Driving home recently, I noticed a truck quite a distance ahead of me. It was conspicuous due to the dark smoke plume coming from its stack. As I caught up to it, I was greeted with a bumper sticker proudly proclaiming, “Just ‘cause there’s smoke, don’t mean it’s broke!”
It’s probably a safe bet the truck was certainly not being properly maintained – no modern highway diesel engine should continually output visible smoke. Indeed, the vast majority of 1994 to 2006 on-highway diesels have peak opacity less than 10% in documents submitted to the EPA. Typically, most states specify peak opacity limits for diesel engines at 30-40% - three to four times more than diesel engines should have when functioning normally. Naturally, the state of California has several programs dealing with exhaust opacity.
More importantly, many enforcement officers use visible exhaust emissions to select trucks to inspect. During my recent discussions with some of them, one officer unofficially told me that 9 of every 10 trucks having clearly visible exhaust opacity will fail inspection and likely be penalized – not particularly reassuring when you drive visibly-smoking equipment that makes you a target for inspections.