IMPORTANT: A check for water intrusion should be performed on all new Marine Engines with thru transom and  Thru Hull Exhaust Systems. Two methods can be used to check for water in the Marine Exhaust Manifolds. The simplest and least time‑consuming way is the exhaust elbow removal method. With this method, however, it is difficult to accurately determine the amount of water in the manifolds. The exhaust manifold tap method provides a more accurate means to measure the water, but requires that a hole be drilled in each manifold. The manifolds must be replaced after performing this check, so the procedure is best performed by a Marine Parts mechanic or specialist.

IMPORTANT: If there is no water in one marine exhaust manifold, do not assume that there is not water in the other manifold. Check each manifold.

Water Intrusion can commonly be categorized into 3 separate and distinct categories; Reversion, Ingestion, and condensation.

1. Reversion – This is commonly caused by the Marine Engines Exhaust tuning characteristics such as camshaft dynamics and valve timing. Mercruiser utilized the Exhaust Resonators to address this specific issue. This problem is mainly experienced on the higher Hp engines with aggressive camshafts such as the 454 Mag and 502 Mag engines. This problem seems to be magnified with thru-hull exhaust  systems as the exhaust backpressure is reduced. This effect can be thought of such that when the engine is breathing better due to the reduced exhaust back-pressure its also able to pull water back through the exhaust system more easily as well.
2. Ingestion – This issue is most commonly caused by incorrect Exhaust Riser Height. In this case your marine engine may need to have exhaust spacers installed. Other common causes for water ingestion are poor exhaust manifold/riser joint design. The stainless steel exhaust riser used by Mercruiser  in the early 1990’s was a good example of a poorly designed gasket/joint. The differential expansion rates between cast iron and stainless steel along with the short mounting fasteners allowed this to be a source of many problems. Mercruiser ultimately designed the Dry Joint Exhaust Systems in 2001 to begin addressing the core issues with water ingestion induced by poor gasket/joint design. Another common casue for ingestion is not utilizing an external exhaust flapper which can allow water to come back through the boat exhaust system exit. Another related item is a failed or missing internal exhaust shutter which serve the same purpose.
3. Condensation – This is probably the most misunderstood and commonly mis-diagnosed issue within this overall water ingestion/intrusion umbrella. In a general sense condensation in and of itself does not seem to be a failure inducing problem on its own, however when taken in context with the overall environment and operating conditions this issue rises to the top as a problem. Warm manifold versus cold manifold systems seem to be the best overall approach to the exhaust system. The primary function is that they route only engine heated and thermostat controlled water through the exhaust manifolds, while then routing the excess cooling water not needed by the engine overboard through the exhaust risers. This then allows the manifolds to run at 160 F in most cases which exceeds the 120 F thresh hold where condensation becomes an issue. While the larger surface area of the exhaust manifolds is now heated while the surface area of the riser is cold but a much smaller surface area in comparison. The other notable benefit with warm manifold cooling systems is that the additional heat in the manifolds not only prevents condensation but also burns out any moisture that may be present due to gasket leakage, reversion, etc.  Mercruiser also utilized the exhaust turbulator on the 496 mag marine engines as a preventative measure for controlling condensation when a warm manifold cooling system was not achievable. Prior to the time that the Dry Joint systems were introduced on the V6 and Small V8 Marine Engines, Mercruiser did develop a retrofit kit which offered a Turbulator Plate to be used with the existing Wet Joint style exhaust systems. If you’ve had any related issues with Water Ingestion or Condensation we would recommend using these Turbulator’s to help deal with the issue…..Turbulator.

Preparation: Consider hose lengths and angle, exhaust elbow height to the water line, water lift muffler exhaust outlet angles, water lift muffler water height, idle relief angles, exhaust collector angles, air temperature, water temperature, and humidity when performing these tests.

Exhaust System Measurement And Testing

EXTENDED IDLE TEST

Checking by Exhaust Elbow Removal Method

NOTE: This test is easier on seawater cooled engines but results are applicable to closed cooled engines also.

1. After the 15‑minute idle preparation, drain the exhaust manifolds of water. If the engine is equipped with a closed cooling system, pinch or plug the hose from the heat exchanger to the exhaust elbow to prevent coolant from spilling.

2. Remove the exhaust elbows.

3. Check for water in the exhaust manifold. Use a flashlight if needed to aid inspection. If water is found, measure and record the amount. Refer to

Interpreting Results. NOTE: A suction device with a rubber hose attached to the end can be used to suck the water from the manifold and discharge it into a container.

4 Replace the elbows using new gaskets. Refer to the appropriate Mercury MerCruiser Service Manual.

Checking by Drilling and Tapping Exhaust Manifolds

1. Drill and tap a 1/8 in. NPT hole into the bottom and center of the exhaust manifold runners for cylinders numbers 4 and 5. Be sure to drill in the area that is not water‑jacketed.

 2. Insert Brass Plugs

3. Run the engine as outlined in the extended idle test.

4. Drain water from the manifolds into a suitable container. Measure and record the amount of water using a graduated cylinder. An acceptable amount of condensation water is 5 ml (0.2 fl. oz.).

5. Replace the manifolds with new manifolds.

Interpreting Results

Water vapor is a normal by‑product of the combustion process, so a small amount of water (5 ml. [0.2 fl oz.]) is acceptable. This condensation has not shown to cause a problem. Water amounts in excess of this could be an indication of a water intrusion problem that requires corrective action. Some of the more common causes for this condition are:

• 1. Operate the engine until it is at normal operating temperature (73°C [160 °F]).
• 2. Place the remote control in neutral. Increase the engine speed to 3000 RPM for 1 minute.
• 3. Bring the engine back to idle slowly after 1 minute.
• 4. Shut off the engine
• 5. Restart the engine and operate at idle speed for 15 minutes.
• 6. Shut off the engine
• 7. Check for water IMPORTANT: If this test is repeated, ensure that the engine is at operating temperature and that the 3000 RPM step is repeated to clear the manifolds of water.
  • Insufficient exhaust riser height. Refer to Measuring Exhaust Elbow Height.
  • Improper exhaust hose slope.
  • Failure to have at least 305 mm (12 in.) between the exhaust outlet and muffler, collector, or first angular fitting. Refer to Exhaust System Design.
  • Missing or mispositioned exhaust resonators. Refer to Exhaust Resonators.
  • Improperly sized or incorrectly installed exhaust system components.
  • Leaking exhaust elbows or risers (i.e. loose screws, damaged gasket).
  • Improperly designed collector or Y‑pipe. Refer to Exhaust System design.
  • Exhaust system causing a tuning effect. Refer to Exhaust Resonators.
  • Improperly functioning or missing water shutters or flappers.
  • Muffler not draining.
  • Engine damage.
  • Poor engine running condition.

Buy the products and parts you need...