PicoScope 7 Automotive
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When relative compression steers you in the wrong direction
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ben.martins
- Pico Staff Member
- Posts: 545
- Joined: Tue May 16, 2017 1:02 pm
When relative compression steers you in the wrong direction
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 about. I’ve always been someone that is happy to admit when they 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 a number of case studies. I wasn’t able to join them in person on this adventure so I supported them remotely.
The machine in question was a little CAT 308E excavator with a Kubota 4 cylinder engine, which had a clear misfire when operating the machine along with black smoke. 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 Pico, a number of 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.
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 a low compression, the peaks are even and cranking speed is 172 RPM. One thing you can see from the signal rulers is that the peak current is 700A. 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 2pm. 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 where 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 fueling system. Maybe the previous technician was on the right track.
Connecting to the rail pressure sensor, a cylinder ID from the current of injector 1 and the exhaust pulse was next based on the ease and accessibility of components. It would have been nice to have the crankshaft sensor as well but as with most things on these smaller machines, access is always an issue. Instead, exhaust pulse gives you the result of everything so anything that looks out of the ordinary will show up in the waveform.
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 the following post. viewtopic.php?p=103861#p103861. 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 that there are 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.
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 1, possibly 2 injectors. Reason being, the second injector could be compensating for the first injector but that is unknown till the first one is replaced. The machine owner was told about the findings and showed the evidence to support the decision for one injector along with the caveat that another injector may be needed and so an injector was ordered.
Coming back to the machine to replace and code the injector, the rocker cover was removed. 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 and so a lower peak in the waveform. How could I have missed this?
It’s at this point I learn that this is a 4 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 during the slow speeds involved with cranking was enough to create enough compression to give even peaks in the relative compression waveform. Once the engine was running though, 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 for getting a second opinion as originally they were looking at 4 injectors costing nearly £4,500 just for the parts, which wouldn’t have fixed it!
Right at the start I said there is always something to learn, so what can we take from this. Firstly, technical information is a must. Try to find out as much as possible about the machine and the engine you are working on prior to starting any diagnosis. Use the initial customer complaint to work out what measurements you are likely to make and familiarise yourself with component location. Sometimes this isn’t always possible and the engine is often unknown till you take a look but at the very least make sure you know where to find the information once you are with the machine.
Next, try to ignore any prior diagnosis from another individual. It’s very easy to try and fit the data to 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!
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 the extreme, if you have a 4 channel scope, your setup could look like the following. Relative compression on channel A (voltage or current), channel B for exhaust pulse, channel C for intake with a pulse sensor and for channel D if you know the relationship between cylinder 1 and the camshaft sensor signal you then have a cylinder ID.
With this job every day is a school day! I hope this helps.
This happens to the best of us and should never be something to be ashamed about. I’ve always been someone that is happy to admit when they 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 a number of case studies. I wasn’t able to join them in person on this adventure so I supported them remotely.
The machine in question was a little CAT 308E excavator with a Kubota 4 cylinder engine, which had a clear misfire when operating the machine along with black smoke. 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 Pico, a number of 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.
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 fueling system. Maybe the previous technician was on the right track.
Connecting to the rail pressure sensor, a cylinder ID from the current of injector 1 and the exhaust pulse was next based on the ease and accessibility of components. It would have been nice to have the crankshaft sensor as well but as with most things on these smaller machines, access is always an issue. Instead, exhaust pulse gives you the result of everything so anything that looks out of the ordinary will show up in the waveform.
One thing we can do to highlight the rail pressure sensor changes is to use the derivative math function as shown in the following post. viewtopic.php?p=103861#p103861. This can highlight changes to the signal that might not be obvious in the original signal.
By increasing the speed and load on the engine we can see the effects this has on those signals.
Coming back to the machine to replace and code the injector, the rocker cover was removed. 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 and so a lower peak in the waveform. How could I have missed this?
It’s at this point I learn that this is a 4 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 during the slow speeds involved with cranking was enough to create enough compression to give even peaks in the relative compression waveform. Once the engine was running though, 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 for getting a second opinion as originally they were looking at 4 injectors costing nearly £4,500 just for the parts, which wouldn’t have fixed it!
Right at the start I said there is always something to learn, so what can we take from this. Firstly, technical information is a must. Try to find out as much as possible about the machine and the engine you are working on prior to starting any diagnosis. Use the initial customer complaint to work out what measurements you are likely to make and familiarise yourself with component location. Sometimes this isn’t always possible and the engine is often unknown till you take a look but at the very least make sure you know where to find the information once you are with the machine.
Next, try to ignore any prior diagnosis from another individual. It’s very easy to try and fit the data to 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!
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 the extreme, if you have a 4 channel scope, your setup could look like the following. Relative compression on channel A (voltage or current), channel B for exhaust pulse, channel C for intake with a pulse sensor and for channel D if you know the relationship between cylinder 1 and the camshaft sensor signal you then have a cylinder ID.
With this job every day is a school day! I hope this helps.
Re: When relative compression steers you in the wrong direction
That's a great read Ben thanks for taking the time to tell us about it as its definitely something I would consider in the future, like you say there is always something to learn.
I try to think would this have caught me out and the answer would have to be yes, the fact this was spotted before changing the injector rather than oh it still has a misfire after replacement says alot
I have to wonder if the original tech would have sourced and fitted the rocker without admitting the error and continued replacing the 4 injectors.
I try to think would this have caught me out and the answer would have to be yes, the fact this was spotted before changing the injector rather than oh it still has a misfire after replacement says alot
I have to wonder if the original tech would have sourced and fitted the rocker without admitting the error and continued replacing the 4 injectors.
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maniekdaniel
- Newbie
- Posts: 8
- Joined: Thu Nov 17, 2022 7:38 pm
Re: When relative compression steers you in the wrong direction
Why didn't you prefer to use a pulse sensor in the intake pipe? Would that also show the problem?
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ben.martins
- Pico Staff Member
- Posts: 545
- Joined: Tue May 16, 2017 1:02 pm
Re: When relative compression steers you in the wrong direction
Hello both,
Firstly - thank you Sharpy and yes, always something to learn! As I said above, being able to adapt the process following an experience like the above shows that we can grow as technicians.
Maniekdaniel - thank you for your comment. You are right that an intake pulse would have highlighted a possible issue when running but given the relative compression test came back showing no obvious concerns with a compression, the process moved away from a mechanical issue. Coupled with the slight anomalies in the rail pressure sensor, focus was on the fuel side. As I’ve mentioned, the diagnostic process has now been adapted to include at least the exhaust pulse, as this is as non intrusive as it comes. Even with an intake pulse you need to remove some components to gain access for your pressure transducer or pulse sensor and as the exhaust pulse is the result of everything you will see something here.
My worry here is that even with only 1 intake valve operating as normal, given the engine speed is much slower during cranking would there be enough of a change to highlight the problem. Based on the relative compression test, there is enough air in the cylinder to show the current demand is the same as the other cylinders so would this change the exhaust and intake pulse significantly enough to show the issue? Maybe keeping the exhaust pulse on throughout the diagnosis for these types of symptoms is required? Something that I think needs to be revisited.
Thanks all.
Kind regards
Ben
Firstly - thank you Sharpy and yes, always something to learn! As I said above, being able to adapt the process following an experience like the above shows that we can grow as technicians.
Maniekdaniel - thank you for your comment. You are right that an intake pulse would have highlighted a possible issue when running but given the relative compression test came back showing no obvious concerns with a compression, the process moved away from a mechanical issue. Coupled with the slight anomalies in the rail pressure sensor, focus was on the fuel side. As I’ve mentioned, the diagnostic process has now been adapted to include at least the exhaust pulse, as this is as non intrusive as it comes. Even with an intake pulse you need to remove some components to gain access for your pressure transducer or pulse sensor and as the exhaust pulse is the result of everything you will see something here.
My worry here is that even with only 1 intake valve operating as normal, given the engine speed is much slower during cranking would there be enough of a change to highlight the problem. Based on the relative compression test, there is enough air in the cylinder to show the current demand is the same as the other cylinders so would this change the exhaust and intake pulse significantly enough to show the issue? Maybe keeping the exhaust pulse on throughout the diagnosis for these types of symptoms is required? Something that I think needs to be revisited.
Thanks all.
Kind regards
Ben