Detonation killed this piston? Bad injector?

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Detonation killed this piston? Bad injector?

Post  Alpine on January 21st 2017, 1:53 pm

This is a picture of #5 piston that was in a bone stock 93' 351W. The remaining 7 cylinders look just fine. I'm not sure what happened here, but my guess is that an injector failed, melting the piston, and from that point on detonated from aluminum build up in the chamber and on the plug. This engine was driven for roughly 300 miles with a misfire, so the damage was likely caused over a long period of time.

My apologies for the photo orientation, I think you'll get the point!

Thanks guys, looking forward to your opinions.


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Re: Detonation killed this piston? Bad injector?

Post  pmrphil on January 22nd 2017, 3:14 pm

Man, that's ugly, but I think you have already hit the nail on the head. Did the block survive? I've seen quite a few with severely distorted (or cracked) cyl bores after a failure such as this.

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Re: Detonation killed this piston? Bad injector?

Post  rmcomprandy on January 22nd 2017, 4:45 pm

Looks like a very lean cylinder problem to me.
There was SOME fuel present though, in order to cause that cutting torch type heat.

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Re: Detonation killed this piston? Bad injector?

Post  maverick on January 22nd 2017, 5:49 pm

rmcomprandy wrote:Looks like a very lean cylinder problem to me.
There was SOME fuel present though, in order to cause that cutting torch type heat.

....like maybe a vacuum leak might cause?
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Re: Detonation killed this piston? Bad injector?

Post  Alpine on January 24th 2017, 12:40 am

Do you think that there could be a significant enough injector O-ring leak to cause this? If there was a manifold vacuum leak elsewhere, wouldn't I see the effects on more cylinders? The burn on the other cylinders is quite typical. I can't see this being an intake manifold gasket failure. They looked OK anyway.. I've seen this exact failure on another 351W of the same style. We never got down to the root cause of that failure, just replaced the engine with a junkyard unit. I'd like to figure this out.

Thanks for your replies!

Derek.

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Re: Detonation killed this piston? Bad injector?

Post  maverick on January 24th 2017, 8:00 pm

That cylinder ran hot because it was lean. A lean condition can only be be caused 2 ways...
1. Not enough fuel.
2. Too much air.

If you can rule out a problem with the injector and its harness, then it's getting more air than the other cylinders. Since the engine had a definite misfire for 300 or so miles, i'd guess it was an injector problem, too (or injector harness)....that is IF you inspect the entire intake and verify no possible vacuum leaks at gaskets, vacuum hoses, connections, etc. Since this is a '93 (pre OBD2), a code reader won't give you the same good info as the '96 and later stuff.

If it was mine, I'd replace as necessary and move on. After all, it's a bone stock 24 yr. old engine. Wink
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Re: Detonation killed this piston? Bad injector?

Post  Alpine on January 27th 2017, 11:58 pm

Who knows. So much of the vacuum system was brittle and breaking upon disassembly. Vacuum leak, well... Maybe a possibility. You're right, time to march forward and move on, lol.

Thanks for the help guys.

Derek.
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Re: Detonation killed this piston? Bad injector?

Post  DanE on January 28th 2017, 10:11 am

I had the same thing happen to an injected SBF. An injector had leaned out. I rebuilt the short block and replaced the injected unit with an Eldebrock performer intake and a 390 CFM Holly 4 barrel carb. After some tuning, the engine ran stronger and got better MPG. And that was the end of running cheap gas. I run "Tier One" gas only. "Tier One" gas costs a little more, gets better mileage, and is less expensive as to cost of gas per mile driven. AND, "Tier One" gas does not plug injectors.

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