1995 mercury 40hp overheating.. this situation is baffling to me

bowmanka

Recruit
Joined
Dec 3, 2012
Messages
4
I will try to explain this the best i can. I have a 1995 mercury outboard 40 hp tiller handle. my motor has been overheating.. can not find anything that regulates water temp( no thermostat or poppet valve).. water lines seem to be clear because the motor pees and does not overheat when i run it in the yard on muffs and hose, but when i put it in the water the pee stream struggles to keep pressure and the motor starts to overheat. i have no overheat sensor. the water pump is in good condition. I have had a mechanic from a local marina help me out and go through the entire cooling system and he couldn't figure it out. any help would be great thanks!!!
 
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Maxz695

Guest
Re: 1995 mercury 40hp overheating.. this situation is baffling to me

On muffs there is water pressure from the hose and it helps the impellor send water through the block. In the water the impellor must do all the pumping. There are several reasons this could be happening. Focus on the entire water pump first. The Metal plate the impellor rides on must have a good seal. If it is scored or warped it will lack pressure. The stainless steel cup the impellor is housed in must be clean and unscored from sand or the like. The water pump housing must be snug to keep it all together. I have seen the little ring in the impellor slip and could also be a problem. The intake screens must be clear or paint dried seaweed atc. Removing the screens will let you see the intake port and an inspection may reveal something blocking it be it sand or broken impellor fins. Lack of water is one comon cause for over heating. A diagram someone may provide can show if and where the thermostat and popit may be if your engine reuqires one and may need attention. Leaking exhaust through the exhuast port plate baffle will let water into the engine and make it miss hiccup etc. It will also let exhaust gases escape into the water jacket area of the side cover and super heat the water. These are just some causes of overheating and there symptoms.Please indicate serail number 2 stroke 4 stroke 2 cylinder 4 cylinder. This info may be of some help as well
 

bowmanka

Recruit
Joined
Dec 3, 2012
Messages
4
Re: 1995 mercury 40hp overheating.. this situation is baffling to me

serial number- 0G269482
2-stroke
4 cylinder

i have the same assumptions that you do that it may be something to so with the water pump. but i am not a boat mechanic at all and my mechanic friend has went MIA on me and im trying to get help anywhere i can.. i just feel the head is not getting filled like it used too before this problem occurred.. before the problem it only took 5 to 10 seconds for the motor to pee after starting but now its more like 15 to 20 seconds and at first just starts trickling out and gradually gains pressure
 
M

Maxz695

Guest
Re: 1995 mercury 40hp overheating.. this situation is baffling to me

Take down the LU and dis assemble the waterpump. Hopefully you will find any broken fins before they get shot up into the base which would require a power head removal. If fins are missing then be sure to find them all. Look under the metal plate as well. It may be the impellor is just weak. It may look OK but it doesn,t have the tension it did when new. When was the last time it was changed? Have you run the engine at all without muffs? Just trying to find some basic info and starting at the least expensive and most likely place for the cause of the problem. At present it,s a guess and a troubleshoot.I had a 79 2 cyl 2 stroke and dismantled a 4 cyl 4 stroke I have yet to come into contact with a 4 cyl 2 stroke so take it all with a grain of salt. Adding pictures along the way will help alot. The 4 X 4 was a 1997 basicly water pump principals are mostly the same with these mercury,s. Good chance you just need a new impellor about 23.00 ebay has plate and gaskets and housings and base plates if needed or you can get them right here on Iboats.com. Shipping costs are the problem when just purchasing a small item though. I,m usually on most evryday unless i,m out on my boat or shopping so feel free to post away with any concerns you may have. There are alot of decent well educated people in this site as well that enjoy helping a fellow boater.You do not have to be a marine machanic to fix this problem as you can see the one you had is clueless so time to take charge and get your fingers a bit dirty so you know what you have and what gets doen on your seemingly rare engine. I would love to see a pic of the electronics.
 
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