OK so i have a GM mercrusier 468 in a boat that last year dropped a valve on #5 cylinder. So i went through the motor and had it dynoed and all was well. When I put it in the boat it ran great. I have stainless marine exhaust with short risers. I was concerned with reversion so after running it for a bit I noticed that I had alittle water leaking from the band clamp that holds the riser to the manifold. I pulled the manifold and one side looked good and dry. The other side looked like it had water beads in the #5 cyliner exhaist runner. I pulled the manifold and made a pressure testing rig from a automotive pressure tester. I dunked the manifold in a big tub of water and presssured it up to 30 psi. No bubbles or leakage. Could the riser leaking be a sign? If so why would it leak into only one cylinder? Have asked to many so called professionals and am at a loss. Need some input.

did you reuse the old heads?

yes I had them worked over. Pressure tested and installed econell valves. They are racing head service heads.

im not saying it,s the problem,but maybe you have a leak around the exhaust guide on #5 that might not leak until it gets hot so it did not leak when pressure checked.also,keep in mind that an engine on a dyno has no water pressure so it would not leak on the dyno.

Yea I wish I could remember if I put that head back on the same side as before.

If your riser gaskets are leaking it will cause a vacuum and suck water back down. Don't use those cheap green rubber gaskets, use the nice thick carbon fiber ones

Thats the kind of things I want to hear.Man I ready to put this thing in the water but dread having to open the motor back up. It just seems weird to me that it is on the same cylinder that had the problem before. With the exhaust off it looks like that side had some traces of water but # 5 looked like it had been exposed to more that the rest. This was the side that looked like the exhaust riser was leaking the worst. And looking at the riser gasket you can tell it wasn't sealed.

corey,what part of the country do you live in,what state?

Kentucky, we boat on the ohio river

if you were closer you could put your engine on my run stand and get the water temp up to 200 deg and build some pressure in the system,might be easy to find a leak that way.get it hot,remove the header and look in the exhaust port #5.