Dealing with diesel emissions

Nov. 1, 2016
Learn how to properly maintain the growing number of modern diesel engines on the road.

Not so long ago testing diesel emissions meant simply looking at the exhaust coming out of the tailpipe and making sure it didn’t look too bad.

Times have changed.

Emission standards for diesel-powered vehicles have become very strict and complying with these strict emission rules, while still producing a powerful and responsive vehicle, is definitely no easy task for manufacturers. Balancing power and emission control is critical though, so some new diagnostic strategies were developed and are now widely (and successfully) used to keep newer diesel-engine emissions down at the levels where they need to be. And it’s well worth learning about these diesel emission management systems since they’re so widely used and diesel vehicles are indeed gaining in popularity.

Some diesel emission management strategies are fairly simple – like reintroducing EGR valves to reduce NOx emissions.

Some strategies, however, are quite complicated and tamper-resistant, such as installing diesel particulate filters (DPF) and selective catalytic reduction (SCR) systems that use diesel emission fluid (DEF) on newer vehicles – systems that do work well, but unfortunately can cause real headaches if you’re not sure what you’re dealing with or if you don’t know what to expect during service.

Vehicles that spend much of their time idling or driving at low speeds may need the DPFs replaced more frequently than ones that do highway driving.

Complicating matters further, many newer diesel vehicles will go into “limp” or “idle-only” mode (derating the engine so that the vehicle can only be driven at very low speeds until the emission-related fault is fixed) when a problem in the emission system is detected – and it may take time and patience to reset the emission systems back to normal operation afterwards since fixing the problem is not always as simple as it would seem.

In other words, even if the only thing you have to do with diesel-powered vehicles is the occasional oil change, it’s still important to understand the basics of how diesel emissions are managed so that you don’t inadvertently cause problems for your customers, or waste time doing things that won’t repair problems effectively because mistakes can be both time consuming and costly.

Common problems with diesel emission systems
  • SCR / DPF clogged (Typical fix is regeneration, resetting parameters – or replacing the component)
  • Tank heater fails / tank and pump cracked (Typical fix is replacing the component)
  • Contaminated DEF (Typical fix is testing and replacing the DEF)
  • Programming updates (Typical fix is performing updates as required)
  • Running out of fluid (Fixes vary, typically topping up fluid and performing regeneration if required)
  • Stuck in Idle-only mode (Typical fix is correcting the cause and performing regeneration)
  • Won’t go into regeneration mode (Typical fix is correcting the problem in the underlying system and checking for calibration updates)

But don’t worry. Dealing with diesel emission systems is actually manageable and even profitable with a bit of background knowledge and a few hard-earned tips to fall back on.

Here’s how.

Dealing with DPFs

Diesel-engine exhaust emissions, unlike gasoline-engine exhaust emissions, must have the particulate matter (“soot”) removed, which is commonly done by filtering it out with a diesel particulate filter (DPF).

Always consult service information for specific details about the vehicle you’re working on (as with any vehicle system), but in very general terms diesel particulate filters are located inside the exhaust system, much as catalytic converters are, and work by forcing exhaust gas to pass through a substrate that traps the unwanted particles while allowing the gas to pass on through.

And while the system does indeed work to trap those unwanted particles, DPFs (as with most filters) do become restricted over time and require cleaning so that they don’t completely clog up, impede exhaust flow or otherwise affect engine performance.To do this, DPF operation is monitored by a control module and a self-cleaning process (“regeneration”) is commanded by the control module as required to keep the DPF working well. And unfortunately, regeneration is one of the more complicated diesel emission control systems used and it can definitely be challenging to manage.

As a professional touch, clean any spills off the threads so that crystals don’t form and cause a sticky problem.

Regeneration involves heating the DPF to an extremely high temperature, which burns off the particulate and cleans the filter out, returning it to its previous performance level. The control module typically looks to command a regeneration event while the vehicle is travelling along at highway speeds (it takes about 30 minutes of uninterrupted higher-speed driving for this to be successful) so that regeneration process is barely noticeable to the driver if at all and the emission system will, for the most part, keep itself clean and working well.

However, if those computer-commanded events aren’t enough to maintain DPF efficiency – which tends to happen more frequently on those vehicles that spend much of their lives idling away or in sitting low-speed traffic – regeneration can also be initiated manually using a scan tool as required or as directed by service information.

Manual regeneration typically involves running the engine at about 2000 rpm for 30 uninterrupted minutes (again, always consult service information for specific details about the vehicle you’re working on). Making sure the vehicle is outside and there are no combustibles nearby is critical though because it gets very hot as the particulate incinerates away.

Furthermore, if there’s ever a problem within the DPF or emission system the vehicle will “derate” and be limited to very low-speed travel until the system’s problem is successfully fixed. 

But, from experience, things are not always so straightforward with DPF systems.

Unfortunately, vehicles have been known to derate and go into limp-only mode at the worst possible times such as on a job site or off-road, somewhere far from civilization and scan tools.

The fuel filler neck and DEF fill point are different sizes so fuel isn’t accidentally introduced into the DEF – but it still happens, so checking DEF quality can be an essential diagnostic step.

Additionally (from experience) derated vehicles often don’t need just a manual regeneration cycle commanded with a scan tool in order to return to normal operation. Successfully repairing the vehicle may actually involve several diagnostic steps, involving checking, recording and clearing DTCs as well as resetting the modules and even performing the regeneration process manually two or more times on the vehicle before it’s ready to be returned to the customer – which is very time consuming (especially if multiple regeneration cycles are needed) and not something customers tend to enjoy waiting around for.

So it’s wise to understand not only common faults that occur with DPF systems, but also what to expect during the repair – often patience and attention to detail are critical. Also, more than ever, checking for service bulletins, programming updates and revised procedures before beginning anything is essential since sometimes a programming update is the only way to permanently fix a diesel emission system problem.

For example, one of our customers’ work trucks was repeatedly going into limp-only mode while out on service calls and more than once a scan tool was taken out to the vehicle to perform the manual regeneration in the field. Updating the ECM calibration was what finally fixed the problem permanently.

That said, typically the procedure to repair a derated vehicle involves verifying the problem is indeed from the emission system, repairing the fault, performing forced manual regeneration as required, and resetting any parameters with the scan tool. Service information will have the exact details and should always be used, even if you’ve performed the procedure before since updates to tend to happen.

It’s not uncommon for it to take two hours or longer to correct a derated vehicle even if everything goes well, and it’s really important to follow the service procedures exactly as written, which takes patience and attention to detail but is often the only way to repair the problem successfully. There’s often no magic shortcut to success – it just involves being really, really thorough.

The DEF fill points are typically located next to the fuel filler neck – but check service information or the owners’ manual if you’re unsure, as locations vary.

Also, it’s also worth noting that DPFs can only be cleaned by a regeneration cycle (manual or automatic) so many times before this self-cleaning process stops being effective and more serious and expensive actions are needed – which usually means replacing the DPF itself.

True, it is possible to remove the filter and “bake” it clean in a commercial cleaner – and some shops are very good at it – but (from experience) removing and cleaning the DPF can also only be done so many times before it’s also just not effective anymore either. That’s one of the downfalls of the DPF system.

One technician at a downtown dealership has actually given up removing and cleaning customers’ DPF filters since the vehicles he services tend to spend so much time idling in city traffic (and thoroughly clogging up their DPF units in the process) that most of these attempts to save his customers’ money didn’t last very long before the vehicle returned with the same problems.

His policy now is replacing DPFs when they’re beyond regeneration – an expensive but necessary decision to keep his customers’ vehicles problem-free and on the road.

In other words, it’s worth keeping in mind that if you’ve cleared the codes and reset the modules and tried to a few regeneration cycles but the system keeps aborting the regeneration process or continues to go into idle-only mode (and there are no applicable bulletins or updates) it may be time to replace the DPF rather than keep trying to clean it.

Always consult service information and be sure and diagnose any fault with an open mind (and using the most recent diagnostic information), but it’s important to know that DPFs don’t last forever, especially on those slow-moving vehicles that are particularly hard on their emission components.

Diesel Exhaust Fluid (DEF) and Selective Catalytic Reduction (SCR)

Another way diesel emissions are managed is with selective catalytic reduction (SCR) systems which are designed to keep nitrous oxide (NOx) emissions low enough to meet those incredibly strict emission standards.

Again, always consult service information for specific details about the vehicle you’re working on, but in brief SCR systems work by spraying diesel exhaust fluid (DEF) into the exhaust stream where it converts to ammonia on the catalyst, causing a reaction that converts NOx into nitrogen and water thus keeping the vehicle’s NOx emissions within an acceptable range.

DEF does need to be checked and topped up regularly (a warning on the instrument cluster lets the driver know when the fluid is low) and the control system will derate the vehicle if it runs out of fluid or if a problem with the fluid or system is detected. Many manufacturers make the DEF fill point much smaller than the fuel fill port to reduce the changes of accidentally putting diesel fuel directly into the DEF system. Actually, having a very clean funnel just for topping up the DEF is an excellent way to avoid contamination and avoid spillage during routine service. Also be sure to wipe any excess DEF off the threads so that they don’t become sticky and cause problems with the cap getting stuck to the threads – a nice, professional touch indeed.

Inside view of the (troublesome) DPF that was removed from a Ford truck

Overall, SCR systems and DEF are actually quite simple to work on and around (in theory) and need very little input other than regularly topping up the fluid and avoiding contamination.

The service-world reality, however, is that it’s a bit more complicated than that.

One service advisor at a GM dealer says she regularly sees customers bring their vehicles in idle-only mode with their DEF fluid warning lights on and absolutely no idea what’s gone wrong. She also says that if there was even a single thing technicians should know about SCR systems it’s how to reset the systems when they run out of fluid – it’s just become that common of a problem.

Fortunately, fixing a vehicle that derated because it ran out of fluid may be as simple as topping up the fluid and releasing the vehicle – but more often it may involve filling the fluid, then performing a regeneration, then resetting various parameters and codes and then finally performing a test drive before releasing the vehicle to the customer. It’s important to check service information and be sure before starting any work.

Additionally, not only can running out of fluid be problematic, contaminated or poor quality DEF can also cause problems in the SCR system, leading to warning lights and derated vehicles.

Manufacturers do have various ways to test DEF quality such as test strips, refractometers or hydrometers if you suspect contaminated DEF is an issue – or if you suspect DEF has accidentally been added directly to the fuel rather than to the DEF tank.

And if it turns out the DEF does need to be replaced (and the problem is not just an outdated PCM recalibration – a surprisingly common problem) consult service information for the specific procedures, but expect to remove and drain the tank, reinstall everything and then refill the system and road test the vehicle to confirm that the repair was successful. And of course, use only high quality DEF.

We learned this the hard way when one of our customers brought a vehicle to us in idle-only mode. The vehicle was serviced and returned to normal operation – and then returned six hours later once again in idle-only mode which was very embarrassing.  The problem was traced to poor quality DEF from a new supplier. Removing the DEF tank and replacing the contaminated DEF in the vehicle with known good fluid fixed the problem permanently – a mistake we won’t make again.

Additionally it’s worth noting that DEF can freeze (it’s a mixture of deionized water and urea so it’s important not to overfill the tank), and frozen or semi-frozen DEF affects sensor operation and can cause the system to malfunction. True, reductant tank heaters are used to prevent this problem from happening – but, from experience, the heaters are indeed known to fail.

In one instance, a customer brought in a 2011 Ford F-350 with the check engine light on.

DTC P20BA “Reductant Heater A Control Performance” was stored and was quickly diagnosed as being caused by the heater’s resistance being out of specification (the specification is 1.0 - 2.5 ohms and actual reading was 3.8 ohms, so the problem was fairly obvious).

Vehicles that idle excessively and seldom reach higher speeds – like this Ford F-450 service truck – are especially hard on their emission systems.

Sure enough, replacing the tank heater (after cleaning caked-on mud from the tank to avoid contamination issues) and performing a regeneration cycle fixed the problem and after road-testing it the vehicle was released to the customer without any further issues.

Actually, sometimes diagnosing a frozen heater is even easier than that.

One vehicle simply came in for regular service during the winter months and the tech noticed during the routine inspection that the tank had frozen and grown much larger in size, wrecking the tank and actually cracking the pump as well – replacing the components and the faulty heater corrected the problems.

And although it’s nice when problems are straightforward, SCR-related problem are, unfortunately, not always so easy to diagnose.

Another Ford truck (this time a 2014 Ford F-550 6.7L) being diagnosed for low-power simply would not go into regeneration mode when commanded with the scan tool, which was getting very frustrating. The DPF was reading 491 percent, indicating that the SCR had indeed plugged right up and regeneration was required.

However, updating the calibration and resetting the DPF and SCR parameters still would not allow the vehicle to go into regeneration mode. Frustrating indeed.

It took some patience, but in the end actually replacing the SCR (and related parts), reflashing the NOx module and then finally resetting DPF and SCR parameters fixed the problem and the vehicle returned to normal operation and has been working well ever since – a bit more challenging to solve but the vehicle was fixed correctly in the end.

So while SCR systems are indeed complicated, problems can usually be solved by checking for TSBs and calibration updates, following service information exactly as written and by having patience and paying careful attention to detail – but if not there are old-yet-reliable diagnostic tricks that still may help.

One memorable 2011 Ford F-550 pickup came in as a no-start with DTCs that just would not be cleared (P200C “Diesel Particulate Filter Over Temperature Bank 1” and P200E “Catalyst System Over Temperature - Bank 1”).

After diagnostic tests were done, new sensors were installed and the vehicle was retested – and DTC P2471 “Exhaust Gas Temperature Sensor Circuit High Bank 1 Sensor 4” appeared and the “Number 4 EGT Sensor” was now reading off the scale.

Great.

So number 4 EGT sensor was swapped with a known good sensor and the vehicle was retested – and it was OK, proving that “swap-nostics” is still a very valid diagnostic aid.

The parameters were reset and the vehicle was road tested to verify that it was indeed fixed before it was released to the customer, and it’s still on the road to this day.

Swapping known good parts may not be glamorous but it can help verify a problem with a component.

Even though times change, some diagnostic strategies will still work well when things get tough.

Conclusion

No doubt there were negative news stories surrounding diesel emission management and unfortunately that caused problems for all of us in the industry, not just those directly involved. The whole industry looks bad when one player gets caught cheating.

And the fact is that diesel emission systems exist and aren’t going away – which maybe for the best since it’s nice to breathe – so it’s wise to understand at the very least how the systems operate and how to avoid causing problems, and maybe a few common problems and fixes to make things easier, to go along with old and reliable diagnostic techniques.

Diesel emission systems can be challenging to service and work around so it’s important to look for bulletins and updates, follow procedures exactly as described and have extra patience since service procedures often need to be attempted more than once to successfully restore normal vehicle operation, and that’s even under the best circumstances. But it’s nothing you can’t handle.

There are no shortcuts, but by understanding how modern diesel emissions work and managing them like a professional you can keep your customers happy, minimize problems and ensure their vehicles run trouble-free and pollutant-free for many years to come.

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