3 Symptoms to Detect Mercruiser Exhaust Flapper Failure

A boat with sterndrive motors like Mercruiser needs exhaust flappers to function properly. But it is normal for the exhaust flapper to degrade over time, possibly harming the engine. So to prevent the engine from being damaged, you need to detect the exhaust flapper failure symptoms and replace it. 

Unusual noises are one of the Mercruiser exhaust flapper’s main signs of failure. In addition, water in the exhaust manifold is another typical sign of a faulty exhaust flapper. 

There is much more information that we are going to discuss in this write-up that you need to know. Stay with us till the end to get to the deep down of the matter. 

What are the Common Mercruiser Exhaust Flapper Failure Symptoms?

Any engine has flappers at the top of the exhaust that attach to the exhaust hoses, which then clamp to the exhaust manifold. 

The exhaust flapper acts as a one-way valve and helps to out-flow the exhaust of the engine. 

At times when the exhaust flapper goes bad, it causes a hydraulic lock as well as some damage to the engine. 

So, it is better to detect the bad Mercruiser exhaust flapper before it is too late. Here are common symptoms of the bad Mercruiser exhaust flapper that indicates its failure.

Symptom 1: Peculiar Sounds From the Inboard Engine

The most common symptom that indicates a faulty Mercruiser exhaust flapper is the sounds coming from the manifold. 

When the flappers get old, they start making “Clackity” noises. Most of the time, the sound issue is diagnosed as a bad drive, bad lifter, or for gimbal bearing. 

But the bad flappers can also be the reason behind this. Besides, when the flappers go bad, they can break loose eventually and get stuck in the drive exhaust passage or pipes.  

Symptom 2: Water in the Exhaust Manifold

One of the main functions of the Mercruiser exhaust flapper is to prevent the water from rushing to the exhaust. Water starts getting into the oil when the flapper is bad or out of their function. 

So, if you notice water in the exhaust manifold, there is a possibility that the exhaust flapper has failed & needs replacement. 

The issue can also happen for the bad manifold itself, the bad head gasket, or other related problems.

Symptom 3: Engine is Running Hot

When the engine of your Mercruiser gets hot, the owners often need clarification on whether the bad exhaust flapper can cause the issue even when the temp gauges show normal temperature. 

Over time, when the exhaust flapper goes bad or broken & lodged itself somewhere in the exhaust. 

The water can’t flow freely to that particular side. This obstruction easily results in overheating of the manifold. 

On the other hand, the temp sensor of your Mercruiser is located in front of the engine, in the thermostat housing. 

That is why the manifold only gets overheated but not any changes in the temp gauges.

4 Possible Causes of Mercruiser Exhaust Flapper Failure:

As you know,  the exhaust flapper prevents water from entering the motor under certain circumstances. 

When the waves come from the back of the boat, the water pushes against the exhaust flapper & the flapper prevents it. 

The flapper’s importance depends on the engine’s location in relation to the boat’s water lines. 

The engine is at the bottom of the hull & the pressure of water can directly go forward to the engine if the exhaust flapper goes bad. 

Here are some common reasons why Mercruiser exhaust flapper can go bad:

  1. Overheat condition of the engine 
  2. Prolonged exposure to the hot exhaust 
  3. Extended use
  4. Loss of row water flow or low water flow 

Replacement Process and Repair Costs of Mercruiser Bad Exhaust Flapper

Once you detect the exhaust flapper of your Mercruiser to be bad, the only way to fix the issue is to replace it. 

Your engine would have 3-piece exhaust Y-pipes if you owned an older engine. 

In newer engines, the Y-pipes have shutters or flappers on the top of the pipe. In this case, you just need to detach the exhaust elbow from the manifold. 

In case you have thru-transom exhaust, your boat has both external & internal exhaust flappers. Now you have to take replacement steps according to the engine of the boat. 

Replacement Process:

The replacement process is pretty straightforward. With concentration, you can easily fix the problem. To do so- 

  1. Loosen the stainless clamps from both the upper & lower boots.
  2. Remove the stainless clamps. You may also need to use a screwdriver to pry the boot. 
  3. Now try to rotate the aluminum tube or the intermediate elbow. To avoid hazards, wear mechanic gloves. 
  4. Push the upper boot as far as you can, as well as the lower boot. Now remove the tube. 
  5. Pull out the flapper & replace it with a new one. 

Replacement Cost:

However, replacing the exhaust flapper Mercruiser can cost you $30- $48. Hre is the exhaust flapper replacement costs with its fitting models for your rough idea. 

In case you don’t understand which flapper will fit your particular engine model, you should contact the manufacturer of your boat. 

Exhaust FlapperCostFits For Models
60930 Flapper $32 ● Mercury & GM engine [1982 & up]
● Mostly V6 and V8 GM engines [1985 and up] 
99370A2 Flapper $30 ● 185 V6 262 cid [1985-87]
● 185 V6 229 cid [1983-84]
● 200 305 cid V8 [1985-86] and others.

Frequently Asked Questions

We tried our utmost to cover all the information about the symptoms to detect the Mercruiser exhaust flapper failure. Still, some queries are often encountered in the owner’s mind. That is why, we are going to answer some of the most searched queries here.

Where is the exhaust system located?

The exhaust system of your boat is located below the water lines. The exhaust can be dry & wet. It does not require a main muffler but requires a bypass muffler. Through the stalk, the exhaust exits the boat.

What are the 3 parts of the exhaust system of a boat?

Although there are a handful of components in the exhaust system of the boat, there are three major components. They are the exhaust manifold, exhaust muffler & exhaust catalytic converter. This is the standard factory exhaust system by the manufacturer.

What is through the hull exhaust?

The thru-hull exhaust system is the most cost-effective exhaust option. It is the system that allows the exhaust to exit full-time through the side hull or the transom. While doing so, it does not have any control over the sounds or exhaust flow.