Exhaust Flappers

[sr79]

Re: 1999 305 Mercruiser fresh water cooled
I'm sure this subject has been discussed more than most people care to remember but, it's still a question for me. When I removed the outdrive for normal service I noticed what was left of an exhaust flapper lodged next to the shifting linkage. I removed both risers for inspection and noticed both starboard and port side flappers were missing. My questions: 1). I bought new flappers, should I install them or leave them out? 2). Do exhaust flappers cause back pressure or other issues?

Thank you,
Dennis

 

[dubs283]

install them or leave them out, its your boat...

the purpose of flappers is to prevent water from entering the engine through the exhaust manifolds/valves. in the case of coming off plane, the wake wash that hits the transom can force water through the exhaust and into the engine, flappers prevent the water from entering. also high seas where waves come against the transom may have the same effect

it is best to have them in place to prevent a hydrolocked engine, i cannot see how they create back pressure since they remain open when the engine is running

 

[santino44]

Of course put! They will protect your cylinders from water ingress.

 

[Lou C]

agreed if Merc could have saved money by eliminating them they would have, so if they were there, they are needed.

 

[ronaldreagan]

Yes, install them. It's not fun to rebuild an engine over missing $40 worth of parts.....

 

[Scott Danforth]

you only need to install them if you want to prevent hydrolocking from the following:

• coming off plane
• boating in rough seas
• backing the boat in the water.
• improper timing causing the engine to diesel or run-on

as others indicated, if Mercruiser could have gotten away with not using flappers, they would have decades ago.

the most likely cause of failed flappers is an over-heat or running without water.