How to relieve exhaust back pressure

[jholcomb]

I have a late model A4 that has been running great until a couple of weeks ago. The motor will start and run fine if I shut off the raw water inlet valve. Once I open the valve the motor will quickly die. There is also a lot of water coming out the exhaust when the valve is open. While trouble shooting I determined that there is water in the first cylinder after the valve has been opened.

I'm currently in the process of removing the head and replacing the gaskets at which time I'll have a better idea of what's going on with the motor. From the looks of it I don't think this engine has ever been overhauled. Having read many of the threads I also suspect, but haven't confirmed yet that it could be elevated exhaust back pressure. What steps need to be taken to lower the pressure? I've searched the forum and have not been able to locate a specific answer regarding this. I realize I'm going in two different direction here to resolve the issue, but would still like to know how to reduce the pressure.

[s/v Dearbhail]

Don has mentioned this from time to time. they have seen issues where the exhaust is seriously clogged up. I replaced the water exhaust between the manifold and muffler and found a lot of thick chunky stuff in mine. I next pulled the muffler and found the same thing in there.

[rigspelt]

If the design of the exhaust is such that back pressure is not a problem when the pipes and hoses are new (number of bends, inside diameter of pipes and hoses, height of lifts in the lines, and length of exhaust system from manifold to discharge), then increased back pressure would next most likely be due to clogging downstream of the exhaust ports, as I understand it. Such clogging is not uncommon in older systems.

[Don Moyer]

Based on reports over the past couple years, the most common place for
exhaust back pressure to build up is where the engine cooling water enters
the hot section (two examples attached). Minerals apparently precipitate
out as the engine cooling water encounters the elevated temperature within
the hot section and builds up along the ID of the pipe. A second place
where elevated exhaust pressure can occur is within the first several feet
of rubber hose just downstream from the water lift muffler. The inner
layer of rubber breaks away from the rest of the reinforced hose until it
crumples up on itself and blocks the flow of exhaust and cooling water. In
these cases, engine temperature also elevates, but not necessarily in less
dramatic cases of hose breakdown.

Don

[jholcomb]

Don,

Thank you for the photos. The pipe from the manifold to the muffler looks fairly new so I don't think there is corrosion in that area, but I will check it anyway. From the muffler and beyond it doesn't look like it's ever been touched. I will not have a chance to work on the boat until this weekend. I will let you know what I find. Thanks to all for pointing me in the right direction.

[sastanley]

Even though it looks fine on the outside, it could be blocked on the inside, or corroding from the inside out..Hopefully you have a few sections of 'soft' hose in the system there so you can disassemble and inspect it. We seemed to need to take apart this area and clean it out every 3 or 4 years on my step-father's Tartan 3000 (M-15 diesel), as it made a dramatic difference in engine performance.

It is one of the first things I checked on my A-4 this fall when I took ownership, since it was an unknown.

Good Luck!

[rgoff]

In my 25 years of owning my Ericson 27 with an A4, I've replaced the exhaust system several times. The exhaust pipe from the engine goes to a vertical riser (made of galvanized pipe). At the "L joint" where the exhaust pipe enters the riser, the carbon and rust from the inside of the riser fall down and collect over time. Eventually, this point gets packed with stuff and the engine will run only at idle speed.

[sastanley]

Ralph,
thanks for that post. Mine currently doesn't really have a riser, and I am considering replacing it with a better designed system (which is being discussed in some other threads here) to possibly reduce the chance of water ingress thru the exhaust (it would have to go up and over, instead of basically straight across as it does now) - I didn't think about scale falling into the pipe just outside of the manifold and clogging it..it might be nice to have a threaded plug (or maybe a "T" with on end plugged as an access port) to open up and clean that out without disassembling the thing.

[Don Moyer]

Rgoff,

Thank you for this important post. I'm intrigued by the fact that you've owned your boat long enough to have settled into a pattern of replacing the hot section of your exhaust system (approximately every 10 years), watching another blockage develop, and then changing the hot section again to restore normal engine operation.

Considering that many Atomic 4 powered boats are running around with their original exhaust systems, you can understand why I say that half of the Atomic 4 fleet is probably suffering the effects of some level of elevated exhaust back pressure, sooty plugs, sticky valves, low power, etc.

Could you share with us more details of your symptoms as you transition from normal operation to the engine running only at idle?

Don

[rgoff]

In 1983 I bought my boat used from the original owner of 10 years. I know the first time it happened, it was very baffling. Would start and idle fine, but when I reved the engine in neutral it would sputter and die. Had a friend come over and we went through all "on the engine" possibilities (carb, points, etc.). No dice.

I finally called the Ericson plant (was still in business then) and they said it sounded like a clogged exhaust system. I disconnected the exhaust pipe from the engine flange and it reved up fine (but made one heck of a racket).

When it occurred again a few years later (maybe 6 or so), I knew what it was.

Over the years my engine has been used mostly to get out the harbor and back for a day sail. Limited trips at cruising speed for long periods. Also I would usually start the engine and run it at idle for 10-15 minutes when I visited the boat, but didn't go out. I suspect all this "low RPM" activity might have contributed to the problem.

I don't remember much advanced notice of the problem. Seems like it just "got bad" quickly.

Fortunately, now my A4 is running great. I was away from my boat for 4 months while RVing. When I got back I ran the electric fuel pump a bit to prime the system then hit the starter. It started in about 2 seconds!