That looks all too familiar. I'm going to replace the knock sensors just to be on the safe side but the question of just what caused it is a nagging one. I was also planning to put in pressure switches that would alert me it the fuel pressure dropped below a certain pressure.

I guess I have to consider myself lucky. The damage to my engine was much less than yours. I can't wait to hear what Mercury tells you.

I was bouncing around on E-Bay and came across these heads. Maybe they will help you if Merc doesn't cover you.

http://cgi.ebay.com/ebaymotors/ws/eB...category=31286

I overheated a BB Chevy engine badly many years ago at Watkins Glen and burned a pathway between two adjacent cylinders. My guess is overheating and/or preignition. Just a blown head gasket usually does't melt pistons.

that motor got hot,the pistons and walls were washed clean w/unburned fuel,the head gaskets have been leaking for a while,that didnt just happen in a few minutes,you need to open your cooler up and tear your pump down and inspect it,youll probally find the cause there!

I have a couple of sets of iron rect. port Merlins for sale in the bulletin board classifieds. Upgrade when you repair.

I was freaking out over this and called a good friend that is a certified Merc mechanic that has treated me right and I really trust.

He told me to calm down, that he is NOT seeing anything like this out of 500's. In fact he menetioned several with EFI motors with 300+ hours he knows of running strong.

He said the only time he sees this type of problem is when the engines have been overheated!

Any thing can happen to any engine at any time, but I am going to get running mine. They are under warranty and hope for the best.

Troutly,

I am not doubting you, but I would think that if that were the problem, you would see the same issue with Mercs' other motors. Why just the 500???

Water pressure at idle is around 3-5 psi and while running is between 14-20 i think. But there was diffently water presurre and water flowing out the side of the boat from where it leaves the dry exhaust. I do have one strainer for each motor. The motor never got above 190 degrees and was then shut down. We have also checked the impellor in the water pump housing and it looks good also. Although it was replaced at one time as the wear on the housing and impellor are not the same. Not sure why someone would just install an impellor only and not the hole housing, but the proof is there. There is not signs of overheating on the block or head, but that is the only conclusion that we can come up. Is there a way to have the knock sensor bench tested. The truth will come when we pull the heads of the opposite motor and see those gaskets. If they look similar, then something is really screwed up.

3DO

Same exact thing happened.
 

98 500 carb motor 220hrs, dont have firing order, but it
was the back 2 on port side. 5/7?
Burned a hole in block and head just like yours.
Had the boat traded in on my fountain just
waiting for it to come in, Gave the boat and motor
in pieces with a pile of cash to keep the deal.
Sucked, I believe it was mercs. problem but
good luck with that. Hope it goes better for you.
John

It doesn't take long for cylinder temps to spike and not be noticed on gauge. Steam pockets form and severe localized overheating occurs. Inadequate water volume and pressure along with over-propping and/or overloading boat will cause high cylinder temps. Cool water will pass through block and around steam pockets and show normal engine temps. The 500 EFI has been pretty forgiving overall because of it's low compression and moderate horsepower. It may be time to review overall setup and build a little margin into the system.