140 mercruiser water in oil

[1970LasVegas]

Hello all. I have an old trihull with a 140 mercruiser in it. I have water in oil. The compression test with wide open throttle is #1-140 #2-145 #3-150 #4-130. It idles pretty good but under load I can only get about 3000 rpm out of it. My manifold does have a crack on the side. Would the manifold cause water to get in oil or am I looking at a head gasket problem. Also getting blow by thru the pic valve. Thanks for any help.

 

[fishrdan]

The manifold could have an internal crack, remove the manifold, fill it's water jacket with acetone, then closely inspect inside the ports for leakage. Water can get in the oil from a cracked manifold, but it has to get past the piston rings. Pull the spark plugs and look to see if any of them are rusty, also look up into the head ports to see if any of the ports or valves are rusty.

Since the manifold is cracked, you could also have a cracked block. While the manifold is off, check the block behind the manifold, they like to crack just below the block's deck, about 1/2" to 1" down. The block can also be pressure tested to check for an internal leak, see stickies at the top of the forum for procedures.