The block and Starboard exhaust manifold on my 1988 4.3L OMC froze and
cracked a few winters ago. I purchaed a NEW 4.3 Vortec over the
internet to replace the block and had a reputable welder repair the
external crack on the manifold. Last fall, I started the engine (on a
stand with a cooling setup) and all seemed well. I drained the block
and then filled it and all manifolds with green prestone adntifreeze.
The spring, I installed the engine in the boat, hooked up the water
hose to the lower unit and went to start it. for "some" reason. The
motor was locked up - as if a rod was broke and blocking any rotation
fot he engine. I could not even turn it by hand wiht a socket on the
harmonic balancer. I went to pull the plugs (to make it easire to
turn) and clear water came out of several of the holes. I opened the
draincocks and GREEN prestone came out, Putting the plugs back in - it
started and after clearing the remaining water - A week later, I took
it to the lake - went to start it - and locked up again. I pulled the
plugs, cranked the engine and water came out, Put plugs back in - and
ran - went for ride on lake - ran very well.

I have people telling me the exhaust is cracked inside (also - outside
was welded earlier) but if that was the case - would not the green
antifreeze from winter storage gotten into the cylinders instead of
clear water. Found some new (later version) manifolds on eBay but
seller sad they woudl not work on vortech engines. He sent me to
manufacutrere when I inquired. Manufacturer claims that Vortech
engines have a long exhaust valve overlap and ti will actually suck
water back into the cylinder. and I woul dhave to use newer mercruiser
manaifolds (but no guarantee it will work) and modify them to mount to
OMC backend.

Has any one ever heard of this (vortech engine sucking exhaust water
back in due to its overlap) if so, how do you get around it? Vortechs
have been used in boats for several years,

How can I check if the manifold is cracked inside? My local boat
dealer says thie single piece exahust cannot be pressure checked?

Thanks in advance for any help!