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O2 fails, water in oil, bent valve

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Mwc7805

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Have a 2018 Sea Ray SPX 210 stern drive with a Mercruiser 5.7L 200 hp. Bought used in 2021 in Florida. Salt use vast majority of 250 hours. O2 sensor alarms after slowing down for wake zones in icw areas only, never lakes.

Local dealer replaced o2 pre and then after more alarms post. Second round of o2 alarm notices, independent service diagnosis guy said major internal engine issues. No rough idle or running, no white smoke. Safe mode alarmed in but typically could run under 3000 rom to get back to dock.

Dealer then said Mercury would offer 4” riser parts for exhaust manifolds since no risers oroginally installed this model year. Sea Ray picked up labor after pushing dealer. Oil replaced because dealer said filter was wrong and oil pressure ran little low. O2’s not replaced after riser installs. Ten year old blog on exact mercruiser lack of riser issues exist but manufacturer evades design fault.

Next time out just weeks later, engine started slow but did start and run but then wouldn’t start. Also stern drive unit trim sticking.

Charged battery and ran it on earmuffs in driveway 15 minutes. Alarms returned. Trim alarm and o2 again.

Dealer fixed trim and said o2 not related to old riser. Other o2’s not corroded after asked them to check. After dealer lake test, boat alarmed again with oil warnings. Now water in oil, zero compression in one cylinder, another cylinder low compression, camera scope shows bent valve.

Could this be hydro lock pre riser install? Can water get through that catalyst?

Should I try just checking or rebuilding heads or bite bullet and replace engine with exhaust manifolds etc…?

Mercruiser or aftermarket heads and engine?

Appreciate it.
 
You mention 200 hp but i think the 200 hp engine is the 4.5 L that is mercs engine.

several options depending on how much you want to spend. Most expensive is a bobtail engine from merc, but it will come with a warranty suppoerted by any merc dealer.

lesser option especially if you can do some wrench turning look at a place like michigan motorz get a new base engine. They also sell remans But a new base engine is the same gm engine merc and VP use to make their power plants. I got an extended base engine from Mich Motorz in 2015 has worked flawlessly since then.

5.7 https://www.michiganmotorz.com/5-7l-350ci-vortec-base-marine-engine-1996-current-replacement

i wonder if you have salt water eating through the manifolds Or heads.
 
Ok, yes, 4.5L. Imagine same referrals for engines. Only 250 hours but 6 years in salt, likely backwash up exhaust since no risers plus normal corrosion. Appreciate advice.
 
The main thing with a problem like this is to figure out WHY it happened to avoid a recurrence. Can you just replace the cyl heads & exhaust instead of the whole long block? Well it depends. I had water in 2 cyls (not hydro-locked though) & blown head gaskets. I was able to install 2 reman cyl heads & new exhaust & the engine has run fine since then (2017). Water in the cyl & blown HGs was from a past overheat.
If you had an actual hydro-lock the risk is that you bent a connecting rod. Sometimes you can tell because the engine makes abnormal noises when cranking it over but the only way to tell for sure is to remove the heads & crank it manually so each piston is at TDC & then measure how far below the block deck each piston is. They should all be the same if one comes up shorter than the others that rod is likely bent.
I don’t think you can legally replace this engine with the similar GM 4.3 due to emission laws but I guess that is both a state & federal issue.
If you compare parts diagrams you can see that Merc seems to have just updated much of the old GM 90* V6 design, some of the parts look nearly identical. But since they are Merc parts the price doubles!
I for one would NOT buy any inboard with cat converter exhaust in a salt water area. You have to replace exhaust every 5-7 years here and those systems are 5000-6000 just parts cost. No thanks.
I’ll stick with older boats!
 
Thanks for responses, problem is I bought boat with 80% of its current hours already on it (had 200 hours on it and I put 50 hours on it) so it could have overheated or hydro locked with old owner. I personally have never heard any bad noises at start up, no white smoke, etc..since I’ve been running it.

If no cheaper engine replacement options for 4.5L then I have to flush water out of oil, get heads pulled and check pistons.
 
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