98 502 MagMPI . . .
First time firing up in about 4 mo. . . as I am detaching jumper-box cables, I see water spitting out of the bottom of the stbd eng exhaust manifold ( stbd bank ). Hose comes from the waterpump assy to the bottom of the exhaust mnfld. Suspicious of cracked hose, I cram my hand down and under the mnfld ( real tight area ). Hose is fine. Seems like a drain-cock or something is open - or partially open immediately adjacent to where hose tees into mnfld. Water press. and temp are fine at 1k rpm for 20 min.
Pretty sure it wasn't doing this AT ALL before sitting for 4 months.

Check to see if you have a block drain left open from draining for winterization. Mine has fittings installed which have an integral valve. Others may just use an NPT plug. They are located about 2/3 of the way back, so the right side one is just over the forward end of the starter motor. It might be spraying onto the bottom of the exhaust manifold, making it look like its leaking from there. It's worth checking. If that's not it, then you may have a cracked manifold from freeze damage over the winter. Hopefully that is not the case.

I've accidentally left them open before, and discovered water leaking to the bilge just after startup. Oops. shouldn't hurt anything at idle...

That hose is hooked to a tee, which should have a blue end on it that you can open and drain the ex. manifold. Check to see if it is closed. Just finger tight. they break easy.

Or mybe you have the same thing I had and just fixed this winter. Seems kinda common with the cast iron manifolds. There are 2 plastic pipe plugs on the inside of the manifold(side against the heads) that leak. The manifold will rust and scale at those threads, which in turn deforms the plastic pipe plug, allowing it to leak. Usually 1st signs are rust stains in your bilge floor. Bad thing is, the exhaust has to be removed to repair. I sandblasted mine then repainted those areas, then new plugs. The Plastic plugs from Merc have been superceded with brass ones now. Hmmm.
You can see those plugs looking down the top along the inside of the manifold.
Just a thought

Get a mirror and a flashlight and locate the exact location of the leak. If your rig was exposed to freezing temperatures, you may have a cracked manifold due to incomplete winterization.

I hope that's not it but that wouldn't be uncommon.

If it is, look at the bright side... Haven't you been wanting a bright new set of Stellings???

OK - I got the mirror under the manifold, and found that the leak is coming from the 'elbow drain' assembly.
Looks like its the #14 - elbow drain plug, or #15 - the elbow plug gasket.

Anybody ever replace one of these ? I'm hoping I don't have to remove the manifold to do it.

Piece of cake. #14 and 15 is just a plastic screw in cap. Maybe it came unscrewed just a hair?? Those are removed to drain the manifolds for winterizing, they break easy. Just finger tight, no cheater bar needed to tighten.

But if it is the elbow itself, it's just plastic assembly, so it will unscrew pretty easy if the whole thing needs replaced. Can be done in the boat. No need to remove the manifold. Once removed, be sure to use a mirror and recheck the pipe threads in the manifold. I ran a tap in mine to clean them up and remove any rust scale. If it is leaking at the threads, I'll put money on the rust scale causing the elbow to deform and loose it's sealing.

Could it be just the "O" ring on the drain screw ?

Very odd that the drain plug would fail. Here is Southern California nobody winterizes (no need to) so it should not have been messed with, and could not be freez damage. Something else may be going on. If your manifolds are original from '98, you better pull them and replace anyway.

Dave

Update:
Just changed out the plastic elbow. All is good. Not sure how it failed, but 20 hours later, still good.