[Entabeni]

From Entabeni: 2006 Gen3 Pajero 3.2 DiD LWB GLS Auto.

Having suffered a 'hole' in piston 2 of the Pajero, I extracted it with the motor still in the vehicle. The 'hole' turned out to be a crack running from the back towards the front, across about 3/4 of its diameter. Examining the piston carefully I found that two of the three manufactured holes that can be seen in the bottom view of the piston in Photo 1, run into some sort of cavity just below the piston's top surface. Furthemore, these holes are linked as proved with compressed air and the cable tie in Photo 3.

The best way to find what this strange (to me) structure was, was to complete the piston break-up by sawing with a hacksaw towards the crack through the remaining unbroken metal. The results are seen in Photo 2. The large black area shows that the crack had gone nearly all the way across the piston --- only the lower parts of the gudgeon pin sockets were still uncracked. Perhaps the crack had been developing for some time before the white exhaust smoke suddenly appeared in December. It seems that the crack finally burnt a hole through into the piston's 'cavity' (Photo 2) so discharging mixture directly from there into the crankcase. From there it would have gone via the timing chain cover into the tappet cover and through its breather into the clean air intake of the Turbo unit before emerging from the exhaust as white smoke.

Another very interesting feature shown by this cross-section is that the top compression ring and slot are tapered inward. Furthermore, the colour of the metal surrounding this ring is differrent from the rest of the piston. This difference is so uniform and regular, as seen also on the piston's surface, that it is clearly part of the manufacturing process --- quite a metallurgical achievement by Izumi! On the piston top there is the following identification: B, ME191470.

What is amazing is that cylinder 2's wear proved to be slightly less than cylinder 4's! But I will post measurements of all cylinders in 'Engines and Engine Management" later.

Further info and comment very welcome.

[andrew.ashton]

Hi Entabeni,

Thanks for keeping us informed.

I am no metallurgical expert, but I just wonder whether this is really a failure due to thermal stress caused by a leaky injector (per initial speculation), or whether it could not be caused by excessive clearance in the small end or big end, resulting in repetitive shock loads on the gudgeon pin area and then ultimately leading to fatigue.

I guess the casting design (cavity) is for reduced weight and improved cooling (lower mechanical and thermal inertia).

[CATS]

Amazing! Thanks for sharing and please keep us updated on your progress.

Sent from my GT-I9500 using Tapatalk

[Entabeni]

Hi Entabeni,

Thanks for keeping us informed.

I am no metallurgical expert, but I just wonder whether this is really a failure due to thermal stress caused by a leaky injector (per initial speculation), or whether it could not be caused by excessive clearance in the small end or big end, resulting in repetitive shock loads on the gudgeon pin area and then ultimately leading to fatigue.

I guess the casting design (cavity) is for reduced weight and improved cooling (lower mechanical and thermal inertia).

Thanks Andrew,

I don't think failure can be due to loose small-end bearings, for I was impressed with how beautifully new they looked and felt! I guess that I could also replace the con-rods without the pistons just to feel how much play there is, but the shells and journals look perfect!

Entabeni.

[4ePajero]

I have little doubt that the failure was due to overheating, which could only have occurred due to over-fueling on that cylinder.

Have you had the injectors tested, especially that cylinder's one?

[andrew.ashton]

I have little doubt that the failure was due to overheating, which could only have occurred due to over-fueling on that cylinder.

Have you had the injectors tested, especially that cylinder's one?

I have read that there are other reasons for this:

1. Detonation - this quote:
"
Cracked Piston
Detonation is the most likely cause of a cracked or broken piston. The hammer-like blows of detonation can literally beat a piston to death. The causes are similar to those that can burn a piston: a lean fuel mixture, over-advanced spark timing, a bad knock sensor, low octane fuel or anything that causes the engine to run hotter than normal.

Loss of exhaust gas recirculation (EGR) is a common cause of detonation (spark knock) because EGR has a cooling effect on combustion temperatures. Consequently, if the EGR system isn’t working, combustion temperatures may exceed the octane rating of the fuel causing the air/fuel mixture to ignite spontaneously before the spark plug fires. The knock sensor should detect the rattling noises produced by detonation and signal the PCM to back off spark timing. But if the knock sensor isn’t working, or the PCM fails to retard timing, detonation may continue unchecked and eventually damage the engine. Possible causes for loss of EGR include a bad EGR valve, loss of vacuum to the EGR valve (due to a leaky hose or EGR vacuum solenoid) or carbon buildup under the EGR that restricts exhaust flow back into the intake manifold. If you suspect an EGR problem, check the operation of the system and remove the EGR valve to inspect the intake manifold passageways for carbon buildup (clean as needed).
"
from here.
2. Knock caused by burning of excess oil from turbo seals.
3. Excess small / big end clearances.
4. Hydrolock from leaky injector.

Although by far the most common reason is excess thermal cycling, such as that caused by incomplete immediate combustion and then combustion in low oxygen environment - typically, as you say, from a leaky injector or faulty pump.

It is interesting to read how many marques and models have cracked pistons reported - just Google ford transit cracked piston as one of many examples.

The piston manufacturers acknowledge that the high temps and forces in modern diesel engines with higher specific outputs are putting more strain on pistons and they are developing new manufacturing techniques to address this. See for example New, nano-reinforced alloy improves car diesel engine pistons.

[Entabeni]

I have little doubt that the failure was due to overheating, which could only have occurred due to over-fueling on that cylinder.

Have you had the injectors tested, especially that cylinder's one?

Hello Gerhard,

I have had all 4 injectors tested and reconditioned by Brandwacht Diesel & Turbo in Worcester who apparently have sophisticated equipment for this job. I asked if I should also have them check the injection pump and any other items that might be the cause of the piston cracking. I took the Turbo Charger unit to them too but, after looking into the input and output ports and turning the impeller with a finger, they said it was fine. They were adamant: the cause was due to overfuelling by the injectors, so I accepted that. I have also decided to replace all pistons and rings to try to avoid premature failure on future trips into the bundu.

Entabeni.

[Entabeni]

From Entabeni to Andrew.Ashton,

Thank you Andrew, for the quote about spontaneous ignition being the cause of piston break-up. However, they talk about spark advance too, so is it perhaps about petrol engines? Nevertheless, explosive as opposed to progressive burning of the mixture can probably occur in Diesels too, perhaps giving rise to the knocking sound many people have heard, including myself.