Relative Compression newbie learning to trust the wiggles

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Sharpy
OneWave
OneWave
Posts: 49
Joined: Sun Dec 23, 2018 9:59 am

Relative Compression newbie learning to trust the wiggles

Post by Sharpy »

I am by no means an expert on all things Pico but 10 months in to the learning process I'm far more confident in making a diagnosis using the scope now.
I thought I'd share something that happened the other day in case it helps others getting started to maybe trust the Pico.
I had yet another Scania truck come in with engine issues and although from the symptoms I knew the engine was going to come apart regardless but I gave it a once over with the Pico just to get some before captures and to see what was going on before taking it apart.
As usual I decided to start with a relative compression and wasn't expecting any issues to show up much as there were no running issues or fault codes plus the cranking cadence was nice and even and the known issue was primarily coolant loss from the head gaskets and liner seals but it did also have unexplained excessive oil consumption.

Well this is what I captured (straight 6)

Relative before 25-10.psdata
(2.61 MiB) Downloaded 164 times

Well I actually thought I'd done something wrong so I done it again and it was the same so I decided to put the Wps in the dipstick to check for crankcase pulses and it didn't show anything so at that point rather than try and get a sync I discounted the relative captures and set about stripping the engine. Sadly I haven't took any pics yet but I was surprised to find it had a broken top ring on 4 out of the 6 cylinders so I learned yet another lesson and that's TRUST THE WIGGLES

Steve Smith
Pico Staff Member
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Joined: Sun Aug 25, 2013 7:22 am

Re: Relative Compression newbie learning to trust the wiggles

Post by Steve Smith »

Hello Sharpy, thank you again for posting another adventure.

I completely relate to your comment "I actually thought I'd done something wrong so I done it again and it was the same so I decided to put the Wps in the dipstick to check for crankcase pulses and it didn't show anything so at that point rather than try and get a sync I discounted the relative captures and set about stripping the engine."

Quite often, time does not allow for intrusive diagnosis when you know in your heart of hearts that the engine has to be stripped. "Hind-sight is a wonderful thing" and there are multiple tests you wish you would have done but never the less, we have the unique relative compression waveform and the hard evidence to link/ relate to such captures in the future.

If you could post the photos we can add them to your waveform to bring about clarity and serve to train others

Thank you again, take care......Steve

Sharpy
OneWave
OneWave
Posts: 49
Joined: Sun Dec 23, 2018 9:59 am

Re: Relative Compression newbie learning to trust the wiggles

Post by Sharpy »

Sorry its took so long to update but I did forget about this one, I can at least confirm that after a couple of weeks and with 3000 km under its belt the repair was a success, in the heat of battle and the usual pressure from the customers to have their vehicles back by the day before I didn't get many pictures during the rebuild apart from the ones I took to remind myself what went where despite doing the job many times lol I also got some of the damaged parts that were removed.

Despite a steam clean its still dries looking filthy you can also see the staining low down the block from the long term coolant leak,
IMG_3307.jpg
The pistons and liners note the 4 with top rings missing these come out in at least 2 pieces,
IMG_3440.jpg
This liner was cylinder 3 which had by far the worst scoring which would have contributed to the excessive oil consumption and in turn 3 DPF filters fitted by the main dealer.
IMG_3443.jpg
This is the erosion on the water jacket side of the liner and you an just see the top liner seal where the coolant was escaping, this is usually caused by a lack of Antifreeze/corrosion inhibitor, in this case the customer was lucky in it wasn't that bad and the block wasn't damaged as it would normally mean having the seats cut and over-sized liners fitting.
IMG_3445.jpg
The last one was the most surprising and that was the state of the big end shells being the truck is fairly young at 4 years old and roughly 150000km which is nothing, I mentioned the excess oil consumption and at one stage the vehicle came in and I had to put 28 litres of oil in to get it back to the max despite the driver having checked it lol.
IMG_3449.jpg
I have seen far worse but never on a vehicle this young and apart from the oil and coolant loss there were no other symptoms it actually sounded ok too.

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