Water in Cylinders

I have a Mercruiser 898 (305 Chevy) that started running rough and I found water in the 4 & 6 cylinders. I pulled the head, had it surfaced and magnafluxed and replaced it. I put it in the water and the same cylinders filled. I pulled the exhaust manifold and riser and ran water through it. No leaks into the exhaust ports which are black w/no rust. Compression on both cylinders is 140. I am at a loss as to where this water is coming from. Any suggestions?

 

Manifold ports, or head ports? Check the ports in the head carefully, I've seen machine shops miss rust throughs in the exhaust ports. Some remain hidden until you clean the port out (wire brush), which they probably didn't do. If the manifold is good, this is where it is coming from.

 

The exhaust manifold was black w/no rust and I shined a light into the exhaust ports of the head which were black w/no sign of rust in the port or the exhaust valves. Could a crack there leak into both cylinders? If so, could the compression be 140?

 

The soot tends to hide the signs of the leakage.

Yup. Cylinders 4/6 and 3/5 have "siamesed" exhaust ports, which will allow water to travel from one to the other through the manifold, or the corrosion could have came through on both ports. These tend to run hotter than the single ports of 1,7,2,8. If your engine is raw water cooled and run in salt, this really becomes a problem, as the hotter it is, the more corrosive salt water is.

Compression wouldn't be affected because the exhaust port is after the valve.

 

It is sea water cooled, but only run in fresh water. Can I plug the water hoses and pressurize the water inlet and detect this leak while the Exhaust manifold is off or do I need to pull the head?

 

I would try that first. If nothing, clean out the ports with a wire brush and inspect.