CAT 308E Excavator | Misfire Diagnosis on CAT 308E Excavator with Kubota Engine

What I love about working in this industry is that there is always something to learn. Unfortunately, the most important learning moments are when you get something wrong, overlook something or let your diagnosis be led by previous reports.

This happens to the best of us and should never be something to be ashamed of. I’ve always been happy to admit when I get something wrong, and fortunately now at Pico, I have a platform to share this with others which will hopefully help them going forward. The incident in question comes from a support call with someone I’ve worked with on several case studies. I wasn’t able to join them in person on this adventure so I supported them remotely.

Another technician had been out to the machine and recommended replacing all 4 injectors. Given these injectors aren’t the cheapest, and when being told it needs all 4, the owner of the machine asked for a second opinion.

Armed with a PicoScope, several tests were carried out, including a relative compression test. This is very much a ‘go-to’ test for a lot of us looking at misfires or engine performance issues, as the simplicity of the test coupled with the data that follows can quickly give you direction.

Relative Compression Test: Performed as a standard test for misfire issues. Results showed even waveforms, indicating no significant compression problems.

As you can see from the above image there are a few things to note. Firstly, from a uniformity point of view, the waveform looks pretty good. No obvious issues with low compression, the peaks are even and the cranking speed is 172 rpm. One thing you can see from the signal rulers is that the peak current is 700 A. 

This is significantly higher than we would expect, but it’s worth pointing out when this was captured. By including the time and date stamp we can see that it was taken on the 6th December 2023 at 2 pm. This allows us to look up weather events for that day and we can see that London had an average of 4℃ (39℉). 

The machine had been standing overnight when temperatures got down to -1℃ (30℉), which would have meant the engine was cold, leading to a possible heightened cranking current. The way to prove this would have been to let the engine warm up and then recheck the starter motor current.

Given the engine would start, even with this current draw, the diagnosis moved forward. With relative compression out of the way, it is hard not to start thinking about the fuelling system. Maybe the previous technician was on the right track…  

Rail Pressure Sensor & Exhaust Pulse Analysis: Identified deviations in rail pressure and exhaust pulse for cylinders 1 and 4, suggesting potential injector issues.

With the engine at idle, the biggest thing to stand out is the exhaust pulse. There is a clear repeating pattern where the waveform is not uniform. When lining this up with the cylinder ID and the 4-stroke cycle, this exhaust pulse can be attributed to cylinder 1. Using the rail pressure and a 1kHz filter, at this same point we can see that we have a slightly lower drop in pressure for cylinder 1 as well as cylinder 4 when compared with cylinders 2 and 3.

One thing we can do to highlight the rail pressure sensor changes is to use the derivative math function as shown in this post. This can highlight changes to the signal that might not be obvious in the original signal.

As you can see during this section, the maths channel shows two points where the signal changes, which line up with cylinders 1 and 4. This doesn’t always repeat though, but it is there through the rest of the capture and always with the same cylinders. This gives us more evidence to support a fuelling issue caused by injectors.

By increasing the speed and load on the engine we can see the effects this has on those signals.  

Derivative Math Function: Applied to highlight changes in rail pressure signal, further supporting an injector fault.

We still have the repeating event in the exhaust and we can still see a deviation in the rail pressure sensor, in both the maths channel and the raw signal. For this reason and after talking it through with my contact, we came to the decision that we are looking at one, possibly two injectors. The reason being, the second injector could be compensating for the first injector, but that is unknown until the first one is replaced. The machine owner was told about the findings and shown the evidence to support the decision for one injector, with the caveat that another injector may be needed, therefore an injector was ordered.

Back at the machine, whilst the repair was underway, my contact noticed that one of the rocker arms for the intake valve on cylinder 1 was broken!

But hang on, our relative compression peaks were even, surely a broken rocker would have been highlighted in the signal? Not getting any air into the cylinder will mean less compression, leading to a lower peak in the waveform. How could I have missed this? 

It’s at this point, I realized that this is a four-valve per cylinder engine with independent rockers for each valve. This meant that one of the intake valves was still opening and allowing air into the cylinder, which was enough to produce even peaks in the relative compression waveform, during the slow cranking speeds. Once the engine was running, the amount of air wasn’t enough to complete combustion, causing the black smoke and the misfire. 

Whilst being slightly embarrassed for missing this, the machine owner was happy to get a second opinion. Originally, they were looking at replacing four injectors, costing nearly £4,500 just for the parts, which wouldn’t have fixed the issue!

In diagnostics, everything is a learning experience, and so, what can we take from this? Firstly, the correct technical information is a must. Secondly, try to find out as much as possible about the machine and the engine you are working on, before starting any diagnosis. Lastly, use the initial customer complaint to work out what measurements you are likely to make, and familiarise yourself with component locations.

Sometimes this isn’t always possible, and the engine is often unknown until you take a look, but at the very least make sure you know where to find the information once you are with the machine.

Finally, try to ignore any prior diagnosis from another individual. It’s very easy to try and fit the data into an initial report, which might be lodged in your subconscious. This isn’t easy to ignore, especially when the data you start to see ties up with the initial diagnosis. Stay strong though and take nothing for granted and never assume!

What could we have done differently? 

If you have additional options for non-intrusive testing, like a pressure transducer, use it! I know relative compression is a simple test, and we talk about it a lot as the first measurement, but why not add in exhaust pulse or intake pulse at the same time? Given the engine is an air pump during this test, you’re simply seeing the movement of air through the engine. I’m almost certain that if we had an exhaust pulse and/or intake pulse at the beginning of this test, we would have seen something that would have given us a different direction. 

Taking it to the extreme, if you have a 4-channel oscilloscope, your setup could look like the following: 

•    Channel A: Relative compression (voltage or current),
•    Channel B: Exhaust pulse
•    Channel C: Intake with a pulse sensor 
•    Channel D: Cylinder ID.

With this job, every day is a school day! I hope this helps.