It is caused by putting too much fuel into a Diesel engine by reprogramming the ECM ( computer ), and it would cause a vehicle to fail an emissions test. The ECM can be reset to factory spec for an inspection though.
Unfortunately, you can't legislate against stupidity.
And I have had it done to me.
So does the driver rev the engine to cause the smoke or - flip a switch on the dashboard?
And having reprogrammed the ECM, does the engine run that way all the time? And if so, isnt that, well, I was going to say crazy, but I guess the whole thing is crazy, nuts and as you say - should be illegal.
Do the cops ever stop anyone for it?
If the engine is not giving out smoke all the time, I guess they would have to see it - or have it reported to them?
Matt