According to current diesel smog regulations, every diesel motor produced since January 2008 MUST have an OXS catalyst, a DPF, an NOS catalyst, a H2S catalyst, and a dual EGR system. “Catalytic Converter” is ofttimes shortened to “Cat”. Lots of stray letters, let us explain the CR diesel emissions control components. From the engine and turbocharger towards the rear of the car: The oxidation catalyst (OXS) is the old same thing since 1996, but now it lives above the diesel particulate filter (DPF) inside the same canister. Most like to call that first canister the DPF, although it does contain the first OXS cat. The diesel particulate filter captures soot, which it later burns into ash during a regeneration cycle. Some call that “regen” for short. The DPF canister has an oxygen sensor in the top to check the mixture. The DPF canister also has a dual pressure sensor, one before the DPF and one after the DPF. The pressure sensors are used to check the back pressure as the DPF clogs with soot, or the soot load. The soot load can be viewed in measuring value blocks (MVB) using a factory scan tool or computer with VAG COM software.
Second in the system is the NOS cat which reduces oxides of nitrogen or NOx. Then a third H2S cat which reduces sulfur emissions. There is another oxygen sensor between the NOS cat and the H2S cat that monitors the NOS cat for condition. The system has 4 exhaust gas temperature sensors (EGT) total. One EGT in the exhaust manifold before the turbocharger, and another EGT after the turbo. These 2 EGT’s monitor engine exhaust for overheat and high pressure EGR temperature. One EGT sensor is inside the DPF canister after the OXS cat but before the actual DPF, and another EGT in the pipe after the DPF canister. These 2 EGT’s are used for warm up and regeneration cycle monitoring. There is a EGR temperature sensor for the low pressure EGR system, along with a potentiometer to control and monitor exhaust gas flow into the turbo inlet.
In CR diesel emissions control, the exhaust gas recirculation (EGR) has two forms, a high pressure EGR which is very much like the earlier PD EGR system. It leaves the exhaust manifold before the turbo and goes straight to a valve on the intake manifold. A throttle body does about the same thing as the earlier BRM system, allowing exhaust gases to be injected into the intake manifold under partial boost and at idle. A second low pressure EGR system has a temperature sensor and a electronically controlled valve after the OXS CAT/DPF and before the NOS cat. The valve has a potentiometer that reports the valve’s position to the ECU or engine control unit. This system meters the exhaust gases into the inlet side of the turbo, allowing EGR to occur under full boost. No earlier engine could do that. In between the NOS cat and the H2S cat is an exhaust throttle body to pinch off the exhaust flow a little bit while the NOS cat and DPF regenerate, as well as assist the low pressure EGR if need be. There is a low pressure EGR pressure monitor to assure that EGR can occur at the proper boost levels.
CR Diesel Emissions: The Regen Cycle: Waste of Fuel?
The regeneration of the Diesel Particulate Filter occurs several different ways, and all involve the DPF running at very high temperatures. At exhaust temperatures between 650-950 F, the soot is converted to ash and Carbon Dioxide (CO2) by reacting with Nitrous Oxide (NO2). During active regeneration, the exhaust temperatures will be above 1000 degrees and may exceed 1200 degrees.
- Warm-Up Phase: Soon after you start a cold engine, the injection system starts giving each injector an extra electrical pulse to put fuel into the combustion chamber post injection, or after the main injection event. This fuel burns inside the cylinder, and increases the exhaust gas temperature to warm up the OXS cat and diesel particulate filter. A temperature sensor after the DPF monitors the outlet temperature and stops the warm up cycle at the proper time.
- Passive regeneration: During high speed driving and/or at higher loads, the exhaust temperature stays high enough for the DPF to regenerate itself constantly. As long as the computer detects soot loads between 6 to 18 grams, active regeneration does not take place, although distance regeneration will occur .
- Active regeneration: This occurs when the engine computer either detects the pressure difference of a soot load of over 18 grams in the DPF, or it calculates the soot load using the driving profile and the temperature and oxygen sensor signals.
- Distance regeneration: Every 466 to 621 miles, the fuel system will initiate an active regeneration cycle if no successful cycle has already occurred. This is to prevent possible clogging of the DPF due to short distance driving.
- Service regeneration: This is an emergency regeneration that must be performed by a service technician trained in CR diesel technology and repair.
When active regeneration occurs, the following actions are completed: The throttle valve closes slightly to restrict air flow, which helps to control exhaust temperature. The EGR is disabled to increase exhaust temperature and oxygen levels. The injection signal is slightly delayed, and a post injection pulse causes extra fuel to burn in the combustion chambers just like in the Warm-Up Phase. Another post injection pulse occurs which injects fuel that does not burn, but gets evaporated in the combustion chamber, then gets expelled into the diesel particulate filter. The oxidation cat above the particulate filter inside the DPF receives the evaporated fuel. The unburnt hydrocarbons cause a reaction in the catalyst that heats up the soot filter element to near or over 1000 degrees F. A temperature sensor located just after the OXS cat and before the particulate filter in the DPF helps control amount and duration of the unburned fuel cycle. The turbo boost is adjusted so the driver does not feel any power loss. The cycle lasts until the pressure sensor detects a reduced soot load that falls to around 6 grams. If the soot load in the filter is allowed to exceed 40 grams, the cleaning cycle could generate so much heat that the diesel particulate filter might be damaged. The CR diesel engine control unit will stop trying to perform active regeneration. In this case, the service regeneration must be done, and SOON. Failing to have a qualified service shop perform a service regeneration will definitely cost you for a new DPF.
Diesel Particulate Filter Warning Light
There is a DPF Indicator Light on the dash, it does NOT come on during normal regeneration. If you see the filter indicator come on, the soot load has exceeded 24 grams. You need to take a freeway drive for long enough to get the system hot and allow it to go through a complete high temperature active regeneration cycle. If the DPF filter indicator light stays on, and the Glow Plug Indicator Light come on as well, you MUST see a pro, NOW! The dash display will indicate “Check Engine – Service Shop”. For some reason, regeneration was not successful, and the engine computer has quit trying to regenerate as it is possible to damage the DPF. Waiting a few days may cost you a new DPF, and we don’t have to scare you with the mid 4 digit $$$$ cost. If the calculated soot load gets to 45 grams, the diesel particulate filter must be replaced.
P0401 EGR Insufficient Flow Code
There appears to be a lot of misconceptions about the P0401 EGR code. EGR is the acronym for Exhaust Gas Recirculation, which recycles a small portion of the exhaust gases back into the intake system. A Factory Technical Service Bulletin (TSB) advises that one check for the presence of soot just after the Diesel Particulate Filter, and if any soot is observed, replace both the DPF and the EGR filter. This will cure the problem 98 out of a 100 times, but the price and frequency suggests that this shotgun technique may be overkill. And the internet shows many reports of owners who have had to change these two expensive parts about once a year. Here is the reality check.
The fuel system checks the signal from Mass Air Flow signal (MAF) when the EGR is turned on, and expects to see a reduction proportional to the requested EGR flow. The allowable range is tightly controlled and checked. Since there are two EGR systems, and they can work even simultaneously, this cross check is not able to verify which system is at fault. The high pressure EGR valve, which inlets directly after the throttle valve, can have a build up of debris. Since this EGR valve opens just very slightly, a small amount of debris can impede the flow enough to be detected. This however is rarely the case. Usually the culprit is the EGR filter of the low pressure EGR system that dumps in just before the turbocharger. But the reasons and degrees of failure are not well identified. A knowledgeable technician can check the restriction across that EGR filter, and determine just how clogged it might be. Readings showing around 100 millibar and above of pressure differential across the filter will cause the EGR flow to be reduced enough to throw the P0401 code.
The filter can be removed and cleaned, but the question becomes just how bad is the DPF leaking soot to cause the EGR to clog. Contrary to most literature, all DPF’s leak some soot. A relatively new car with less than a year of driving will show a tiny amount of soot in the tailpipe. Understand the the DPF has a ceramic element in a steel container, and it must fit very tightly to seal the tiny particles of soot. During regeneration the filter gets to temperatures hovering around 1000 F. The steel case will expand more than the ceramic element as the expansion coefficients are wildly different. If an engine is shut down during a regen cycle, the steel case will contract faster than the ceramic element, risking slight loosening of the core to case clearances, and possible microfractures of the core. This may also happen from shutting down immediately after a hot weather freeway run.
How does one quantify the problem, and decide whether to replace the DPF and EGR filters or not? We recommend checking the soot as per the TSB first, and checking the EGR filter restriction electronically. If heavy soot is found similar to a diesel from last century, then the DPF is definitely damaged and must be replaced. If the soot is minimal, then clean the EGR, and retest the restriction. If the reading drops to under 60 to 80 millibar, then it is good to go. Retesting at future oil changes will indicate whether the EGR filter is getting clogged again, and if so, the DPF should be replaced. And of course if the P0401 code is back next week, the DPF probably needs to be replaced.
A last note is that the P0401 code can also be triggered by other problems, like a leak. Several times, we have found that some shop replaced a turbocharger, and failed to properly reinstall the exhaust hangars, causing the EGR filter to break. Other fuel system components can get damaged or fail, causing the exhaust to run too hot or way too dirty. The EGR’s exhaust restriction valve in the exhaust after the DPF can fail or get disconnected. Bolts can be left loose. Brackets can get broken. The list goes on.
Diesel Fuel Additives in the Common Rail Diesels
Customers should know that Karmakanix advises the continuous usage of diesel additives such as Stanadyne. Aside from lubricity issues that can cause total system failure due to metal flake from failed pumps, these additives also reduce the overall amount of soot generated. This will clearly increase the service life of the Diesel Particulate Filter.
We also recommend adding a molybdenum disufide additive such as Liqui Moly MoS2 additive to the engine oil every 4th oil change to prevent wear. The friction reduction properties are reputed to increase mileage, which is likely true and also likely hard to measure. However the ability to reduce wear to almost nothing is well worth consideration, and consistent usage.