Based on the carbon buildup around the pintle I suggest that the exhaust passage under and through the floor of the intake manifold is similarly caked if not worse. The telltale is whether or not lifting the pintle completely off of its seat at idle stalls or nearly stalls the engine. So you are still faced with finding a means to accomplish that. If you can't figure out an alternative, thoroughly clean the caked carbon from the old valve and reinstall it; then you can lift up on the old EGR diaphragm to evaluate exhaust passage restrictions through the head and intake manifold. If the passages are in fact clogged with carbon, there are various means to de-kludge them along with the combustion chambers (recall Viles reference to generating copious plumes of smoke). Beyond the application of very select chemicals are stiff but flexible metal rods, compressed air, and worse case, removing the intake manifold and having it hot tanked.
FYI - oxides of nitrogen are produced during the combustion process exacerbated by high combustion temperatures. Various means of lowering combustion temperatures reduce NOx emissions (e.g., EGR, modest ignition timing, decreased compression ratio, etc). Specifically, the EGR valve dilutes the intake charge with pre-burned gases decreasing the volume of oxidizable compounds. Decreasing the available fuel and air to burn equates to lower combustion temperatures, hence, lower NOx emissions.
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