGR valve, or from EGR valve to intake manifold discharge ports are plugged with carbon.

EGR is the FIRST suspect when there's high NOx.

the o2 sensor is switching.

How often? Is it old and lazy, so it doesn't switch fast enough? What are the cross-counts when the engine is producing excess NOx?

What else is there to check?

An overheating engine, or an engine (one or more cylinders) that runs lean can produce excess NOx.

What are your short- and long-term fuel trim numbers when the engine is producing excess NOx?

I did a cat temp test at 2000 rpm inlet 354 outlet 248. I'm thinking cat converter, although the smog ck paper work say's pass, maybe that's the visual inspection.

If the inlet is hotter than the outlet, the catalyst is dead, or the exhaust stream is excessively lean. If the engine is running excessively lean, that'd cause high NOx.

What is your fuel pressure?

I don't think the catalyst on a '95 is intended to reduce NOx. Maybe I'm wrong.

 

[RichLo]

^ Agreed with everything

Also, NOx starts at the spark plug (High temp/High pressure). If you haven't replaced them in a while, try that also.