How do coal rollers control when their vehicle spews black smoke and when it does not?

There are people who - intentionally or not - have vehicles that spew out thick clouds of black smoke. Sometimes they purposefully release the black smoke to harass protesters or someone they don’t like.

But - how do they control it specifically? I’ve been behind some of these vehicles, and they didn’t spew smoke all the time, not, for instance, every time the driver pressed the gas pedal. What is the combination of pedals (or other things) they do to belch smoke on command?

They install a switch that causes the engine to run extremely fuel rich. Most of the fuel goes unburnt and ends up as particulates in the exhaust. There are various means of implementing the switch, but most of them involve lying to the ECU in some way. At least one device plugs into the OBD-II port (a diagnostic port available on all vehicles) and sends commands.

This is specifically an issue with diesel engines that tend to produce a lot of particulates, especially if the diesel particulate filter is removed; running the engine fuel risk produces a large mass of diesel particulate matter (DPM) that are the characteristic of “coal rolling”. Note that DPM emissions are classified as a IARC Group 1 carcinogen linked to lung and bladder cancers as well as general emphysema, COPD, asthma, and cardiac disease. Other Group 1 substances include asbestos, hexavalent chromium compounds, polychlorinated biphenyls, and cadmium, all substances you wouldn’t want to be inhaling or ingesting.


It starts with a “DPF delete” kit, which allows the user to remove the diesel particulate filter in the exhaust system. These have been commercially available for some time, championed largely by truck owners who don’t like the exhaust restriction, power loss, and maintenance that results from having that filter in place. Modifying emissions components like this is illegal, and the EPA has been working hard to get these things off the market.

The DPF delete kit by itself is not enough to roll coal. AIUI, the “lie” told to the ECU that makes the magic happen has to do with manifold pressure: if you convince the ECU that the engine is at high turbo boost, then it’ll inject all the fuel you ask it to when you step on the accelerator. If the boost isn’t actually that high, then you’ll end up with a rich mixture and make black smoke. I expect you’d need to spoof the manifold pressure sensor, and also do something to limit the boost - either open the wastegate, or disable the variable geometry feature (if it’s present on the turbocharger in question).

It goes to show that coal rollers are not merely opportunistic assholes. They’re people who have spent time and hundreds or thousands of dollars to modify their vehicles. It’s premeditated assault.

The ones I have seen have the dual overhead pipes.

I’ve seen plenty in the normal location; however, they’re always oversized pipes - 2-3x the diameter of a standard exhaust pipe.

Given that the modifications are illegal and substantial and that coal rolling is used offensively, it’s surprising that the perps aren’t charged very quickly. I would have thought their plates would be reported by the first victim.

Being in a vehicle nullifies many consequences, whether explicitly or implicitly. When a dangerous action is normalized by society, and driving a vehicle absolutely qualifies, it’s operating heavy machinery after all, it is no longer deemed dangerous by the law. Thus many behaviors that would get you quickly arrested if you were doing it as a pedestrian on the sidewalk are brushed off if you’re driving a vehicle. Coal rolling is assault, so is swerving towards cyclists, or passing too closely, even accelerating too quickly. But there’s a “no touch no crime” mentality from police when it comes to vehicular threats, even though assault, by definition, does not require contact. Very much a double-standard.

Understood but I would have thought that reporting someone coal rolling to those responsible for vehicle emission standards would result in action.

States don’t usually check individual vehicle emissions unless they have a problem meeting NAAQS:

If a state or region is found to be in violation of EPA-mandated NAAQS, then they’re required to develop a SIP:

A SIP is a state’s plan for achieving NAAQS compliance and often targets privately owned vehicles, either with an annual smog check requirement, or (as Denver does) a drive-by roadside emissions check.

If a state is in NAAQS compliance, then they often aren’t equipped to do emissions checks. As noted upthread though, the EPA is targeting bigger fish, with some success:

Like who? The EPA doesn’t really have any law enforcement capabilities toward personal vehicles. As noted above they are seeking a fine for the Windham dealership for making alterations to some trucks, but they can’t levee a fine against personal vehicles on a one by one basis. No manpower or enforcement capabilities.

Large areas in the US have no vehicle inspections at all. My area in Oregon does not. In the 20 years that I have had my muscle car the Dept of Motor Vehicles has never even seen the car. I just go on line to apply for new registration tags, get them in the mail, and put them on myself. They have no idea of the modifications I may have made.