Merc. 50 4-stroke overheating

thomas.moth

Cadet
Joined
May 22, 2003
Messages
6
I've just bought a Merc. outboard 50 HP 4-stroke to fix of a friend. He says it overheats at high spead and that it was a factory problem caused by corrotion at the joint between the engine and shaft (at the big gasket). In the shaft this should result in exhaust being forsed into the cooling water. The story also tells, that the corroded part was exchanged for free by the factory if claimed before an now expired date. Can anybody recognise any of this story?? or give me an advise - the motor should start and run fine at lower work load.
 

graddy

Petty Officer 2nd Class
Joined
Apr 3, 2003
Messages
150
Re: Merc. 50 4-stroke overheating

A failed P/H base gasket could cause exhaust to get into the cooling system and overheat.When the engine overheats at high RPM's is the water pickups on the L/U completly submerged under water. What kind of boat is this engine mounted on?
 

thomas.moth

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May 22, 2003
Messages
6
Re: Merc. 50 4-stroke overheating

The engine is 1998 and was mounted on a 16 ft centerconsole Boston whaler trifoil type- I dont know the real name. It now has a Merc 4-stroke 70 HP and has no problem. The Impeller on the 50 HP is sound and as new. The leg is so long I would doubt the intake was above water at high speed.
 

BillP

Captain
Joined
Aug 10, 2002
Messages
3,290
Re: Merc. 50 4-stroke overheating

Moth,<br />The impeller on a 50x4s Merc is about the same size as that on my 3hp Johnson...in other words it is marginal even when new. You cannot look at the impeller on this motor and tell it is "little" worn and a "little worn" means a hot running engine. I don't think anyone would believe they are so close in size so a pic is attached. http://www.momentoffame.com/snapshot.html?id=38868 <br /><br />My Mariner(Merc)50x4s had a cooling problem and I could barely tell the difference between the new and old impeller. I've changed dozens of these over the years so have some clue on what to expect. The new one fixed the problem.<br /><br />My local Merc mech says there was a problem (big problem) with the early 50hp 4 strokes getting corrosion build up where the water tube joins the power head. This happens whether you flush or not. I suppose it it a hot spot and makes deposits. He also said Merc repaired many for the rental fleet guys and sorta ignored private owners. The fix is $300 at my dealer. He removes the powerhead, cleans the tube and puts it back together...pixx poor design to me. <br /><br />I bought the first year Mariner(Merc)50x4s and still have it so I'm waiting to see if it happens to this one. My mech says it normally shows up the first season or two. Mine is rarely run over 4000 rpms so this may be the saving grace.
snapshot.html
 

thomas.moth

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Joined
May 22, 2003
Messages
6
Re: Merc. 50 4-stroke overheating

My 50 HP must be a 'big-foot' then because the impeller is much bigger and, I would say, identical to a Alpha-one Gen II outdrive, that can serve several hundred HP Mercruiser engines. Anyway - I will change it as you suggested when I'm in there and clean the waterpipes junction to the 'lower powerhead chamber'. Thanks for the good advise.
 

thomas.moth

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Joined
May 22, 2003
Messages
6
Re: Merc. 50 4-stroke overheating

Thank you Capt BillP!<br />You were right. It was exactly where the (copper!!) water supply tube enters the bottom chamber of the engine. It ends in a rubber grommet that separates it slightly from the aluminum shaft housing entrance. The corrotion of the aluminum whole presses in the rubber, so only 1/4 of the tube diameter is left. The whole in the shaft housing was eaten up, so I need a new shaft housing. What an incredible construction failure!! I'll see if Mercury exchange the housing for free.
 

BillP

Captain
Joined
Aug 10, 2002
Messages
3,290
Re: Merc. 50 4-stroke overheating

Moth,<br />Mine is a standard motor, not a Big Foot. How much of a job was it to get the water tube out? I suspect my motor will suffer the same. Does the repair have different parts to keep this from happening again? <br /><br />This water tube deal is a major flaw and merc should have had a recall...instead they stayed very quiet. Typical merc product and typical merc factory attitude. :mad:
 

thomas.moth

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Joined
May 22, 2003
Messages
6
Re: Merc. 50 4-stroke overheating

Actually it is an eacy - or strait forward job to get into the water tube. I used a hoist to hold the outboard in the top hook. It saves your bag. The tube end rubberseal (wich is deterioated because of the surrounding corrosion of the aluminium housing)is somewhat frozen - or glued together with pipe and housing - you need to work a little on that. If you are lucky the housing is still intact (there is very little material around the hole) and you only need a new waterpipe rubberseal (maybe two new gaskets). The powerhead stays intact - you only have to disconnect two wire couplings and maybe one rubber oilhose and the nut to the gearshaft.
 

thomas.moth

Cadet
Joined
May 22, 2003
Messages
6
Re: Merc. 50 4-stroke overheating

O yes - I have heard they have madde the new housing in cast iron instead - it sounds a bit expensive, but that is what the rumers say.
 
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