Originally Posted by Jhill

How rich are we talking? If it’s constant and high and truly an issue you should have a check engine light by now. You say it’s only from cold start so I would think you may have a slightly leaking injector (if it’s a hard start as well?). This can be checked by doing a residual fuel pressure check and once confirmed you can remove the plugs and check the tops of the pistons for any that are very clean. Other though that comes to mind although I can’t say how it gets effected on Nissan with the vvel but you may clean the throttle bottles and check the air filters. On cold start the ecu opens the throttle more, adds fuel and also retards timing. This is to try and heat the cats quicker so your basically having the air and fuel burn in the exhaust. If you have a restriction on the air inlet (filters/ throttle body) no you have reduced your air to fuel ratio on cold start (you’ll also have a longer warm up time). Again not entirely sure how effected it is with vvel but can be an issue on GM.

Ps scan data won’t show you much if it’s a cold start issue because the AF sensor won’t be accurately reading at that point.