Merc I/O 140 / 4cy - Over heating when in no wake speed

[farrellmh]

First i have no idea what the normal running temp is for a 4 cy 140 but i lknow that pass 180 is not good,,, its only does this wheen im in a no wake area... slower speed.. when i open it up it cools right down.... have heard alot of reasons on why.. but is there a test i can do to see if its the impeller or the water pump... im lost so PLZ HELP ME

Thanks
Mike

 

[Don S]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Easy test. If the impeller is over 2 years old, it should be replaced.

 

[harveje]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Mine runs 140 ish, at speed water is forced in, at idle there's not enough force and a weak or bad impeller can not make up the difference.

 

[DayCruiser]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Yea when you take it apart, you will definitly know if the impeller is the problem--ends curled over and/or chips broken off of it. Thats the way mine looked and I had the same symptoms you have

 

[DayCruiser]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Oh yea the thermostat is 143 deg. Might as well change that too while you are at it if not changed lately

 

[harveje]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Make sure you get all the impeller rubber out. If not your heat problem will come back.

 

[Boatist]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

I do not know much about MERCs' but can not believe that much different than my OMC 3.0 L. 140HP.

OMC My OMC will do the same thing if I forget to put the cap back on the flush adapter. It will over heat at slow speed and be fine at higher speeds. If I look over the transom the water will be running out the flush adapter cap.

There is a very simple test on my boat. Take the hose outdrive comes from the outdrive housing to the thermostat houseing off and put in a bucket. At idle should have a steady flow of 1500 Rpms out. At 1500 Rpms mine looks like I turned the garden hose on.

Only thing is do this test on the water not on muff. Hose pressure on Muff can make it look good OMC outdrive is bad.

My OMC idle water impeller pump works two ways. At planeing it is a displacement centrifical. At planeing speeds it is a centrifical pump. Needs to pump both ways.

 

[cobra 3.0]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

Boatist do you have a cobra or stringer? What's this pumping both ways stuff? (First time I hear this.)

 

[Don S]

Re: Merc I/O 140 / 4cy - Over heating when in no wake speed

He has a stringer, that's the only OMC drive I know of with a hose connection on the drive.
And the stringer drives are nothing like Mercruiser drives.