Has anyone ever noticed if removing the plenum "walls" in front of the EGR ports (behind the throttle body) has any affect on NOx levels? Weird question, but I'm wondering if this is contributing to emission failure (CA) for high NOx at both 15mph and 25mph. 1. No trouble codes set. 2. Mixture is not lean; scan tool shows Integrator at 128 +/- 3 at both speeds. 3. EGR valve and solenoid is functioning as it should be. 4. ECM is requesting EGR. 5. No vacuum leaks. I'm having my doubts that the porting has anything to do with this. Emission test history: 2005 - 85984 miles 15MPH - HC: 110 Max - 105 Measured: PASS 25MPH - HC: 85 Max - 71 Measured: PASS 15MPH - NOx: 772 Max - 772 Measured: PASS 25MPH - NOx: 711 Max - 492 Measured: PASS 2007 - 86175 Miles; Diff - 191 miles 15MPH - HC: 110 Max - 130 Measured: FAIL 25MPH - HC: 85 Max - 55 Measured: PASS 15MPH - NOx: 772 Max - 1233 Measured: FAIL 25MPH - NOx: 711 Max - 384 Measured: PASS 2009 - 86415 Miles; Diff - 240 miles; New Cat 15MPH - HC: 110 Max - 60 Measured: PASS 25MPH - HC: 85 Max - 38 Measured: PASS 15MPH - NOx: 772 Max - 1490 Measured: FAIL 25MPH - NOx: 711 Max - 708 Measured: FAIL The new cat has addressed the HC levels, but I'm stumped with the NOx. The EGR passages are clear. I just removed the plenum and runners to check.

Just because the NOx is high does not mean the EGR is not functioning. NOx is created by high combustion temps, I'd be looking at the timing myself. Might be too advanced at 15MPH.

I know of a lot of cars with those walls long gone and no testing issues so I'd guess that is not your issue.

Checking your timing to make sure it is set for 6*BTDC and make sure it is running on the stock Chip or Timing map.

It would also be a good idea to flush the injectors with Berrymens B12 if you can gain access to the proper equipment. Basically it is a tool that you put it in, pressurize with an air compressor, and feeds the fuel rail through the fuel pressure test fitting. Disable the fuel pump, pressurize it to about 30-35 PSI and start the engine. It will flush the injectors and clean the intake valves and combustion chambers.

My van does not even have the EGR enabled and blew almost no NOx.

Thanks for the suggestions. Timing is set to specs. I've already run injector/combustion chamber cleaner through the fuel rail using pressurized equipment.