Merc 496 blew a piston
 

Hey everyone I'm really looking for some help here. I have never posted before on a forum but really am at a loss. I have a 05 23' outlaw with the mag 496 375 horse. The boat has 300 hours on it and has never skipped a beat until now. I took the boat out and idled the engine for approx a half hour in no wake then got on it maybe 4800 rpm for a short time climbing speed was only up to 55 mph when the engine let go vibrated and died. I pulled the engine and number 7 piston was non existant I have never seen anything like this before. There was about a quart of coolant in the oil I suspect a crack in the block from the rod slamming around. My scare is now I have yet to find the root cause of the problem I am installing a reman long block this week and am very nervous. I filled the exhaust manifolds with mineral spirits and have found no leaks. I sent the injectors out to have them flowed and all came back good I will check fuel pressure when I get the engine back in but am just wondering if anyone has experience with these engines and has seen this problem

I once had a melted piston in a 496 but ho version... never found why. Put a used piston in it, ran it, sold it, still runs fine... go figure!

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One is scan the engine completely and log all the info. Then have someone look at all the data in depth you collected from the scan,

Next info --- fuel rated / fuel pump / fuel pressure regulator / fuel filters / fuel rail corrosion / internal paint peeling from Gen 3 cool fuel if applicable per engine serial number ./ fuel pressure / fuel injectors

Sensors --- Map

real dirty flame arrestor ?

checking fuel injectors depends on how they were check out .plus what machine was used. I have found a small % of these fuel injectors to have inherent problem over time. Also there is a specific amount of fuel volume all injectors need to flow. If that service does not know the specifics of volumes at all rpms and varying m/s pulse widths .thru out - that can be a underlying problem.

Also some 496 pistons just let go. They are not forged.

I have seen all of above when my hey day of seeing 496's. I still see about 25 to 40 per year currently basically is 2006 and up 496's with Gen 3 cool fuel and close to half are DTS apps

What does your spark plugs look like and is # 7 spark plug intact ? .

Are all coils firing ?

head gasket condition and any signs of an issue ?

Thank you for the quick reply and all the information, if the flame arrestors are the exhaust baffles then it doesn't have them it has the straight thru tips. Plug number 7 was destroyed from the piston coming apart I assume, engine always ran smooth as silk before and the head gasket looked as it was seal tight

It sounds like you are leaning towards a lean condition in the engine I sent my injectors to Midwest fuel injection they are a rather large company and I send all my diesel injectors too, I am going to check fuel pressure when I start the motor, the fuel rails like new the lines and and filter look new I will see if I can disassemble the fuel cooler to have a look inside. When I have the engine scanned does it need to be ran on a dyno to get accurate readings
thank you again

You may want to take a fuel sample and test for water and also the condition of the fuel specially if it's been sitting . Apart from this I can tell you I have a customer that has more than twenty of these engines in industrial use and I can tell you the two most common failures are broken pistons and broken connecting rods you can scan them until you're blue in the face and check all the sensors because that is the correct procedure and it needs to be done before you place back into service but they are notorious for those two failures. We rebuild the engines here and replace the injectors as well as pistons with forged JE or CP and connecting rods with Hbeam 4340 rods . Good luck keep us informed. Sincerely Laz

Agree on the forged piston/rod route if you rebuild a 496 MAG/MAG HO. I went with SRP (JE) pistons and Scat H-beam rods. Cheap insurance.

so basically GM built the 502 with nothing but forged pistons and it worked so well that they went back to cast pistons for the entire 496 run.

i will take a fuel sample and send it out, i just i just did not expect this from a bone stock motor with 300 hours on it i have put only 30 hours on it this year but i always keep the fuel tank full and im sure i have ran through all the old fuel from last year which i did stabilize before i put in storage. one second it was running fine the next it just gernaded without any warning,