Finally! Possible DEF relief for diesel-powered RVs, trucks

Last year, untold numbers of diesel truck and motorhome owners were stranded when their rig’s DEF (diesel exhaust fluid) systems acted up. Some sat it out for weeks—even months—waiting for unavailable repair parts. Other RVers left their vehicles parked in their driveways because they feared their engine might act up, leaving them stuck beside the road.

The problems stemmed from faulty DEF sensor readings, which caused engine computers to “derate” or severely reduce operating speeds. At the heart of the matter are pollution control regulations from the Environmental Protection Agency. The agency got an earful about these problems, and now the EPA proposes new rules on engine derating. How might you be affected by these rules?

What’s all this about derating?

What’s behind derating? A little background will help explain. The EPA set new rules for diesel engines manufactured from 2010 onward. They set air pollution standards, and industry was obliged to figure out how to meet those standards. In the end, the industry’s answer was to use DEF, essentially a mixture of water and urea. Pump a controlled amount of DEF into the exhaust stream; there the DEF is converted to ammonia. The ammonia then breaks down nitrogen oxides, significantly reducing pollution.

With the DEF approach, the EPA was concerned that drivers would be reluctant to spend the money or time to keep putting good quality DEF into their vehicles. Perhaps they might just pour water in their DEF reservoirs. So the agency told manufacturers they had to install DEF monitoring systems. If somebody failed to use DEF or, say, watered it down to the point it wouldn’t do the job, they needed a wrist-slap. In EPA parlance, an “inducement” was required—something that would make using quality DEF more desirable than not.

The EPA proposed manufacturers “include a derate of the engine’s maximum available engine torque of a sufficient magnitude for the operator to notice decreased operation.” The agency suggested that “at least 25 percent” might be enough to notice. If DEF wasn’t refilled, then a “progression to further degradation” might be in order. The industry took the EPA’s “suggestions” to heart. That “progression of degradation” for many manufacturers meant failing to respond to warnings would leave a driver with a rig running no faster than 5 miles per hour.

More issues

What industry failed to account for, or at least interpret properly, was additional guidance. The EPA told the industry the idea would be that the “inducement” “should not create undue safety concerns, but should make sure vehicle operators are adding DEF when appropriate.” It’s hard to picture how a motorhome traveling a busy interstate highway, derated to 5 miles per hour, could be anywhere close to avoiding “safety concerns.”

Last year’s debacle of motorhome owners stranded beside roadsides brought up another point, also addressed in the EPA proposal. The vast majority of RVers stuck with derated engines had good quality DEF in their tanks. The problem was faulty DEF sensors, reporting nonexistent problems which triggered the over-enthusiastic derate responses. In most—if not all—cases, the pollution control systems were actually working. DEF was being pumped into the exhaust stream, and nitrogen oxides were being properly reduced. But the technical glitches sidelined RVers who had no control over the problem.

Changes for 2027

The EPA is now working on rules for heavy diesel vehicles that would go into effect in the 2027 model year. Among those rules, the EPA proposes changes to the engine derate issue. Instead of making suggestions, the agency will frame rules for the industry to follow. What’s the bottom line?

For “high-speed vehicles” like trucks and presumably motorhomes, if operators failed to keep the right quantity and quality of DEF in the tank, here’s what would happen. A dash warning system would tell the driver there was a problem with the DEF system, and a refill was needed. If the warning was ignored, then the vehicle speed would be reduced to no more than 65. After six hours of failing to respond, the speed would drop to 60. At the 12-hour mark, 55 would be the maximum speed. And if the driver took no action, then 50 miles per hour would be the top speed after 60 hours from the first warning.

But what about technical glitches, such as the ones that have caused RVers so much grief? Motoring along with a full tank of high-quality DEF, but derated, nonetheless? The EPA proposes that if the rig is still passing nitrogen oxide standards, as measured by the vehicle’s own sensors, then no derate would happen. The driver would still get a warning, but the engine computer should not kick in speed derates. For do-it-yourselfing RVers, the new rules would also allow them to use a generic scan tool to reset the onboard computer, once they’ve made the necessary repairs. That’s quite a difference. At present, you’d need the specialized service of a shop to get rid of those “check engine” lights related to DEF system problems.

But what about existing rigs?

Should the EPA’s proposals become a rule, all’s well and good for 2027-and-beyond rigs. But what about those who are laboring under DEF sensor problems today? Is there any light at the end of the tunnel? Writes the EPA in the March 28, 2022, Federal Register, it may “be appropriate to allow engine manufacturers to modify earlier year engines to align them with the new regulatory specifications.” The agency is quick to add, “We are not proposing to change the regulation to address this concern. We are seeking comment [from industry and the public] on whether and how manufacturers might use field-fix practices under EPA’s field-fix guidance to modify in-use engines.” They add this could include changing existing “5 mile-per-hour” derates to the 50 mile-per-hour over 60 hours proposal.

It would seem the ball will largely be in the engine manufacturers’ court on this. Will Cummins, et al., be willing to spend a few hours of programmers’ time to rewrite the engine computer control codes? If they were, then will they likewise provide the wherewithal to have that new code plugged into existing engines? Only time will tell. For those who worry their existing rig could be sidelined by false reporting equipment, there’s a solution. Check it out in this article, It’s out there – a DEF sensor workaround.

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