Cause for new exhaust bellows failure?

[nll]

My exhaust bellows blew out will pulling a towable at about 3000 rpm. This is the bellows that is inside the transom just past the exhaust elbow (see diagram). We could no longer rev past about 1500 rpm and we noticed a bit of water leaking from the bellows. The boat is about 10 years old and the bellows was in overall poor shape so we just chalked it up to age and replaced it with a V-P replacement. Took a 10 minute test ride and everything was fine.

Next day, after about 15 minutes of travel at about 2700 rpm, the brand new bellows blew out (same side) with a large tear and a "bang". Until that time, the boat was running fine. So now I suspect there may be an underlying issue that is the problem. No problem with the bellows on the other (starboard) side.

Any idea what might be causing the bellows to blow? TIA.

 

[Horigan]

Sounds like you have an obstruction below the exhaust hose (not a bellows). Your boat may have flappers just below that hose that can fall into the Y-pipe and block exhaust flow. You may be able to fish it out from above, but you probably need to pull the drive to find and extract it from the exhaust outlet. Volvo discontinued using them and released a service bulletin to remove them.

 

[Scott Danforth]

My guess is you do not have water flow on that side and are burning the rubber.

 

[flashback]

Recently had a similar problem, the elbows were rusted and were restricting the flow..

 

[nll]

First, there is no evidence of a flapper valve as shown in the first diagram (from a V-P manual). No pin or even holes for a pin.

The new bellows that blew shows evidence of burning. Shown in the 2nd pic. The exhaust pipe itself also shows evidence of overheating on that one side (but not the other). But I was watching the engine heat closely and that was fine.

When the first bellows failed, we did notice some water coming out.

I'm going to get an inspection camera to see what we can see.

 

[nll]

My guess is you do not have water flow on that side and are burning the rubber.

Can you expand a bit on the water flow? Obviously it would be part of the cooling system. The engine was not overheating, and it appears only that one side of the exhaust was overheating. But I don't know where to begin to look for any obstruction.

 

[flashback]

I'm not familiar with the VP system, I have a mercruiser. That said, the diagram you posted looks very close. I found the elbows on mine were rusted to the point that the exhaust was not being cooled.

 

[Scott Danforth]

Can you expand a bit on the water flow? Obviously it would be part of the cooling system. The engine was not overheating, and it appears only that one side of the exhaust was overheating. But I don't know where to begin to look for any obstruction.

If there is any blockage on the side that blew. There would not. E enough cooling water and the exhaust goes from 220F to 900F

 

[Scott Danforth]

Can you expand a bit on the water flow? Obviously it would be part of the cooling system. The engine was not overheating, and it appears only that one side of the exhaust was overheating. But I don't know where to begin to look for any obstruction.

Start at the thermostat housing that feeds the manifold

 

[nll]

We did a bit more testing. We disconnected the water hose running to that manifold. Ran pressurized air through without issue (though we don't have a point of comparison).

Next we started the boat up. The volume of water coming out the good (starboard) side was much greater than the bad (port) side.

As it turned out, the water flow was obstructed in the thermostat housing. Cleaned it out and water flow was greatly increased in the port side.

Won't know for sure until we get another bellows, but it would appear that the problem is fixed. Thanks to all for your suggestions.

 

[Horigan]

You should pull the exhaust elbow to confirm there is not excessive rust in the water passages. You should also get a IR temp gun and confirm both elbows are running around 120degF. I'll bet the port side is hotter. You may need new exhaust manifold and elbow on that side.

 

[bruceb58]

You don't mention your model number or year of your engine but Volvo deleted the flappers in around 99.