We suspect that most everyone within our Atomic 4 fraternity who has a waterlift muffler exhaust system, is aware that if the raw water through-hull valve is left open, engine cooling water will flood back into the exhaust manifold if the starter is cranked too long with the engine not starting, or if the antisiphon valve sticks closed for a prolonged period of time.
Other than creating a lot of messy cleanup work, those scenarios don’t usually create any long-term damage to the engine or the exhaust system itself; nor do they create much debate as to the origin of the water. The sheer volume of water in those flooding episodes is convincing to most people that the water has most likely come from the exhaust system.
The purpose of this newsletter is to alert you to the serious damage that can result when very small amounts of engine cooling water sequentially back up into the hot section and the manifold over time, exhibiting only subtle (if any) symptoms to call attention to the accumulative damage being inflicted on the exhaust system and/or to the engine. If we had to pick a name for this condition; “Exhaust Water Reflux” comes to mind.
Causes of Exhaust Water Reflux:
Design considerations: In a typical waterlift system, approximately one quart of water settles back down into the muffler every time the engine is shut down. While the boat lies peacefully in its slip, this amount of residual water remains safely trapped within the muffler. However, if the design of the exhaust system does not have sufficient protection against water spilling back out of the muffler when sailing in moderate to heavy sea conditions, the residual water trapped within the muffler can spill out into the normally dry hot section and (in worse cases) travel all the way through the hot section to the outlet flange of the manifold.
Notice that in the following photo, the original factory water lift muffler has been replaced with an aftermarket muffler that is somewhat smaller than the original, and (more importantly) the water inlet is on the side rather than on top of the muffler like the original. In this installation, water will likely spill back out of the muffler (much like the contents flow out of a pitcher of marguerites) and head toward the manifold anytime the boat heels even moderately aggressively in the direction of the side inlet of the muffler.
Operational considerations: While sailing in moderate to heavy sea conditions, even in the case of well-designed exhaust systems, the raw water through-hull regularly plunges well below the theoretical waterline relative to its position while the boat is sitting level in the slip. The dimensional relationships of the exhaust system plumbing inside the boat relative to the theoretical water line also change during the heeling and pitching associated with heavy weather sailing; meaning that features which were above the theoretical water line in the slip end up at (or below) the theoretical water line in heavy sea conditions. Both of these factors combine to create momentary pressure gradients favorable to sea water moving in through an open through-hull.
How small amounts of water can create so much damage: There’s a subtle but very important feature in the contour of the exhaust path between the block and the manifold which forms a sump that traps even a small amount of water until it reaches the level of the top of the exhaust valve guide in the 4th cylinder. Notice in the following photograph, that when antifreeze is poured into the back of the manifold, it only took approximately 1/2 cup of antifreeze to fill that sump and bring the liquid level up to the top of the valve port in the block.
For a better look at the actual elevations within the exhaust ports, we cut through a scrap engine in the very center of the rearmost exhaust valve. As can be seen from the horizontal dotted yellow line in the following photograph, the level of the antifreeze, as it passed through the exhaust port in the previous photo, lines up almost perfectly with the top of the valve guide. The yellow circle indicates the opening in the center of the exhaust flange, which (together with the contour of the manifold) establishes the upper level of the pool of exhaust water in the aforementioned sump.
Once it reaches the level of the top of the valve guide, the exhaust water is in position to work its way down between the valve port and valve stem to reach the spring and other attaching hardware of the fourth exhaust valve to leave the evidence as in the following photo. This engine had only accumulated 24 operating hours since a major overhaul, and again, it would have only taken 1/2 cup of exhaust water to reach the top of the valve guide in the 4th cylinder, from where if flowed down between the valve stem and valve guide to leave the rust on the spring and connecting hardware.
In the following photo of the same engine, faint but unmistakable rusty discolorations appear on the bottom of the second and third exhaust valve springs and valve stems. Though subtle, this rusty discoloration is material evidence left behind by the hot moist air that moved down from the exhaust manifold during the short period of time in which the small pool of exhaust water just outside of the fourth exhaust valve was being burned off. It’s this dynamic that leads to sticky valves and hard starting; not only in the case of the exhaust valve in the fourth cylinder, but in the third cylinder as well. NOTE: Contrary to what you might expect from the rusty condition of the fourth valve assembly in the previous photo, the third exhaust valve is the first one that stuck open in this engine.
Damage from smaller amounts of water that never make it out of the hot section and into the manifold: In our most recent report of damage from Engine Water Reflux, after approximately 2 years of trouble-free engine operation, the caller’s initial report was, “the hot section blew out and spread engine cooling water all over the engine compartment”. Initial inspection revealed a one-inch hole in the hot section just behind the exhaust manifold flange.
When poking around with a screwdriver, it became obvious that most of the entire metal piping of the hot section had corroded away, leaving little more than the insulating wrap to retain the original shape of the hot section. The exhaust flange had been largely consumed by corrosion, but as of this writing it appears that the manifold itself will be reusable which suggests that the exhaust water never made it out of the hot section and into the manifold.
Recommendations to minimize the dangers of Exhaust Water Reflux:
1: Develop the habit of keeping the raw water through-hull valve closed at all times when the engine is not running. While it’s difficult to quantify the actual amount of sea water that will flow in through an open through-hull valve during heavy sea conditions, any amount of additional sea water entering into the exhaust system will clearly exacerbate the likelihood of exhaust water backing up into the engine. We estimate that as many as 90% of the cases of Exhaust Water Reflux could be prevented each year if we would all get in the habit of keeping our raw water through-hull valves closed any time that the engine is not running.
NOTE: If you experience any of the following symptoms, and until you discover what may be wrong with your exhaust system, it would be prudent to run the engine speed up to 1500 RPM for a second or two, after closing your through-hull valve, and before shutting your engine off (similar to the time it takes to bring your alternator on line) to purge more of the residual water from the muffler. This precaution will purge more of the residual water from the muffler.
2: Pay attention to the following symptoms of Exhaust Water Reflux:
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- Short periods of steam coming out with the exhaust during the first few minutes after starting the engine.
- Hard starting after sailing, particularly after sailing in heavy sea conditions with the raw water through-hull open.
- Any recognizable rusty discoloration on the spark plugs from cylinders 3 and 4 is an indication of small amounts of exhaust water getting into the aft end of the manifold.
- Any time you have access to the inside of the valve chamber, check for rust appearing on the valve spring and connecting hardware of the fourth exhaust valve. Any rust on the 4th exhaust valve is the clearest material evidence of Exhaust Water Reflux. In this context, the fourth exhaust valve plays the valuable role of a canary in a coal mine – you never want to see this particular canary having a bad day.
3: If any of the above symptoms appear, reexamine the design of your exhaust system to identify any feature(s) that have the potential for engine cooling water flowing back out of the waterlift muffler in the direction of the engine. Be sure to make your projections based on the orientation of the muffler and hot section during the heaviest sea conditions that you are likely to encounter. Most analysis will require only an understanding that water flows downhill, and it also always flows from areas of high pressure to areas of low pressure.
We hope these suggestions will assist you in preventing damage to your hot section or engine from Exhaust Water Reflux.
Once it reaches the level of the top of the valve guide, the exhaust water is in position to work its way down between the valve port and valve stem to reach the spring and other attaching hardware of the fourth exhaust valve to leave the evidence as in the following photo. This engine had only accumulated 24 operating hours since a major overhaul, and again, it would have only taken 1/2 cup of exhaust water to reach the top of the valve guide in the 4th cylinder, from where if flowed down between the valve stem and valve guide to leave the rust on the spring and connecting hardware.
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