well based on my experience with our 3c-t 2.2L Diesel turbo engine, it's not based on emission. I am aware that smoke is unburned fuel but...

our 2.2L-t Diesel Townace Utility type kasi (which I just destroyed just hours ago) has bad emmision quality... I just can't stand it's dark smoke when I want to hatak the van. but... it is really fuel efficient.... and fastest in our townace... we are in business in selling surplus and local vehicles in far south kasi... we used to landtrip all vehicles coming from subic to mindanao. and most of our vehicles are Pajero,Hilux surf and Townace. of all vehicles we traveled my father fell inlove with one particular Townace Utility 5 door type. because it's the most fuel efficient of all vehicles we land trip. it can carry more because it can fold the second and third row seats... I mean really fuel efficient ever... but years later, because it's smoking bad, I suggested it injection cleaning. the result was... smoke is very low... but shifting gear from 1st to 2nd is really bad. so very late... we used to get 10Km/L on city but now barely 7.5km/L.... but on highways, it still could go 13Km/L

I could barely see smoke, yet the consumption is bad compared to our towance 2c-t that we sell.... anyways... I just destroyed the engine today after "over hatak" (bumigay na ata or because it was fueled by gasoline two days ago,though na drain naman...)

... overhaul or change engine will be it's fate... not an expert basta bahala na kuya ko... pabigat na talaga ako sa pamilya...

p.s no kidding...

so the question, fuel efficient = low emissions? my answer is: not really... but most likely...

personal advice, if your vehicle is still drinking normal but smoke is bad... you can touch the air filter and other stuff but never touch the Injection Pump...