Emissions - High NOX GPM

I took my 2000 4.7L in for emissions yesterday. They tried to fast pass it but they ended up having to run it on the dyno. It did pass and everything looked good, minus the NOx GPM readings. The limit is 2.00 and my truck was 1.491. I think when I ran my 2003 Tahoe, the nox readings was like 0.15 so this is high. While it did pass, it has me concerned on how high the numbers are and if it will pass next time in two years. I did some research and looks like it could be a number of things causing higher readings. My truck has been completely gone through since I bought it two years ago. I made sure it was good and hot when I got there but did sit idle for 30-40min since the line was long. Anyone familiar with emissions and where to even start looking? I'll double check all the vacuum lines but they should all be hooked up.

All work done:

• Tune up (spark plugs, fuel filter)
• Timing belt/coolant system (pump, coolant, belts, hoses, t-stat, etc)
• Synthetic oil recently changed. Always change the air filter with the oil
• Cleaned MAF and throttle body

 

Should have an EGR system on it. Make sure vacc hoses are in tact and hooked up with no leaks.

You can also remove EGR valve to inspect for restriction.

NOX is a symptom of high combustion temps.

If CO and HCs were good, then it isn't a tuning, fuel mixture, or timing issue.

Lastly it could be worn out cat converters. But EGR system is far easier and cheaper to check first.

 

I don’t think these old dinosaurs have an EGR, at least from my research. Cats could be suspect as it is pushing almost 200K.

 

Different generation but it would be something like shown here.

DIY: MAF, Throttle Body and EGR Cleaning Write Up | Toyota Tundra Forum (tundras.com)

Searching on rock auto for 2000 tundra does not show any results for egr for a 4.7.

So then it would be a cat converter issue.