Son of a Shogun

A misfiring Mitsubishi Shogun provided Ryan with his latest challenge

Published:  17 January, 2023

 By Ryan Colley, Elite Automotive Diagnostics

Did you know you can fix a misfire with a simple oil change? If you want to know how then please read on. I was recently tasked with looking at a mischievous Mitsubishi Shogun with a 3.5L V6 GDI engine. This engine a rare sight in the UK, which causes issues when trying to research the engine's configurations. The issue is a lack of available service information, as you will see.
    
I was contacted by a local garage who asked me to investigate a misfire this vehicle exhibited when running at idle. They informed me the misfire was permanent and that they had already serviced both the ignition system and the fuel injection system. They also informed me that they had carried out a compression test and that the results were equal for all cylinders so, they were a little stumped as to what the cause could be. In their eyes, they had covered all the possibilities.
    
I started by scanning the vehicle using our diagnostic equipment. A misfire for cylinder #5 was stored. I then used Mode $06 data in the EOBD menu, which records misfire data per cylinder. I could see there were 27 misfire counts for this cylinder, confirming the fault code to be correct. More importantly, no other cylinders were reporting misfires. This was a cylinder-specific issue. Next, I verified that both the ignition system and fuel injection system were operating correctly, as advised by the customer. This was done quickly using an oscilloscope, measuring coil and injector current. I could clearly see both ignition coil and fuel injector current events for the misfiring cylinder. I then carried out a relative compression test using the oscilloscope, and sure enough, all cylinders were equal and consistent. With this data, there was one other option; To connect a pressure transducer to the oscilloscope, remove the spark plug and install this pressure transducer into the cylinder to measure real-time pressure, plotted over time. If there were any valve train issues, we would likely see them using these pieces of equipment.

Pinpointing the cause
As you can see if you refer to Fig.1, there were certain anomalies occurring within this cylinder. We can clearly see that the exhaust pocket is rounded, which indicates a slow change in pressure, over time, as opposed to a quick snap-open of the exhaust valve, which is what is desired. You will also notice the exhaust plateau is shifted to the right, indicating that the fault is related to a valve duration issue. Because the exhaust valve is slow to open, it is also slow to close, causing it to affect the intake valve event. The intake valve opening event appears delayed. Logic should make it impossible for the cylinder to draw into a vacuum if the exhaust valve is connecting the intake manifold to the exhaust via the still-open exhaust valve
    
Knowing there are anomalies occurring in this cylinder I wanted to check a known-good, non-misfiring cylinder. So, I moved the test equipment from cylinder #5 to Cylinder #2, which was also on the opposite bank. As you can see from the idle, in-cylinder capture of cylinder #2 as seen in Fig.2, there is an obvious difference in the valve train events. The exhaust plateau is not shifted to the right, and we can clearly see that the exhaust valve event displayed a quick opening, which is desirable.

Accurate analyses yield confidence
Armed with this information we could then confidently inform the customer that there was an obvious valve train issue for cylinder #5. Before dismantling the engine, I connected a pressure pulse sensor to the intake manifold, and using a current clamp, synced to cylinder #2’s ignition coil event. I compared the intake pulls for each cylinder on this engine. This was simply to back up my diagnosis of a valvetrain-related issue for cylinder #5.
    
If the exhaust valve is opening late and closing late, then this will affect the intake pull and valve overlap for that cylinder. As you can see in Fig.3 and Fig.4, there is a 7-degree difference in the intake pulls between a good cylinder and the misfiring cylinder #5. Please note that the valve overlap event is different on cylinder #5 compared with the other cylinders. This now confirms more so that there is a definitive exhaust valve train problem on this cylinder which is affecting the intake valve event.
    
I then wanted to research this engine configuration to ascertain which type of valve train system Mitsubishi implemented. It was very difficult to find anything conclusive. Therefore, knowing the vehicle had a valve train issue, I advised the garage to remove the rocker cover and inspect the camshaft and lobes for any abnormalities. They reported back that the camshaft and lobes looked in good serviceable condition, with no excessive wear. At that point, it was very difficult to determine whether the valve spring or possibly the hydraulic tappet was causing the problem. I recommended that an oil flush should be carried out. If that rectified the issue, then we would know the problem was within the hydraulic tappet. However, if the fault remained then the issue would clearly lay with the valve spring or valve seat area.

Committed to a correction
After the oil flush and change was carried out, the customer stated that there was an audible ticking noise. However, this noise vanished shortly thereafter. With that, the misfire vanished as well, thus confirming that the issue was within the hydraulic tappet on this engine. The vehicle has run many miles since the flush was carried out and is no longer misfiring. I also took the liberty to check the Mode $06 misfire counters in EOBD data after the vehicle returned for its inspection. This also confirmed no misfires present.
    
Sometimes we are limited in the information we have available to us, as technicians. Although this is an inconvenience, employing logic and understanding can lead to a hypothesis of the likely fault, paving the way for intelligent tests that can flush the fault to the surface for us. This proves, once again, that the most valuable tool a technician possesses is the one between his or her ears.



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