merc 9.9 overheating

[parrothead]

I have a 1998 9.9 merc that I just purchased and the upper shaft on the lower unit just got super hot.The impeller is 6 months old and the water flows out at a steady stream on the muffs(that is also very warm).The heads dont seem to be hot at all ? It pushes a 12ft carolina skiff that is very heavy,just as it feels like it is about to plane I hear it over rev and I throttle back.Thats when I noticed all the steam around the exhaust area.The whole upper shaft turned white from the evaporated salt from the water.Is the moter working too hard?The specs tell me I am way under on horse for the skiff.

 

[plywoody]

Re: merc 9.9 overheating

By the "upper shaft" do you mean the exposed housing above the water line?<br /><br />The impeller and water pump does no cooling of this area. You have a souce of heat that should not be there. Is it possible you have a broken or disconnected exhaust housing that is supposed to direct the exhaust thru the prop?<br /><br />I am a little troubled by your statement that the motor "over revved" and then you shut it down. How long did it "over-rev"? Although I cannot think of a friction point in the shaft above the water line that could possibly cause this kind of heat, except for the exhaust from the engine itself.<br /><br />John

 

[parrothead]

Re: merc 9.9 overheating

Yes the shaft above the waterline.When it revs it sounds like the prop is slipping?Is that getting it hot? it lasts about 2 secs and i shut it down,I will mark the hub to see if it is the prop.Do you tjink the engine is working too hard?Thanks.I have never seen one get soo hot back there.

 

[plywoody]

Re: merc 9.9 overheating

The engine should not be allowed to over-rev. Is this happening on the muffs? Over revving happens in a situation where there is not proper resistance..ie spinning hub, cavitation or other lack of water resistance against the prop.<br /><br />Is the motor working too hard? No, this is not your problem. Your motor is designed to run at whatever the spec is, usually around 5500 rpm. It should not overheat in this circumstance. Over revving occurs when the motor is not working hard enough, ie not displacing enough water as propulsion<br /><br />It sounds to me like you might have two distict problems. One a source of heat that should not be present in the lower unit, and two an over-revving problem. Two seconds of an over revving situation is long enough to do possible damage to the powerhead and related moving parts, but it is not long enough to cause a lower unit overheating problem

 

[parrothead]

Re: merc 9.9 overheating

plywoody:merc 9.9 overheating.Just got back in town.It sounds like it is sucking air in the water.On the muffs it doesnt rev,only when I try to plane out.The exhaust area gets red hot.Even with a good water stream flowing.The prop sits low enough under the hull,flat bottom skiff.It has a ss prop,should I go to the stock prop?

 

[plywoody]

Re: merc 9.9 overheating

You mean the prop is cavitating? If that is the sucking air sound you describe, check the depth of the motor relative to the bottom of the boat. The cavitation plate should be level with the bottom of the boat--no higher. Also check the bottom of the boat for anything that would disrupt water flow under the boat.<br /><br />Now what is exactly overheating? If the water pump is pumping water, and the power head is cool, then the only thing that could cause the heat you describe that I am aware of is a broken and/or improperly installed exhaust tube.<br /><br />Please be as specific and detailed as possible as to what exactly is happening, and where exactly it is overheating, as sitting at the keyboard is not the same as viewing the engine in person and observing symtoms.<br /><br />Good luck,<br /><br />John

 

[plywoody]

Re: merc 9.9 overheating

I don't think this has anything to do with a SS prop vs a standard aluminum prop, unless the diameter and pitch are radically different on the two. Why would you put a SS prop on a 9.9?<br /><br />John

 

[parrothead]

Re: merc 9.9 overheating

The ss prop came with the engine.The original owner installed it to help it plane faster I would guess?The hull is clean and there is ample depth for the plate.I will check the exhaust tube.Oh come to think of it there is some exhaust smoke exiting from both sides of the trim area 9 inches or so lower than the exhaust holes.the area in question has rods that hook up into the shaft used to secure the engine.Hope this helps.thanks.

 

[plywoody]

Re: merc 9.9 overheating

There has to be a reason for a prop to cavitate. It does not do it on its own for no reason.<br /><br />Check the pitch and the dia on the SS prop. If it is dramatically different than what is stock on that engine, change it back to original. Has this engine ever worked properly on this boat at WOT?<br /><br />Exhaust should be exiting out thru the hub of the prop. If it is not, you have a problem that must be repaired.

 

[plywoody]

Re: merc 9.9 overheating

Also, check to make sure you don't have a slipping hub on the prop. make a mark on the outer and inner portions of the hub of the prop, and check to see if they remain aligned after use--specifically after it "sucks air"