165 hp alpha one mercruiser overheating at high rpms

[dabularach]

I was running my engine last weekend and it overheated spewing coolant all over the engine.When I got it back to the marina I replaced the coolant and tried to run it again.It runs fine at low rpms(below 3000)But heats up immediately When I run faster.I took out the old thermostat And I will try that this weekend.I also pulled the water hose before it enters the heat exchanger and it is pumping plenty of water.Any suggestions on how to diagnose this problem would be greatly appreciated

 

[Trent]

Re: 165 hp alpha one mercruiser overheating at high rpms

Heat exchanger is stopped up. Or riser.

 

[loadnet]

Re: 165 hp alpha one mercruiser overheating at high rpms

Yep!..... pull it apart and check it out. It must be clogged up for sure.

 

[rodbolt]

Re: 165 hp alpha one mercruiser overheating at high rpms

hello<br /> You say you have plenty going into the heat exchanger. How about coming out? The old inline 4 and 6 engines and even some of ther v8 had occasional riser clogs. check for water out going to the risers and then look to see if you have about the same amount coming from the relief holes. also when was the last seawater pump change? all the water in wont help if it cannot get out<br /> Good luck

 

[merc 140 pontoon]

Re: 165 hp alpha one mercruiser overheating at high rpms

I am fighting an unexplained high-rpm overheat condition on my 1974 Merc 140. I have ruled most things out. I will be checking the stern drive water pocket plastic cover. It can warp, and at high rpm the water can go to steam and leak out into the drive, reducing flow to the engine at high rpm. I assume you checked the inside feed hose flow at idle or slightly above (as I did). See http://www.marinemechanic.com/site/page150.html

 

[Northern Eclipse]

Re: 165 hp alpha one mercruiser overheating at high rpms

Dabulrach If you have the 3.7L 4 banger with 4 inch heat exchanger, then this motor should also have the exhaust riser/coolant tank combo (casted one piece), issue with this unit is that the riser/tank is aluminum and the manifold is cast iron thus the riser gasket can walk quite a bit during heating and cooling period, because they heat and cool at different rates a leak will occur between the riser and manifold, since we have a closed cooling system there is enough pressure at higher rpms to squeeze out some a/f and temp will increase at high rpms, but should cool down quickly when engine speed is low, anyway Mercury has a revised Gasket for riser/manifold, It was suggested by a very good merc tech friend off mine that the riser and manifold mating surfaces be checked for warpage, before installing new gasket, Most the time there is and thats why its leaking now..The only reason I know this is becuase I'm in the process of fixing My father's 1988 3.7l 165hp for the very problem, after we already changed a fairly new impeller and changed the 160* t-stat. Just a quick note , you can check the surfaces with a staight edge, such as a small steel framing square, if warped can be easily trued by a machine shop for about 1 hrs worth of shop labour.