Mercruiser 140 overheating...

[petrey10]

I have a 1971 Mercruiser 140 and it keeps overheating. I have tried to put in a repair kit for the stern water pump. I have checked all the drain plugs and put new ones in. I can put the engine in drive and run about 7-10 mph and the engine will stayed cooled but when I go to half throttle on up to WOT the engine overheats very quickly and I have to bring it back down to 7-10 mph. Could this be the thermostat being bad? There is also a very tiny crack in the exhaust manifold that lets water drip when the engine is at WOT but I wouldn't think it would make it overheat. I know you guys are good with this stuff so I came here first.

 

[Don S]

Re: Mercruiser 140 overheating...

My guess is either the manifold, or the riser, or both are plugged up with rust and needs replaced. When they get plugged up with rust, they can sometimes crack the cast iron.
It's over 35 years old with water running thru it.

 

[petrey10]

Re: Mercruiser 140 overheating...

is there anyway I can get some of the rust out??

 

[Don S]

Re: Mercruiser 140 overheating...

The rust is from the inside of the manifold and/or riser corroding over time. You could try to clean it, but the chances are it will only leak water into your engine when you shut it off. It's one of the problems with buying an old boat.

 

[petrey10]

Re: Mercruiser 140 overheating...

yeah I understand that but the thing is that the engine only has 187 hours of use on it...I am goign to completely tear down the engine tomorrow...is there any place on line that you can buy used parts for less than a 300 exhaust manifold??

 

[jfl1959]

Re: Mercruiser 140 overheating...

hello and welcome

I would not buy used manifolds or risers, you might have the same problem with them you have know. I am sure they are on ebay. The rust and obstruction is on the inside and you cant see it. Risers are known to have to be replace regularly. (say every three years or so) If they go they can cause nasty problems.
I just bought two risers at about $115 each.
Is your system standard (everything is raw water cooled), full FWC (meaning antifreeze through engine and exhaust manifold) or half FWC (meaning antifreeze through just the engine)?
Overheating underway but cool at trolling speed can be bad risers for sure. You get enough flow at low rpm underway but when the engine has to work you need good water flow to properly cool the engine.

Hope it helps and good luck

 

[petrey10]

Re: Mercruiser 140 overheating...

it is raw cooled for sure....where did you get those risers at?

 

[jfl1959]

Re: Mercruiser 140 overheating...

www.go2marine.com

 

[cheburashka]

Re: Mercruiser 140 overheating...

Another place to check is aftermarket marine. http://www.amarket.com/ They had the best price on the manifold by around $100, and it came complete with the rear block-off plate, gasket set, and front plate with hose fitting. Go2 is cheaper for the riser itself, but by the time you pay shipping, Aftermarket is better. Only problem I had was one stud with poorly cut threads and a drain plug that was the wrong diameter.

http://www.amarket.com/imel140.htm
http://www.amarket.com/imm140r.htm

These two should cover you for right around $300 plus shipping.

 

[petrey10]

Re: Mercruiser 140 overheating...

well guys I took the complete cooling system apart today....blew air through every port with a compressor....my riser was blocked...now it isn't because the air flew threw the channels after cleaning them out... brazed a crack in the manifold...put the boat in the water and it stilled OVERHEATED...went back to square one and saw that the impelled was burnt up AGAIN... I am bettting this was because of the riser being blocked so I am going to replace the impeller again and if it still overheats I will probably have a boat for sale....any suggestions please let me know

 

[cheburashka]

Re: Mercruiser 140 overheating...

well guys I took the complete cooling system apart today....blew air through every port with a compressor....my riser was blocked...now it isn't because the air flew threw the channels after cleaning them out... brazed a crack in the manifold...put the boat in the water and it stilled OVERHEATED...went back to square one and saw that the impelled was burnt up AGAIN... I am bettting this was because of the riser being blocked so I am going to replace the impeller again and if it still overheats I will probably have a boat for sale....any suggestions please let me know

I'd suggest you take the advice that's already been given. Cracked manifolds are bad news. Seems to me that you're already spending a lot just on gaskets and impellers. Now you're going to do the impeller again, run it again, maybe watch it overheat again, etc. You have very little chance of success with a manifold and riser that old. Getting air to move through a riser is very different from having full water flow through it. Might as well save the money on the new impeller and put it up for sale now if you aren't willing to do the repair right.