7.4 litre engine failures

[Shiftchange]

I have a 1988 Sea Ray 390 EC with 7.4 l mercs. I replace my port motor with a re-manufactured one. I broke it in exactly like the manual said. (20 hr oil change, used the merc oil as spec'd, never went under 1500 rpm's or over 3000 rpm's & kept changing speeds etc) 60 hrs later it failed. My boat neighbor went through the same thing. I was wondering if any one else with the 390 EC has had problems. Is there anything that can eliminate this problem from happening again.

The engine remanufacture company told me that somehow water got into #8 piston chamber & caused a hydraulic lock, which bent the connecting rod. My question: Is there a flap or anything in the exhaust manifold that may need to be replaced to keep this from happening again?? Maybe a blockage??? Any ideas or thoughts???

 

[Kevin Smith]

7.4 Engine Failures

You probably already know this already but here goes-manifolds, risers or flapper valves can allow water into the engines. Whenever spending a lot of money on a motor, always consider replacing the manifolds, risers and flapper valves. It can be real cheap insurance and you to know if it was really the engine rebuilders responsibility or not.

Kevin

 

[mrsrobinson]

And replace or rebuild the carbs too

 

[Sundancer]

There were some issues with some boats not having risers that were high enough. Usually this was noticed when dropping them in from the trailers or when coming off plane. The water would rush into the manifold and hit number 8 or number 4 and bend the rod. Usually someone changed the riser and used a cheaper riser instead of the one designed for that boat. It has to do with the engine height in relation to the waterline and at certain times they can ingest some water. The best bet is to crawl inside the engine compartment and drop the hatch to see how much clearance you have. Maybe you can replace the existing riser with one a little taller? The whole point is to prevent water from getting in. ALWAYS check the and make sure the flapper is working!!

 

[Kevin Smith]

That is excellent advice from Sundancer!!!

Kevin Smith, Richmond VA
1991 250/270 Weekender, 454CI, 330HP, Bravo II

 

[rondds]

arent the exhausts on the side of all late 80s searays? wouldnt that make it tough for water to get back up and in?

 

[MarkD]

I think you're right Ron. The boats of that vintage has side exhausts. I havent heard of anything that was prone to water getting back to the engine via that huge muffler they have. Newer ones had that issue.

Like someone said b4, I would also suspect the riser gaskets or risers themselves were the cause. Was the fliud clear or did they have some radiator fluid inside. If so, the cause isnt a bad exhaust system.

 

[Alex D]

Those boats also had log style exhaust system with very little or no ability to hold back water. Not until teh lift style mufflers that positively seal the manifolds from water flushback were introduced were you safe from this issue. Who of course would trailer a 40 ft. EC??

Anotehr issue with the "open" exhaust style of the 80's and early 90's was that sea salt would actually migrate via mist through the open exhaust system into the combustion chambers when teh vesel was moored or docked and cause increased corrosion on valves and otehr engine internals. The lift muffle positively blocks anything from entering the manifolds and combustion chambers.

 

[rondds]

that can occur with any type of exhaust. my dad recently told me the story of how the onan genny on our 1970 chris craft commander got cooked. we had moved the boat from a mooring to a slip (with shore power). as a result, the genny went largely unused for about 2 months. when he eventually went to start it up, it bent the s*&% out of everything.

moisture can work its way in at anytime and cause a valve to rust in place. i always stuff rags wrapped in plastic bags into my exhausts while the boat is sleeping during the winter. some even advocate cranking the engine every now and then during the winter layup to change the position of the valves and pistons. of course, fogging is supposed to prevent all this from happening anyway, RIGHT??

 

[Alex D]

rondds, do you fog your engines??

 

[rondds]

fall 2004, i didnt. i used starbright stabilizer that made a vague claim that it lubricated internal components, etc. fall 2005 the marina did it. this fall i will do it with the combo of 2 stroke and marvel mystery oil into the carb. my previous two boats were stored indoors (out of the wind and moisture), and one of them was FI. those boats i did NOT fog.

 

[Alex D]

I don't fog either and never had a problem, but I only lay up for a max. of three months and try to run her once a month too, get her nice and warm and run on plane for about an hour.

 

[tony lotz]

Water backing up through muffler

I have a 2002 Sundancer 3100 with twin 7.4l mercs FWC running through V drives.
Port engine was cranked but would not start due to a problem with the fuel pump. Unfortunately Mercruiser don't warn that water can come back into the manifolds when engine is cranked but does not fire.
Obviously water lift muffler heights are critical but I am not happy having to re build an engine.
Anyone else suffered this ??