120 HP Mercruiser Overheating

[jlacoure]

I have a 120 HP Mercruiser that is overheating. I have replaced the impeller and the thermostat. When I go slow and just idle it, it runs about 150 but whenever I speed up it goes up over 200 pretty quick. Any reason why. I have spent alot of time on it lately and it is getting frustrating trying to fix everything.

 

[gspig]

Re: 120 HP Mercruiser Overheating

Check the water pump on the engine. There is a water pump on the engine just like on a car and is belt driven. Belt could be missing or loose.

 

[tommays]

Re: 120 HP Mercruiser Overheating

is this a I4 <br /><br /> what year and repiars have been done to this point<br /><br />the water has to able to get in to the engine as well as get out so even if your impeller is perfect if a maniford or riser are cloged you could overheat <br /><br />you need to check water flow at each point<br /><br />if the boats on the trailer does it run right on the muffs <br /><br />tommays

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

How do you check the waterflow. Where are the risers !

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

The boat is a 1983 model. It is 17ft long.

 

[tommays]

Re: 120 HP Mercruiser Overheating

i think you will need a service manual it will have step by step methods to check each part and photos showing parts your are not familar with<br /><br />tommays

 

[Hog Wild]

Re: 120 HP Mercruiser Overheating

Wait, wait, wait.. First off, welcome to boating trying to "fix everything".. learn to enjoy it a bit as it's pretty common stuff if you use it a lot. Was the boat over heating and then you replaced the pump? My guess is "yes" because you replaced the thermostat too (thermostats don't fail often). Ok, if your boat is over heating, it's because your not getting enough water flow (simple enough). First thing that can happen is if you do not get the water pump together right it will get air when you run fast. The pump usually gets air around the pump housing itself. A good way to check this is put a clear hose inline coming from the pump and watch for bubbles at high speeds. Another indicator of this is when you slow down, the engine cools down really fast (because the pump gets water again). Also your new impeller could have failed but is unlikely if you replaced it because of over heating issues.<br /> 2nd, is a plugged up something or other (your most likely candidate), esp. if you are in salt water at all. Corrosion will block passageways (even in fresh water with a 1983 boat) and restrict water flow. Did the water impeller look ok when you changed it? If so, that confirms thoughts that the pump was ok and the water blockage is more the culprit. Does the boat cool ok at 1/2 throttle? 3/4, WOT? The risers are your exhaust system. You may want to pull one off and check them for corrosion. You can pull off a hose and just watch for water flow but that is tough to measure really. also good to check hoses themselves for blockage, kinks, things like that. Maybe add some more detail about when this happens/happened, why you did the repairs you did?

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

Yes I had one when I changed out the imnpeller and the thermostat. Of course the thermostat was pretty easy. The impeller was not that difficult either.

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

I changed the impeller because it was overheating. This was suggested to me. The impeller was in pretty bad shape when I changed it out. it looked like it needed to be changed out. It gets hot whenever I get up to a certain speed. It does not take long and i am not going very fast when it happens. The faster I go the hotter it gets.

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

I have redone the whole inside. I replaced all of the seats and reupholstered them. It was alot of work. I have replaced the starter and battery and the bilge pump. It starts good and runs good even though it show to be hot.

 

[jlacoure]

Re: 120 HP Mercruiser Overheating

I have redone the whole inside. I replaced all of the seats and reupholstered them. It was alot of work. I have replaced the starter and battery and the bilge pump. It starts good and runs good even though it show to be hot.

 

[rattana]

Re: 120 HP Mercruiser Overheating

Were there pieces missing from the impellor? The pieces could be stuck somewhere restricting water flow.

 

[Hog Wild]

Re: 120 HP Mercruiser Overheating

I agree a bit with rattana, missing impeller pieces can get stuck inside of the water hose / housings. Also perhaps like I mentioned above, you might no have the pump together right. The basic fact remains that you are simply not getting enough water flow yet. But think simple. Don't over think or take too much advice, just figure out why your not getting enough water flow. Is it the pump or some water restriction basically.

 

[crazy charlie]

Re: 120 HP Mercruiser Overheating

are you absolutely certain that the impeller vanes are curving to the correct direction.It is pretty easy to put one in the housing backwards.Charlie

 

[Don S]

Re: 120 HP Mercruiser Overheating

Here is a link to a listing of the Merc 120 parts catalogs that should help.<br /> 120 Part Catalog <br /><br />The exhaust elbow or riser (both refer to the same thing) are shown in the image below. It's Item #14. If it gets plugged or the exhaust manifold gets plugged water can not get out and it just sits there and gets hotter and hotter. If that is the case, you need to replace the bad part. Trying to clean them is only a VERY temporary fix, even if it works. Most of the reason for it being plugged is due to rust and corrosion. If you get a hole in either, you will get water in your engine which will cost you an engine.<br /><br />PS: Crazy Charlie. Even if you have the vanes going backwards when you install the pump, when you start the engine they will go the proper direction, unless they are so old and stiff they won't bend, then they needed replaced anyway.<br /><br />