mercury inboard exhaust problem

1989 mercury big blocks, closed cooling, straight velvet drives, fiberglass silencers



1200 engine hours on each. even approaching retirement age, engines still turn over 4000 rpm

random incident last july, when coming up on plane, port engine blew off the 2.5 inch exhaust hose at right manifold

repair was easy ( two hose clamps ), and quickly forgotten

fast forward to last weekend, similiar situation, coming up on plane, at 3375 rpm, port engine blew off left manifold hose

engine starts easily, idles smooth, absolutely no vibration up to 2000 rpm

pulled boat out of water for winter storage yesterday, removed both enhaust hoses on port engine. silencer was full!

I removed silencer, single baffle on top half, bottom of silencer is clear. gasses hit baffle, then run through bottom and exhaust through stern

silencer was clean, very little oily smell, no signs of heat or freeze damage

Re: mercury inboard exhaust problem

While I'm not specifically familiar with your setup, aren't the fiberglass "silencers" supposed to have water in them? On jetskis they commonly refer to them as "water boxes" since they hold water in them to aide in muffling the exhaust noise. I would assume that if it was full or allowed the hose to blow off, that maybe you are having some sort of an exhaust flapper failure.

Re: mercury inboard exhaust problem

you are right, the bottom half of "silencer" is an open cavity.

at idle, I am guessing the gasses bubble through water before exit.

full on, the cavity probably empties and the gasses exit freely.

The flapper is outside the hull. At rest there is a slight gap which I think is normal.

When I removed the hoses the entire "silencer" was full of water even up into the exhaust hoses.

This may be because of the boats orientation when winterized.