Cracked Block???

[chrisw21]

I've got a 1999 5.0L merc. A while back the boat started to run rough, backfire, wouldn't hit full rpm, etc. I pulled the plugs and found a couple were loaded up and had some rust on them. Now the first thing that comes to mind is risers/manifolds. I pull them and they are pretty rusty, but no obvious holes or leaks. Looked into the exhaust ports and saw some rust on one of the valves (CRAP!!!). Pulled the valve cover and found bent pushrod from a stuck valve. Pulled the heads and sent out for rebuild. Get a call saying both heads are cracked!! Guy says I must have overheated pretty bad. Temp gauge never went above 170 - 180 when it was running (I check all gauges constantly when running). So now I buy new heads, risers, manifolds, etc. and put everything back together. Boat starts right up and idles great, but still can't get full rpm. Find water on one of the plugs. Started to pull everything off again and found foamy, brown oil/water mixture covering the underside of the intake manifold and head gasket was blown out. Talked to some people and they said that I likely have a cracked block and when water entered the cylinder, it would turn to steam and blow out the head gasket. I looked at the problem cylinder walls and could not see any cracks. So here are my questions:
1) Would water in the cylinder cause the head gasket to blow out?
2) I check my oil before EVERY trip and have never seen water on the dipstick. Why would the top end have obvious water/oil mix but none seen in the pan?
3) Could the block have gotten hot enough to crack without it showing on my temp gauge?
4) What do you think the chances are that the new head gasket just blew out for some reason? I'm thinking not too likely because it was the same cylinder that had issues before the repair, but I'm no expert.
5) Is there any way to pressure test the coolant part of the engine with it still in the boat?
6) Do you think I should get new head gaskets and bolts and give it another shot? (BTW... what's the deal with these damn "torque to yield" head bolts that can't be re-used?!?! Those things are too expensive to just use once and throw out!)
7) Anyone have a good 5.0 short block for sale?

I know this is a long post with alot of questions, but I've now been boat-less for a year and its KILLING ME!! The whole reason I bought a house on a canal was so I could go on the water whenever I wanted. Now I get to sit and look at my boat on the side of the house while my neighbor's boat is on MY lift!!! Thanks in advance for your advice!

One more thing...If the block is cracked, what would be required to replace the 5.0 with a 5.7? Would the heads be the same (since I just bought 2 new ones)? How about the wiring? Is the ecm for the Thunderbolt ignition the same between the 2 motors? Drive gear ratio same? Anything else??

Thanks

 

[fc3]

Either you have a cracked block, bad exhaust system, or you torqued the head improperly. Of the three, the last is the most likely, however you sound like you know what you're doing.

Got me, but a small block V8 is pretty inexpensive. Unless you know the bottom end is in good shape, why fool with a crank that might need to be worked, etc. You can drop a 350 long block in place of the 305 easily enough. Pretty sure the ignition timing curve is the same. You should be able to use the same ignition module, wire harness, etc. The sell the heads and be done with it.

 

[Turtlesboat]

I heard of a couple of people that put on new manifolds and they weren't true/flat. That's another way to get water in a cylinder.

 

[fc3]

Mike has a point. The blocks deck might be warped from whatever damaged the head. You'll never get the heads to seal properly if that's the case. The only fix is to have the deck machined.

Best regards,
Frank