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Merc Cruiser V6 Valve Problems

S * K

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Well, I finally ripped apart my motor from #1 spark plug blowing up. Piston looks fine, took the head into mechenic and he spotted the valve immediate said was low. He went thru to check everything out and the value very very badley indented. I am not a mechanic so I don't know the terms. He gave me 2 options poor ignition or bad fuel. He said the ethanol could be a factor and said something about cold fuel not burning hot enought? I am redoing all the values / lifters you name it on that side. Has any one had this problem before and I plan on pumping out the 1/4 fuel i have. I put the water seperator on this year and its seems I have never had so many problems this year. Its my 2nd year with the boat. I have been told to only run 89 octane instead of 87 by somebody else. Any input is greatly appreciated.

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Burned valve seats (on the exhaust side) are a sign of very high combustion temps. Check to make sure that your engine is not running to lean or your timing is advanced to far. If you can install a wide band O2 sensor in the exhaust stream to monitor air/fuel ratio.

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My manual for my 1995 Mercruiser 4.3 states for Fuel Recommendations. I'm not saying it's right or wrong to use higher octane fuels though.

Fuel Ratings

Mercruiser engines will operate satisfactorily when using a major brand of unleaded gasoline as follows:

US and Canada- having a posted pump Octane rating of 87 octane minimum. Premium gasoline (92 octane) is also acceptable.

It also states to avoid alcohol gasolines for all of the known reasons and it mentions that the alcohol can clean the lubrication off of the internal metal parts causing adverse wear similar to your problem.

I use Stabil ethanol treatment (white label) with every fill up and haven't noticed any problems. No water in the separator either. The big bottle is about $20 at Walmart, not much when you think of the cost of the 300 gallons of gas it will treat.

Good luck,

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Dear S*K,

The proper term is "valve seat recession". This problem you are seeing is typical of heavily loaded marine engines using unleaded fuel. The cylinder head valve seat and valve seating area is literally eaten away. The best way to minimize the rate of wear is to install stellite exhaust seats and use a premium stainless exhaust valve. BE SURE TO CHECK THE TIMING (Spark Advance) Excessive spark advance can eat valve seats, burn spark plug electrodes, and eat holes in your cast pistons. The Charter Captains are aware of these problems!! 89 Octane is the bare minimum to use in my recommendations. Mercruiser recommendations are only suitable for pleasure use! NOT extreme long term idling and long runs offshore!! I have seen so many burnt pistons, valves eaten and heads cracked (on fishing boats) that it no longer is a mystery!!! ........

Be sure and use the correct spark plug also AC/Delco MR43T in these 4.3 V-6's and 5.7 V-8,s........

These type of failures were documented by the EPA in the early 1980's!! Some engines expired (by burning valves) in about 25 minutes of heavy loading!! The engine makers went to induction hardened seats in the heads and different steel in the valves. But the failures continue. Put better fuel in your boat and keep it as light as possible. Gasoline powered fishing boats do have shorter engine life when loaded heavily and under marginal power output in bigger hulls.......Hope you love fishing!!!

...................Respectfully submitted,.....Jet Boat Bill

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