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Case Study: Misfiring Mokka

Case study by Des Davies AAE MIMI, Top Gear Motor Services Vehicle: Vauxhall Mokka 1.4 LUJ/B14NET Turbo 2018
Mileage: 17,077
Fault Code: P0300

Customer complaint: Loss of power, feels like lack of fuel and engine running poorly

Misfiring Mokka…

I interrogated the customer to get as much detail and information on the symptoms, leading up to and when the faults first developed. I then visually inspected the engine bay for any anomalies or obvious faults.

I confirmed the fault – the engine was misfiring badly, and the check engine light was displayed on the instrument panel. There was no need for a road test as it was running poorly on idle speed.

I connected my scanner to the OBD port to check for fault codes to get some clues that may help diagnose the fault. It flagged up a ‘P0300 Engine Misfire Detected’, so not much help from this generic code.

My next plan of attack was to work smarter not harder by going global and using the OBD11 on my scanner and checking Mode 6 as this has helped me in the past. This mode can be useful as Vauxhall have misfire counters in Test Mode 1.

Mode 6 global information, see Figure 1, recognised cylinder no.4 as the misfiring offender so it was time to break out my PicoScope and investigate. I needed to confirm cylinder no.4 was indeed misfiring as this data can be incorrect. Don’t always trust what you see, I have been burnt before.

Figure 1

Checking the essentials…

For a good combustion burn we need a good spark, at the right time, with the correct amount of air and fuel at the correct time, and good compression. If we have all these requirements then the engine should start and run efficiently.

Planning my next diagnostic approach, I could do a quick relative compression test which is non-intrusive and a good procedure to confirm that the engine mechanicals are in good working order, but bear in mind that it only compares the cylinder work rate against each other and is not an indication of good cylinder compression.

I disabled the fuel to prevent the vehicle from starting and needed to locate the fuel relay or fuse by extracting this information from a technical data source. Figure 2 shows the good relative compression waveform.

Figure 2
Figure 3:
Testing the ignition coil cassette KV output cylinder no.4 with the non- intrusive ignition probe/wand
Figure 4:
Coil secondary KV waveform output from coil no4 with engine misfiring
Figure 5:
Coil secondary KV waveform from coil no2 with engine misfiring, this is a known good output

The relative compression test confirmed that all cylinders were mechanically distributing an equal amount of engine compression, so I decided to test the next easiest system, because of its ease of access, the ignition coil pack assembly.

Working smarter not harder, I now needed a wiring diagram on this system to help me test the ignition system and to confirm the correct wiring for this vehicle, see Figure 6.

Figure 6

These ignition systems use an ignition coil pack, also called cassettes, where the four ignition coils are housed as one coil unit. Each coil is individually controlled and switched with a square wave signal wire by the ECM.

Unfortunately, the diagram information did not give me the wiring locations from the ECM pins to this coil pack. I needed to locate the terminal for no4, so I had to test all the ECM coil driver wires to confirm that they were being switched by the ECM coil driver and were all operating correctly.

Figure 7:
Ignition coil pack module 12V feed was good
Figure 8:
Ignition coil pack module ground/earth good
Figure 9:
Square wave signal wire controlled by the ECM to coil pack was good. All 4 signal wires were tested with the same results

We now needed to take a closer look at cylinder no.4’s coil secondary output extension lead and the spark plug to check for any anomalies. In Figure 10, we can see evidence of the results of the spark plug firing voltage for no.4 with carbon tracking on the spark plug insulator arcing across the spark plug terminal to the cylinder head, causing the plug not to fire inside the engine cylinder and causing our misfire.

Figure 10
Figure 11:
Ignition HT lead connection coil no.4
Figure 12:
Coil pack/cassette with no.4 HT connection eroded

A new coil pack was required as well as new spark plugs. We then needed to retest the ignition system waveforms to confirm the faulty coil pack and spark plugs had fixed the misfire. Figure 13 shows the waveform of the secondary ignition system after fitting the new ignition components. Figure 14 shows the ignition coil pack current was good.

Figure 13
Figure 14

In summary…

  • Testing ignition systems in a logical manner, understanding the fundamentals and using the correct technical data for these systems helps to reduce mistakes.
  • It does not matter where you decide to start your diagnostic testing for a misfiring engine, or any other fault on the vehicle, because the test results that you have chosen to perform should guide you towards your next test for elimination of that system or component to help you locate the fault or faults in the system.
  • I look for the easiest point to check a faulty system and use the appropriate tools and equipment for testing these components. If my test results are not what I expected, then I need to either backtrack my testing procedures and methods or test that system or component in a different manner, using other test equipment to achieve the correct results or manufacturer technical data.
  • I do not rely too heavily on my scanner results and information, but I do use them as a guideline. The scanner’s refresh rate can be slow and it has to make a quick judgement and diagnosis of the fault and can easily get it wrong.
  • Always test don’t guess, as this will only create expensive mistakes, is time consuming, and produces poor customer relationships.
  • If you have not got a labscope for this problem, finding this fault is very difficult and laborious. Changing and swapping parts is a recipe for disaster and very expensive for the customer and for your reputation.
  • Think before you leap, as it’s a long way down and very lonely at the bottom!
  • Diagnostics can be rewarding but we need to be kept on our toes by keeping up with the latest vehicle technology.

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