Mercruiser 120 water in #4 cyl

[russnord]

Last October when I was winterizing I took out the plugs, shot some oil in each cylinder, and turned it over. Water shot out of the #4 cylinder. I didn't have time to work on it then, so I started it up with the #4 plug out, which blew out the water pretty well, then I left them all open for a couple of weeks to dry out, oiled them up, and put it away for the winter.<br />Now it's time to get at the problem. I've been reading the postings, and see head gasket and manifold gasket and manifold crack as likely suspects. <br />Can anyone explain the construction and workings of the Mercruiser exhaust manifold. And what is the "riser"? I know my way around car engines, but this stuff is different.<br />Thanks.

 

[agefhx]

Re: Mercruiser 120 water in #4 cyl

Ive had this problem before, the riser sits on the exhaust manifold usually with a big hose on it(usually 4 or 5 inch hose) which is kinda like the exhaust pipe. mounted with 4 nuts on studs with a gasket between riser and manifold. its where the cooing water and exhaust gases mix and exit the boat. the riser and manifold has a water jacket around the center exhaust portion. well after a while the water jacket corrodes, especially bad in saltwater and allows water to enter the cylinder at low rpms. high rpms blow the water away from engine and out the riser.<br /> Im betting its time for new riser or better yet manifold and riser. Not a hard fix at all, I think it cost me $250 for parts on a v8 for 1 side of the engine. If 1 side needs replacing wont be long until the other side will give out too.<br /><br />If you take off the manifold and run a garden hose to where the cooling water enters it, (usually in front of manifold) you will probably see water running out the exhaust port on #4 cylinder of the exhaust manifold. In which case it is toast.

 

[Boatist]

Re: Mercruiser 120 water in #4 cyl

I agree with agefhx. The exhaust manifold on a boat is kind of like the one on a car except it is surounded by water to keep it cool inside the boat. If you run in salt water rust and scale build up on exhaust tube. With the heat and time it will rust thru and allow water to enter the inside of the exhaust tube and it can then run back to the exaust valve and get into cylinder. If you do not find it in time it will cause exhaust valve and inside of cylinder to rust. First thing you will notice is a low speed miss when the exhaust valve starts to leak compression. Since the number 4 cylinder is normally the lowest it is the fist to go. As the exhaust on the inside and water on the outside exit the manifold it goes into riser which go up high enough to stop any sea water wave from coming up exhaust pipe into cylinders. Riser will go up then turn back down at which point water and exhaust will exit together.