250Hpdi piston

[soakin wet]

What might be the cause of this to the #5 piston

[CatCam]

detonation and/or pre-ignition.

Detonation occurs when the temperature and pressure inside a cylinder exceed the fuel’s octane rating. Instead of igniting when the spark plug fires, the air/fuel ignites spontaneously much like a diesel engine. This creates multiple flame fronts within the combustion chamber that collide and hammer the top of the piston, producing a metallic knocking noise called “spark knock.”

[ausnoelm]

correct, except for explaining that the ignition of the fuel can be way before top dead centre, often caused by a lean running cylinder, which in effect tries to push the piston back down while the others are keeping it going up, sometimes called 'pre ignition" and often accompanied by a "ping" sound, called "pinging"

[TownsendsFJR1300]

Hows the other pistons look?

That ones pretty bad...

[boscoe99]

Detonation generally occurs after the normal ignition process, which begins with the firing of the spark plug firing. As noted, detonation is a violent explosion of the fuel air mix, as opposed to the normal burn rate following spark ignition.

Pre-ignition is the ignition of the fuel air mix prior to the firing of the spark plug.

Detonation can lead to pre-ignition and pre-ignition can lead to detonation. They are the twin ugly red hair step sisters of spark ignition piston engines.

A 250 HPDI is a big block HPDI. The big blocks are noted for being susceptible to detonation related issues. Usually it is the number two cylinder. It is seldom seen in the small block HPDI's.

Other than making sure the motor is properly propped, and that the fuel is clean and fresh, about the only thing that the user can do to avoid detonation is to use gasoline with a higher octane rating. Many big block HPDI users use 89 if not 91/93 octane rated gasoline to try and mitigate the onset of detonation.

Here is a photo of a piston from the number two cylinder. Your motor seems to have fared better than the one this piston came out of.

[TownsendsFJR1300]

Looks like the damage to that piston(^^) was caused by the broken off piston crown rattling around the combustion chamber.

[boscoe99]

Looks like the damage to that piston(^^) was caused by the broken off piston crown rattling around the combustion chamber.

Yep, detonation broke the top of the piston and then broken fragments damaged other parts of the piston plus the cylinder head as well. Injector was destroyed. Shot peening by piston parts.

Crap can also get back into the block via the intake system and make a further mess in there as well.

[soakin wet]

the other pistons look great. The cyl head did not show any signs of anything anything being in the cyl it was damage free. I just bought the motor, so I don't know the complete history. this is the 3rd powerhead failure on this motor in200hrs. he had the injectors cleaned and tested, replace ecm. He had about a 180hrs on this rebuild. Last trip made was from Maime to the abaco bahama's The side skirts show sign of heat scoring.. Maybe bad fuel or excessive heat on the trip back. He did replace lower unit when he got back. said it showed signs of water in it. Just trying to figure it out before I rebuild it need to fix the cause So I can try to keep from happening again. going to replace all the filters. may even send injectors out to have the cleaned. thanks for all ya'll help

[ausnoelm]

One thing is 100% certain, if you don't fix what cased the problem first, then you will rebuild the motor over and over again.

[ausnoelm]

OH, I forgot, just because the seller said he had injectors cleaned, and motors rebuilt and all sorts of other 'stuff" don't believe it.......