Cracked exhaust manifold chevy 350/260

[RandyJ]

1978 Hammond ski boat, 350 chevy (260 mercruiser) exhaust manifolds are cracked from over heating and restarting before cooling. Anyone know if they can be fixed or how to do it or will I have to buy new ones? <br /><br />I'm thinking of blocking off the incoming water and running some radiator stuff in it (such as liquid glass) to see if that will seal them but I don't have a clue it it will work or if I'm taking a big chance of getting that crap on top of the pistons and messing up the motor. Right now it runs FANTASTIC but I have to pull it out of the water, pull the plugs, and blow the water out of the pistons after I shut it off because the water is coming in thru the exhaust valves.

 

[Don S]

Re: Cracked exhaust manifold chevy 350/260

Replace the manifolds, and risers. Probably need a raw water pump impeller that caused the lack of water to begin with. You probably also burned up the exhaust hoses, the exhaust shutters, and the water pickup housing and seals in the outdrive.<br />Try your radiator stuff, and you can add an engine to the list also.

 

[waterone1@aol.com]

Re: Cracked exhaust manifold chevy 350/260

Randy, this isn't a car. The engine operating temps for that engine should be about 150 degrees, not 220 degrees. Unless there is something very wrong, there is no reason for your manifolds to crack from cool water entering them. Replace everything Don posted and see what temp she runs at. If it gets over 150, you still have a problem. After what you have described about water in the cylinders, you might also want to change the oil and do a compression test to see if the exhaust valves are still seating properly.

 

[RandyJ]

Re: Cracked exhaust manifold chevy 350/260

Thanks guys. I've already replaced the impeller and thermostat. When I got it home from the guy I bought it from the first thing I did was check out the circulating water pump and the thermostat after blowing water out of the cylinders. Thermostat had blue soot and appeared to not be operating. I was riding in the boat when it ran hot (this time). It was running great for about two miles then it started cooking. I did the work then put it back in the water and it ran fine until I stopped (never ran hot on me) when I tried to start it again there was water on the cylinders which got past the rings (I hope that's how it got there) and it now has some water in the oil. compression is fine.

 

[KaGee]

Re: Cracked exhaust manifold chevy 350/260

New manifolds and rsers for sure!

 

[Don S]

Re: Cracked exhaust manifold chevy 350/260

Did you replace the raw water pump impeller, the one in the outdrive, not on the engine.

 

[f_inscreenname]

Re: Cracked exhaust manifold chevy 350/260

Originally posted by KaGee:<br /> New manifolds and rsers for sure!

Thats a start. Hopefully that will be it but if it was me I would do the impeller and thermostat just to be safe. Then go from there.<br /> A 1978... Hummmmmmmmm... I hope you have a big budget. A 27 year old boat may seem cheap at first then something goes wrong and it goes on from there.

 

[RandyJ]

Re: Cracked exhaust manifold chevy 350/260

Yep...before I put it in the water I pretty much gave it a cooling system overhaul (new thermostate, even a new temp sender, and a new impeller, and flushed everything then pulled plugs and blew out the water) she ran like a champ till I shut her down then the cylinders filled with water thru the exhaust valves that were open (3,6,8, and 7 would have filled up but the piston was near TDC). That pretty well fits the firing order so I figure it was open exhaust valves that let the water in.