Blocked exhaust, but found nothing?

[chiron]

I'm stumped on this one!
Went to take my son out sailing for fathers day only to have the engine not start. Did the usual check for all things needed for an engine to run and when I got to the spark plugs I noticed water in the pistons. Dumb me forgot to close the fresh water valve. Though I have cranked the engine many times for too long and never had water back up to the manifold, I learned my lesson this time.

So going back the next day with new spark plugs and a few other maintenance items I began trying to get the water out and get it running. Here is where I got stumped. With the water shut off, spark plugs out, and cranking the engine to get the water out of the manifold I finally gave it a shot. The engine still would not fire up. Repeated everything a few times just to still fail on firing it up. So I pulled the exhaust off to see water pouring out. While the exhaust was still disconnected I got the engine to fire up till I was certain all water had left the manifold. But when I reinstalled the exhaust the motor would not start. Again I removed the exhaust and got the motor running and by hand pushed the exhaust flange up against the engine, causing the engine to die.

Obviously I have a blockage. Removing the flex pipe from the stand pipe, which goes to an inline muffler and out the hull, I got the engine running and the fresh water back on. So the blockage must be down hill of the stand pipe. However once I took it all home I found absolutely nothing blocking the flex line or the muffler, save a half penny size chunk of carbon scale that came out of the muffler.

Any thoughts as to what could be going on here?

[Al Schober]

Not clear to me what you have for an exhaust system - water lift? moyer type mixing can? or just a hot pipe going up, then water injection downstream?
Most of my experience is with the mixing cans, and they can fail in two ways - both leading to cooling water going back down the hot pipe to the mainifold.
Just to be sure check outboard - perhaps someone gave you a potato in your exhaust?

[chiron]

Ha! I checked for a potato and found nothing.

My exhaust system is as follows.

Directly off the exhaust flange is a bell shaped metal thing, much like a stand pipe, where the water gets injected.

2' of 1 1/2 flex line to a fiberglass muffler/water lifter

then 5' of flex with a loop just before it exits the hull.

[Al Schober]

I'm suspicious of that mixer/standpipe thing. No idea what's in there, and all the downstream stuff seems pretty straight forward.
I'm guessing, but aft seems to be to the left in your photo.
I see the water hose leading into the mixer. What's that thing between the hose and the manifold?
I suspect that whatever magic inside that mixer that used to keep water out of the manifold is no longer doing it's thing.
Any manufacturer's mark on the thing? You may have to go to someone like Yanmar or Westerbeke for a substitute.

[Al Schober]

A little research shows that your mixer is probably a Barr part - made of aluminum! - also known as corrodium.
Also check Centek and Vetus for equivalents.

[ndutton]

Analyses and recommendation

1. The component you're calling a standpipe is actually a mixing elbow. It functions more like our hot section than a standpipe because it does not elevate the raw water prior to discharge. These elbows are known to have issues as they age, specifically internal rust that blocks water flow (see attachments) and in some cases, salt crystal build-up precipitated out of sea water at elevated temperatures. The former affects water flow and system cooling that can manifest itself into internal hose collapse downstream (see second analysis). The latter simply restricts the exhaust gas flow. Neither are good.
2. Despite your examination on the bench, it's possible you have internal hose collapse downstream of the mixing elbow that occurs in operation due to temperature and exhaust flow.
3. It's possible you have some degree of both that combine to cause objectionable restriction.
4. The exhaust design has inherent backpressure because of the rise at the end and no waterlift to push the water past it, no batching, just exhaust pushing against built up water inside the hose. It sounds like a system guaranteed to backflood to me.

I'm a full monty guy so as long as you have the system this far apart I recommend replacing the mixing elbow with a conventional hot section, adding a waterlift and replacing the exhaust hose entirely. When done your wallet will be a little lighter but the system will be the best it can be.

[Al Schober]

Ain't a band saw a fun tool!!

[chiron]

The muffler down stream of the water mixer is a water lift muffler.
Also I ran the engine with just the water mix elbow and it ran just fine. Made a hell of a mess of water and exhaust in the engine compartment but worked just fine. To add I rebuilt my A4 a couple years ago and at the time gave the mixing elbow a good inspection, it was surprisingly clean for an old engine.

I just had a thought though. If my anti siphon valve/loop in the water supply has stopped functioning it would be possible the the mixing elbow and water lift muffler could have filled completely with water. I would think this could create a situation where there is too much pressure in the water lift and thus simulating a blocked exhaust. The exhaust would have to push a couple gallons or more of water up 2' before it cleared itself.

[roadnsky]

...If my anti siphon valve/loop in the water supply has stopped functioning it would be possible the the mixing elbow and water lift muffler could have filled completely with water. I would think this could create a situation where there is too much pressure in the water lift and thus simulating a blocked exhaust. The exhaust would have to push a couple gallons or more of water up 2' before it cleared itself.

... Bingo!

[chiron]

Come to find out it was the mixing elbow all along. A closer inspection showed that corrosion had broke the gap between water and exhaust right where the injection point is. About the same as attaching a hose right to the intake manifold.

Found a new one for $185 but think I will just make a new hot section out of black pipe and save some money. Question though is how well does the black pipe hold up downstream where the water is injected? I read here that SS is not the best choice for the immediate hot section right after the exhaust flange. But I was thinking I might do a combination of the two metals.

[roadnsky]

Question though is how well does the black pipe hold up downstream where the water is injected?

Well, YMMV here, depending on your environment...
But on average you can expect 7-10 years of a trouble free installation.
Mine is already 10 years old and doing an inspection back in February it looked almost brand new.
Disclaimer... I'm in a fresh water lake.

[edwardc]

I rebuilt mine with stainless pipe purchased from McMaster-Carr, and its in year 8 of trouble-free operation.

If you use pipe, be sure to use schedule 80. Its thicker walls will last much longer than common schedule 40 pipe.