Most Marine engines use a water cooled exhaust manifold. Each manifold will use a riser or elbow to connect to the exhaust muffler or y-pipe. Very few people are aware of the life expectancy and symptoms of manifold and rider failure.
Marine exhaust manifolds and risers are normally made of cast iron. Depending on the marine engine, there can be one or two sets of manifolds and risers. The manifold and risers are usually painted or coated to help prevent rust and are located on the side of the engine. V-8 engines have two sets of manifolds and risers which are located alongside each cylinder bank. Smaller 3 and 4 cylinder marine engines normally have one manifold and riser locate near the top and along one-side. Risers, sometime called elbows, which look like an inverted horseshoe, are sometimes located at the aft or on top near the middle of the manifold. The riser is then connected to the exhaust hose.
In order to combat the heat of engine exhaust, manifolds and risers have two compartments, one inside the other. The inner most compartment is for the engine exhaust, which is surrounded by a water-filled compartment, called the water-jacket. The water-jacket prevents the manifold and risers from over heating and keeps them cool to the touch. The two compartments combine prior to the exhaust reaching the exhaust hose to ensure that the engine exhaust is cool enough to not burn through the exhaust hose.
Although it is important to keep the engine exhaust cool it is even more important to keep the water and exhaust gas separate until the exit the riser. If the water-jacket starts to leak and water gets into the exhaust compartment when the engine is not running, the water can end up in the cylinders and seize the pistons with rust or create hydro-lock. Hydro-lock occurs when pistons try to compress the water in the cylinder. Since water cannot be compressed, the engine can suffer extreme damage. This usually results in bent engine rods or extreme damage requiring an engine overhaul.
The main cause of manifold and riser failure is time. A common held belief in the marine repair industry is that most manifolds have a useful life of 8 to 10 years. However this is highly dependent on engine use. Manifolds and risers that see heavy use in saltwater can have life expectancy as low as 3 years, while manifolds and riser in engines rarely run and used in freshwater can last 15 to 20 years. No matter how you use your engine, the older manifolds and riser get, the chance of failure increases.
The environment that manifolds and risers live in helps to limit the useful life of manifolds. Internally, manifolds and riser are continually slammed with high velocity hot exhaust and water. Externally, manifolds and riser live in an environment that has perfect conditions for corrosion. Splash water, heated by the engine sporadically ends up on the manifolds and the humidity in the engine compartment, all take part in corrosion. The fact that manifolds and risers violently vibrate during use and then sit for long periods of time allows water and humidity to eat away at the manifolds and risers.
Failure Symptoms: One of the most common symptoms that your manifolds have failed is difficulty in starting or your engine will not start at all. One of two things may have happened; You have a hydro-lock situation in your cylinders (water will not compress). · Or one or more of your pistons have rusted inside your cylinder(s). These are definite signs that your manifolds have leaked and water has entered into your cylinders. If your engine does start but produces white smoke or runs roughly, you may have water in a cylinder, which can foul the spark plugs. Another sign that your manifolds may be failing is your engine overheating at high R.P.M.s. This usually means that you have a blocked water-jacket that is not cooling the engine exhaust and thus causing the engine to overheat at high R.PM.s. Sometimes the blockage is not reflected by the temp gauge and will simply cause one cylinder to run hotter than the others. When you have a subsequent engine failure, the culprit may be difficult to identify.
Manifold Inspection - Probably the easiest way to check if your manifolds are working correctly is to look at your exhaust and see if your water output is lower than normal, if it is then you may have a blocked water-jacket. A blocked water-jacket can lead to a water leak in the manifolds. If you have two manifolds, you can compare the temperatures from each manifold. You can use your hand, but it is better to use an infrared pyrometer. If one of the manifolds is hotter than the other, you probably have a clogged water-jacket. When inspecting your manifolds it is a must that you visually inspect the manifold riser joint for corrosion. Corrosion at the manifold-riser joint means that the manifold-riser gasket has failed and water is at the very least seeping out. If the gasket has failed there is a high chance that water may also be seeping along the inside of the joint and possibly into the exhaust chamber. When visually inspecting the risers you should also check the hose-riser connection. On some boats the exhaust hose is connected to the riser with a stainless steel hose clamp that does not have a stainless steel screw. Thus the screw can rust, causing the clamp to fail and you can end up with your exhaust emptying into your engine room. Note: Clamps with stainless screws usually have "All Stainless" stamped on the screw housing of the clamp. These inspections can help flag a problem with your manifolds. You should periodically remove the risers and inspect them, along with the manifolds. How often you check your risers depends on where you live and how you use your boat. A good rule of thumb is to check them every year before the start of the boating season.
When you remove the riser you need to look for corroded metal, done best with a large flat screwdriver scraping the surfaces of the water jacket. If it flakes off it is time to replace. You should also look for signs that water is entering the exhaust chamber, signified by rust in the chamber. Look for pitting and or rust through out the riser and on the coupling faces, any of which signifies a leak that can lead to hydro-lock.
It is common for your marine mechanic to suggest cleaning the manifolds and pressure testing at inspection. This process of scouring loose rusted metal from the manifolds can cause a number of new problems. (a) the process may actually move particles within the manifold water jacket and cause a blockage which will not be evident until you run your engine again, (b) after cleaning, the thickness of the water jacket is reduced and fails soon after manifolds and risers are re-installed, or (c) the reduced thickness of the water jacket passes the pressure testing with cold water but fails in use due to high temperature and porosity in the cast iron being exposed due to the cleaning process.
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