Over heating Mercruiser 120 sterndrive

"Months ago I posted that I ha

"Months ago I posted that I had changed the water pump in the foot, the thermostat, and flushed all the lines after the motor ran hot. I took it to a shop and they found a piece of melted baffle (backflow baffle) still in the manifold area. With all that it runs fine on a hose at home and then on the lake at increased RPM it runs hot. I get past the no wake zone for the landing and apply power to about 2000RPM and the gauge begins to rise. Before I get back to the no wake zone it is at 250 and rising. What can be wrong? The lake water is colder than 250 ;-}"

"clogged manifold , riser , th

"clogged manifold , riser , therm housing , kinked/collapsed connector hose at transom"

"Put boat in water, remove sea

"Put boat in water, remove sea water hose at heat exchanger or if sea water cooling, remove hose where it connects to engine and observe amount of water being pumped with engine running. Time how long it takes to fill a gallon bucket and report results. Sould be 30 sec or less.
If the flow is weak, you could have assembled the pump wrong or have a pluged or kinked hose."

"it runs fine on a hose at

"it runs fine on a hose at home and then on the lake at increased RPM it runs hot"

The hose is creating the water pressure needed to cool the engine.

"they found a piece of melted baffle (backflow baffle) still in the manifold area"

What about the rest of the flapper? Did they find it? Do you have sufficient power when it overheats or does it seem to bog down? "Y" pipe may be plugged from exhaust flapper.

Raw or river water cooled. No

Raw or river water cooled. No bogg down effect. Runs while overheating fine but I of course fear damage so idle back to landing during which it runs fine.
Will check the water flow with a gallon jug. I am assuming you mean to sit at idle in still water letting the motor pump what it will instead of having the boat moving. I therefore wonder if a water barrel would do rather than the river since one landing is open on the lake and that one is so shallow now that putting in is tricky. Lake Wylie is down 30 inches below normal which is about 5 feet or more below full pond so all the normally deep obstacles are near the surface.

A barrel will work if you keep

A barrel will work if you keep the water level sufficient to cover the OD intake. Take the prop off.

"Do you have any signs of wate

"Do you have any signs of water in the oil, milky oil on dipstick, inside the filler cap or around pcv valve?
Using a plastic garbage can is fine. If plastic can is a bit small, take prop off and squish can over lower unit. Fill so water level is above water pump. Idea here is to observe water pump output without aid of externl pressure hose. At idle, with raw water hose disconnecte from engine it chould have volume to produce about a 4" stream past the end of hose. Rev it up to about 2500 and it should squirt 2-3 feet..or more.
At 250 or more degrees, there should be some signs of boiling water.
Could it be possible the thermostat be installed backwards?
Or, bad gauge or temp. sending unit?
If you end up droping the lower unit, before doing anything with the pump, pour water down the top of the outlet tube on the pump assy and look for leaks anywhere around the pump assy.
Worse case senerio, being a raw water system, you could have some major issues with rust and crud in the coolant passages in block/head or a crack in the head in which exhaust gasses could block coolant flow. That would require a partial tear down so do the easier, nick picky tests first and report your findings."

"The o-lady has horses and the

"The o-lady has horses and therefore a horse trough. I just back up the barge to the water trough and when its over, I live on the boat for a day or two."

Thanks Bill. I took the list o

Thanks Bill. I took the list of todos to the shop and gave it to them. We discussed it and they had taken off the hoses but with water pressure from the hose and engine running. They will try the barrel thing and we discussed putting it in the river but they still are afraid of hitting something with the foot in the shallow water. I will post the results when I hear from them. I half wish I had brought it home to do myself but I live in one of those controled neighborhoods and it could not stay in the yard over 72 hours at a time. I hate to drag it back and forth to work just to beat the system.

"Hi, I have the same identical

"Hi, I have the same identical problem. Was hoping you would post the solution soon. Just replaced the lower unit impeller & tube. It makes plenty of water on its own in the lake but when I power up - it starts getting too hot."