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What would cause piston to melt/crack?

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Saturdee

10+ Year Contributor
405
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Nov 9, 2010
Duncannon, Pennsylvania
Almost done rebuilding my engine from when it blew almost a year ago. Well anyways, the cause was a melted/cracked piston, and I'm not exactly sure what would've caused it. I have some hunches, such as faulty o2 sensor, bad knock sensor, or maybe just real bad knock at wot (car was stock and never had a tune). But I'm no expert and these are all just hunches and I'd like to know if there is a common culprit to this or not. Any input would be much appreciated, thanks.
 
Having a lean A/F under load will cause excessive heat and/or knock and destroy the piston. A bad o2 sensor won't have much to do with it since it functions during cruise and idle, not WOT. A bad knock sensor may have contributed, but it being bad in itself is not enough to cause knock and melt a piston. I would thoroughly check your fuel system (injectors, pump, filter) and get ahold of some sort of logging device so you can monitor things more closely on your new motor.
 
Where was it cracked? If the crown or ring lands were cracked, it was caused by excessive detonation. Lots of things cause that. For example: Faulty Knock Sensor, Bad Fuel, Too Much Boost, Too Hot of Spark Plugs, Improperly Set Timing, and Faulty Fuel System.

The 02 sensor won't cause detonation. It has NO effect on the engine at WOT. It could lean it out at part throttle enough to cause issues, but there would have to be other issues such as vacum leaks or faulty injectors to get it lean enough to cause damage.

If its a skirt thats broke, that is usually caused by to much piston to wall clearance, usually meaning the motor is wore out.

Melted piston is usually caused by a combination of being too lean, and detonating. It's caused by many of the same issues from above.

If you have a melted and cracked piston, it sounds like you were running lean. Make sure your fuel filter isn't plugged, you fuel pump is adequate for what your trying to do. Obviously check everything in the list above too, however most of that will be addressed when you rebuild the engine.
 
I suppose I will just buy a new fram fuel filter and use that. As far as tuning goes, I will have a wideband before I drive my car anywhere, period. I will have to wait a little to get link though, due to my low income (part time..), but it will be my next upgrade when all is said and done with this motor. How would I go about cleaning the fuel pump?

Also, here is a picture of my piston.
 

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It's hard to tell from just that picture. Just a head gasket failure between cylinders, and keep running it can cause similar looking pistons. I've done that my self.

Make sure you check things out good before you get it back together.
 
wow, ive never seen one melted that bad. yea, like the other guys said, it was run extremely lean most likely. definitely get a WBo2. good luck with your new engine
 
WTF whoa thats really bad! Something got really freakin hot in yoiur combustion chamber! To me that could either be some serious detonation or a lot of carbon build up on the piston causing some serios pre ignition! Damn I hope all goes well with the new engine and tuning!:thumb:
 
IF it's just in one cylinder than you might have a bad injector in that cylinder or something along those lines.
 
Lean for whatever reason as stated before, but that is the least of your problems now those black marks on the piston skirt are known as black death, you will now have that aluminum inbedded on the cylinder wall, check all other cylinders for similar marks, It might clean up with honing but in your case it does look good.
Is your head also damaged?

You are probably looking at a complete rebuild.
 
I suppose I will just buy a new fram fuel filter and use that. As far as tuning goes, I will have a wideband before I drive my car anywhere, period. I will have to wait a little to get link though, due to my low income (part time..), but it will be my next upgrade when all is said and done with this motor. How would I go about cleaning the fuel pump?

Also, here is a picture of my piston.

wow that looks baddd
 
I had the same problem after my first build. I melted the crap out of a weisco. It was only in my #3 cylinder. When it happened i was looking at my wbo2 almost the whole time, and it was rich all the way through my rpms. But after later investigating i noticed my fuel injector plug for my #3 cylinder was melted a little bit and the wires turned black. I narrowed it down to high resistance in the fuel injector harness, witch means that cylinder was getting less fuel leaning it out in that cylinder. In term causing the piston to heat up and melt.

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Sorry for the late response, I have been busy lately. And just to let people know, the block was bored .020 over, block and head resurfaced and hot tanked, and crank journals were polished.



IF it's just in one cylinder than you might have a bad injector in that cylinder or something along those lines.

Yes, it was only one piston that was damaged

What cylinder was that out of? How much boost were you running?

It was my number 1 cylinder. All others were fine. Car was bone stock, other than a K&N filter, and test pipe. Oh, and the BCS restrictor mod. No boost gauge so I couldn't determine an exact number.

It's hard to tell from just that picture. Just a head gasket failure between cylinders, and keep running it can cause similar looking pistons. I've done that my self.

Make sure you check things out good before you get it back together.

I did overheat the car once by accident when I first got it. An accessory belt broke, and I thought I'd just drive it home, until about ten miles down the road I look and see my temp gauge maxed out. So I don't know if I could have slightly warped the head, which would cause damage to the headgasket, which in term could've eventually caused this?

I had the same problem after my first build. I melted the crap out of a weisco. It was only in my #3 cylinder. When it happened i was looking at my wbo2 almost the whole time, and it was rich all the way through my rpms. But after later investigating i noticed my fuel injector plug for my #3 cylinder was melted a little bit and the wires turned black. I narrowed it down to high resistance in the fuel injector harness, witch means that cylinder was getting less fuel leaning it out in that cylinder. In term causing the piston to heat up and melt.

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I went out and checked all my fuel injector plugs/wires, all four looked good. But wow, that piston looks very similiar. Thanks for the information.
 
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