Exhaust fumes inside the car

[Filthy Filbert]

Just a wide band air/fuel ratio meter.  There are lots of options out there.

This is the one I am currently using:

https://www.amazon.com/gp/product/B00N3VGPYS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I'll never try to dial in air/fuel ratios without one of these again.    Tune your idle? easy enough to do.   but to properly tune your part throttle cruise or WOT, you have to shut the engine off and come to a stop, pull a plug because otherwise, as you return back to idle and pull into the garage, your idle mixture may clean up the plugs and make them look "good" again. 

the wideband O2 sensor allows you to monitor actual air fuel ratios real time.  So go drive down the road and see what you're getting.   read what you're at for idle, see what you're at when WOT acceleration.   I use an LM-2 which is a bit older now, but still works great.   The cool thing about the LM-2, is that if it's a modern vehicle, I can plug into the OBD port and capture anything from the engine computer--vehicle speed, engine rpm, coolant temps, intake air temps, intake manifold pressure or mass air flow, etc.   And I can add analog inputs to the LM-2 for an older vehicle, which allows me to capture the same things. 

And I can record it to view it later on a computer screen as graphs, so I can see that at 2500 rpm I'm lean, then rich at 4000 rpm, etc. without having to watch the wideband while driving, which can be dangerous at WOT.