Vehicle affected - 2010 Chevrolet Express 3500 6.6L Diesel
Issue – Check engine light illuminated, will not perform diesel particulate filter (DPF) regeneration (regen). Fault Code P2463 (DPF soot accumulation)
Tools used:
· Vehicle-specific information
· Scan tool
· DMM
· Diagnostic smoke machine
· Various hand tools
Catalytic converters have been used on vehicles to reduce emissions since 1975. Since they were introduced, converter technology, along with the technology that controls the engine’s exhaust emissions, has gone through significant changes. The most significant changes are actually in the engine management system’s ability to better control the amount of fuel to the amount of air entering the engine. This results in less emissions entering the exhaust stream, and therefore, less demand on the catalytic converter. In fact, that is the foundation of the OBD-II system, the ability to better control the engine to reduce exhaust emissions, and to recognize when a reduction of the catalytic converter’s efficiency has occurred.
Catalytic converter efficiency has been greatly improved for gasoline engines by the engine management system’s ability to tightly control the amount of fuel entering the cylinder to maximize power and reduce the amount of unburned or partially burned fuel entering the catalytic converter. A gasoline catalytic converter is designed to reduce Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), all of which create air pollution.
Diesel engines do not create HC’s and CO nearly as much as a gasoline engine due to their ability to burn a leaner fuel mixture; but due to the leaner mixture and the properties of the fuel, diesel engines produce more NOx and a significant amount of soot, or Particulate Matter (PM). To combat these emissions, various designs of exhaust aftertreatment systems for diesel applications were introduced throughout the mid to late 2000s when the EPA determined that exhaust emissions, specifically PM generated by diesel engines, were a health risk.
The most common systems for diesel exhaust emission reduction are a DPF, primarily for reducing PM; a Diesel Oxidation Catalyst (DOC) for reducing HC, CO and NOx; and a Selective Catalyst Reduction (SCR) for reducing NOx. These systems generally use high exhaust temperature to restore (regenerate) the system to its ability to reduce emissions. Another difference between gasoline and diesel systems is that the engine management system has the ability to reduce power, or de-rate performance, until the exhaust system has been restored to full functionality.
The effectiveness of exhaust emission reduction systems, whether gasoline or diesel, primarily rely on one thing: a well-functioning engine management system controlling the amount of fuel entering the engine to produce the lowest amount of emissions possible entering the exhaust. With that in mind, one of the first things to check when diagnosing any emission control problem is the engine management system’s ability to properly control the amount of fuel entering the engine.
Step 1 – Gather information
Questioning the customer about the details of their issue and obtaining fault codes are always a good place to start any diagnostic process. There are times when the customer will be able to provide details that can help with determining the cause of the problem. In many cases, they can provide information about work that has been done recently, or even – in the case of this vehicle -- work that has been done to try to solve the current problem. The customer in this case stated that the current issue had been occurring for a few weeks, and they had taken it to a different shop where they had performed a regen of the DPF system. The customer also stated the vehicle had only been driven about 100 miles since it had been at the other shop.
This vehicle is used for a bus route, so we understood that the conditions it operates under, stop-and-go driving, will not always allow natural regen events to occur.
Scan readings showed a P2463 fault code and the sensor showed 85 grams of soot. We used Mitchell 1 [SVR1] to obtain information on the system. This fault code sets if the sensor reads more than 70 grams for more than30 seconds.
Even if you are familiar with a system, it is a good idea to access vehicle-specific information to fully understand how the system functions, the conditions necessary to set the fault code and some of the common causes of the failure. We again went to Mitchell 1 where we found the operating specifications for the system, as well as some interesting possible causes for the code.
The possibilities included:
1. Faulty exhaust pressure differential sensors, which could cause incorrect soot load readings
2. Pinhole exhaust leaks, which can reduce exhaust temperature and pressure
3. Incorrectly installed or operating exhaust temperature sensors, which can affect regen
4. Low engine compression, which can cause excess soot
5. Faulty fuel injectors, which can cause excess soot or improper exhaust temperatures
6. Water in the fuel, which reduces the available energy the fuel produces
7. Excess carbon on the aftertreatment fuel injector, which can reduce the amount of fuel used during the regen process
8. Stop-and-go driving, which does not allow regen to occur due to low engine speeds
Step 2 – Diagnose the vehicle
Armed with the fault code and the list of possible causes, we started the diagnostic process by driving the vehicle to see if there were any performance issues that could be contributing to the issue. We drove the vehicle for about 30 minutes over 45 mph, which should have triggered a regeneration event, but did not. The road test also did not indicate any performance issues.
Due to the vehicle’s inability to perform a regen on the road, we used the bidirectional controls of the scan tool to command a manual regen. Even though all of the conditions were correct to perform the function, the vehicle was able to start – but not complete – the process.
Since the regen was unable to occur either during the road test or when commanded by the scan tool, we were fairly certain there was an issue in the emissions control system, rather than a mechanical or engine performance problem. Because the regen did not occur, we scanned the vehicle again and found another fault code, a P02E7, which indicates a fault in the intake air flow (or throttle valve) position sensor, which is located in the throttle body. Yes, you read correctly, the throttle body. On a diesel. Normally, diesel engines do not use a throttle body, but GM uses a throttle body to control and monitor air flow into the engine allowing engine RPM to increase and assist in controlling exhaust temperature during the regen. Regen occurs when fuel or Diesel Exhaust Fluid (DEF) is injected into the exhaust stream to raise exhaust temperatures as high as 1,000 degrees F, necessary to burn off contaminants in the system.
Following the diagnostic fault tree for the P02E7 code, we used a scan tool to obtain sensor values and our DMM to compare reference voltage and ground at the throttle valve position sensor, and to check for excessive resistance in the circuit. Using our scan tool, we commanded the throttle valve open and compared the desired position to the actual position. There was consistently a variance between the commanded and actual position of the valve. We also noted that there was an occasional ratcheting noise coming from the throttle valve when it was commanded to open or close.
Step 3 – Repair the vehicle
To repair this vehicle, we needed to replace the throttle body and then attempt to perform a manual regeneration. After the throttle body was replaced, we were able to command a regen to occur. We then used the scan tool to read the soot level on the sensor, which indicated two grams – well within the specification.
We still had some concerns about the ability of the vehicle to recognize when a regen needed to happen and then perform it while driving. This part is important because we wanted to make sure the vehicle would not return with the same issue. In order to confirm the system’s ability to perform a regen on its own, we decided it would be worthwhile to eliminate concerns about any of the other common failures that were noted in Mitchell 1.
To eliminate these possible causes, we checked the values for the exhaust pressure and exhaust temperature sensors. All of the readings were within specifications listed for the current operating conditions. The final test was to perform a high-pressure smoke test to make sure there were no intake or exhaust leaks in the system. Intake air leaks will affect engine performance and create excess soot due to unburned fuel caused by lower boost pressure. Exhaust leaks can cause reduced exhaust temperature as well as lower exhaust pressure.
We used the Redline HD Power Smoke tester because it is a compact unit able to generate high-pressure smoke that easily indicates where even the smallest intake or exhaust leak is occurring.
There was an intake leak in one of the tubes from the air cleaner to the charge air cooler, where the hose had folded down under the clamp. This required replacing the hose. There was also a leak at one of the clamps around the turbocharger flange which was easily repaired by tightening the clamp.
Either of these issues individually would most likely not cause regeneration to occur more frequently, but the combination of the two could have that result. The vehicle was returned to the customer with the request they return in a few hundred miles to make sure everything was working properly.
The vehicle returned after it had been driven 215 miles. Scan tool data showed the last regen occurred 215 miles ago, and the sensor showed 16 grams of soot. Sixteen grams of soot would be expected of this engine because a regen is triggered approximately every 400 miles, or with a soot load of over 30 grams. We again requested the customer return after it had been driven another 200 miles. Upon its return, scan data now showed a regen had occurred less than 20 miles ago, and the soot load was reading three grams.
Whenever a catalytic converter issue is present, whether it’s a gasoline or diesel engine, diagnose the immediate problem with the system, then take time to make sure there is not a cause of excess emissions that could cause the system to fail again.
[SVR1]ProDemand? Or a different product?
