Hi everyone. So my 89 2.9L 4X4 did not pass the NOx portion of smog check. There are no vacuum leaks, no leaks in cooling system, heads are new as well as engine recently rebuilt and have only 38K miles on it. It's a wheeling toy, so I do a lot of idling, which makes me think it might be the CAT. Is there a way to rule it out or is that a shop kind of thing? What else can be causing a high NOx? O2 sensor has less than 20K on it and is a BOCH O2 sensor. Thanks everyone.

 

Nox comes from high combustion temps, or a red hot cat, but I doubt that is the problem.

What was your CO number? If well below the max, you are probably looking at an EGR problem. EGR cools combustion temps, which reduces NOx.

If you were running rich and causing the cat to overheat, you might also have high HC and CO. If both of those are at the high end, run it and check the cat temp.

So, you are looking at the EGR and/or mixture, which could be sensors, throttle sensor, temp sensor, t-stat too cold, possibly other items.

If your CO number passed, you do not need a new cat.

 

...unless your state requires visual inspection, in which case you could leave the cat housing on and just knock out the internals. Best of both worlds.

 

I corrected my comment--he does not need a new cat if he passes CO.

You won't pass CO without a cat unless you are meeting some early high allowance regulation with a really small engine.

 

89 2.9l doesn't have EGR... I think they did away with EGR on '87 and newer 2.9's

 

Where were you with CO and HC? If you were near the max allowable, you may have high temps in the cat causing the high NOx. Any missing? Any vacuum leaks? How about the t-stat?