Re: Mercruiser 350 mag head gasket
It could be a head gasket (try a compression check), a cracked head, a cracked block, a leaking manifold to riser gasket, a cracked or corroded manifold or riser, a missing exhaust flapper (for through hull transom exhaust) and probably a few other things. I'd start with the compression check and if you find one or more low, pull the head. If the compression is good, I'd next pull the risers and inspect the sealing surfaces of the risers and manifolds. If they look good and no signs of water leaking past, check them for cracks. The manifolds are fairly easy to pressure test for leaks but the risers aren't(once they are removed from the engine). Using a thick piece of rubber for a gasket between the man. and riser and a 3/4" MPT to hose thread adaptor (PVC fitting from Home Depot)and a garden hose (which will be putting out 50 + psi. Bolt the riser to the manifold and connect the garden hose. If no water is present in the inside of the manifold after 10 minutes, your golden. The risers are very difficult to pressure test (it can be done, but some custom fabrication is necessary. Risers aren't too expensive and in salt water applications should be changed every 4 -5 years anyway.
