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2003 Merc 150 over heating

J

jamlew

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"over heat alarm sounds off if

"over heat alarm sounds off if engine is ran at anything over 1200 RPM. Has brand new water pump assy, new thermostats, and only has 130hrs on engine. Plenty of water coming out of pee hole. Any ideas would be helpful."
 
"Depending on whether you have

"Depending on whether you have a 150 Opti, 150 EFI or 150 4 stroke it (or they) will be in slightly different locations.

The cooling system on larger Merc's is a 3 stage type system.

At low rpms (and low water pressure from the water pump) the flow of cooling water is controlled completely by the thermostat(s). They open/close to get/keep the motor at it's preferred operating temperature (around 140 degrees give or take a bit).

As you give it a little more throttle (1500-4500 rpms - again give or take a bit), the motor produces more heat, and the water pressure rises.

At that stage the thermostats are bypassed and water flow is controlled by one or two poppet valves which are a diaphram/spring design that open and close according to water pressure to regulate the amount of water flow cooling the heads.

As you approach wide open throttle and the water pressure rises above 12 psi (if I'm rememberin' correctly), the thermostats are bypassed, the poppets are (locked) open and cooling water flows "unrestricted" to cool the motor.

If you plan on doing the work yourself you really should pick up at least a Seloc's manual for your model - it will save an "oops" that could prove costly..."
 
"That is very helpful info, I

"That is very helpful info, I have recently purchased a manual for this engine I'm waiting for it to get here. I"m sure will help in the locating of the poppet or poppets. I am mechanic by trade so it shouldn't be to hard. Just very frustrating to do all the prep work to take the boat out only to have problems to cut the day short. This is a carbureted engine do you know if it also has the poppets."
 
"Ok, you have the 2.5L model &

"Ok, you have the 2.5L model (XR6 or Mag III version).

Yes, has 1 poppet. Looking at the back of the engine (facing spark plugs), the poppet is around on the right hand side of the motor, right near the base of the powerhead, on the back half of the block, just immediately behind where the front and back halves of the block come together (over the crank shaft).

It's cover will look like two disks joined to form a lop-sided 8 laying on it's side - with the disk towards the front of the engine slightly larger than the back disk (but it's all one piece).

If you decide to undo it before you get your manual, be prepared for all that will come out - there is about a dozen pieces burried under that cover
"
 
"You should however, also chec

"You should however, also check out the temp sensor. They are a pretty simple design and do fail regularly.

Fortunately, when the sensor fails the alarm goes off which is much better than having it fail quietly and toasting your engine.

Again from the back of the motor you will find it on the head on the left side.

It's just a little sensor with two wires coming from it. One will be grounded to the block, the other has a bullet style connector on it.

The sensor works on simple resistance - as the temp rises, the resistance drops - when it hits zero resistance, your horn goes off. That should occur at about 175 degrees (alarm range is 175-195 approx).

So if you have an ohm meter, a pot of water on the stove and a thermometer you can test it out. If you hit zero resistance before 145 degrees it's toast - a new one is about 35 bucks..."
 
Thanks for all the great info

Thanks for all the great info I will be digging into it further after the easter holiday. Will let you know what I find. Thanks again
 
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