Unraveling a problematic Isuzu DPF issue
The Diesel Particulate Filter (DPF) is a regular conversation starter when mentioning diesels. Some people prefer to make quick cash by illegally and unnecessarily removing a DPF rather than investing time in training to rectify the faults. I’ve found diagnosing diesels are much easier when they have a DPF fitted. It’s almost like having a scan tool permanently fitted to the engine.
When a DPF fault occurs, its simply informing us the conditions are not being met for the vehicle to perform a DPF regeneration. Incorrect conditions can be something as simple as a fuel gauge not working, valve clearances incorrect, injector washer failure, turbo actuator failure or the customers driving habits. We’ve experienced hundreds of causes of DPF related fault codes and in this article, I will share a simple cost-effective diagnostic and repair solution that I vested years in to determine the cause.
The case involves the popular fourcylinder Isuzu models: NPR300 chassis code NPR75 2006 4HK1-TC 5.2L, NLR85 4JJX-1 3.0L 2006 – 2012 experiencing repeated regenerations.
Symptoms
The driver is reporting excessive DPF regenerations required. Often full regeneration every 40kms which involved the vehicle being stopped and DPF button would have to be pressed to activate. This truck should be able to reach 300km distances with only a manual regeneration occurring whilst the vehicle is still driving.
Failure/Issue
Failed Mass Airflow Sensor (MAF) resistor. The voltage feed to the MAF was too high causing inconsistent airflow readings. This truck is fitted with a 12-volt MAF sensor often found on the Isuzu D-Max utility. Most trucks including the Isuzu use 24 volts electrical systems and therefore a resistor has been installed to reduce the voltage so to avoid damaging the sensor. Unfortunately, Isuzu factory wiring diagram isn’t much help as there is no resistor indicated on this model.
Diagnosis and/or early detection of the fault
First test the DPF for restriction to confirm it is not blocked by comparing your scan tool data to real pressure readings using an analogue gauge. Ideally, we want to see less than 1 kPa at idle an no greater than 20 kPa under load. This information and more is available to Diesel Help members including access to the dedicated DPF technical bulletin TB1087- DPF forced regeneration not required and our DPF training video.
Once confirmed the DPF was not blocked, start to diagnose the possible cause. Fortunately, our technical bulletin guides the technician towards the testing of the failed component. First check the voltage feed to the Mass Air Flow (MAF) sensor. This should read between 12 and 14.8 volts at idle. At idle, the failed resistor was sending 16 volts to the MAF. This truck is a 24-volt system fitted with a 12-volt MAF. 16 volts is too high for a 12-volt system and too low for a 24-volt system. To locate test area, refer to Legend in MAF wiring legend No1 MAF sensor power supply.
Solution
Remove and replace the resistor with genuine only.
Special notes
One of the case study vehicles reported an earlier fault two years prior to the DPF issue.
After working back through previous work files, it was found that this fault was related to the resistor and at the time was found to have broken one of the wires caused from wear and corrosion. The code present at that time was P0560 is defined as System Voltage Malfunction, meaning an abnormal voltage reading has been detected from the battery, charging, or starting systems. At the time they didn’t think anything of it and repaired the wiring on the resistor.
The Ohms reading can be as low as 3 ohms difference. For example, the resistance shows 48 ohms and results in too high voltage. A new resistor is replaced and a reading of 51 Ohms is suitable.