EGR(exhaust gas recirculation) system is there to lower NOx when engine is under load, so a vacuum leak in the EGR system could prevent EGR valve from opening enough, so higher NOx and then extra air coming in, lower CO
High NOx can also be caused by carbon build up in the EGR Valve and tube from exhaust manifold, this build up reduces the exhaust flow into intake when valve opens.
This can be cleaned by removing EGR valve and tube.
High NOx is cause by high cylinder heat which happens when you accelerate or go up hill, engine under load.
The exhaust gas actually cools the cylinders, as odd as that sounds, lol, it also reduces the chance of pinging/knocking.
If you ever get pinging/knocking you will also have high NOx as that raises cylinder temp
Last edited by RonD; 10-28-2014 at 06:20 PM.