Overheating

[imastevesmith]

I have a merc 120. It is a 1967 4 cyl. eng. The eng is a replacement on a 1980 Ebko. I just spent a ton of time replacing a bad eng valve. With all of you guys and your help I got the engine purring!(see Merc 120 hitting on 2 cyl.) Took it out today. My 13 yr. old with ski's in hand and a big grin and the darn thing overheated. We spent some time ( 10 min) idling off and on trying to teach another kid to ski. After 45 min on the lake I noticed the temp gauge was pegged. After letting it cool down a little (temp gauge in the green again) I headed back to the dock. The temp gauge did not increase when heading back (not wot but clipping along).
I've read alot of postings on overheating but how do I begin testing to find my source. I did garble up my (new, of coarse) prop last fall in low water at a ramp. I wonder if I sucked up something in that episode. It was the last time out. How will I test if I just have something plugged or a bad impeller. Also how hard is it to change out the impeller and/or any instructions on that process.
I watched a video on Utube of a guy testing his impeller by running the prop in a big bucket of water and watching the water spray from the little holes on the lower unit... is this right?
I'm determined to get these kids skiing... hopefully before winter!

 

[achris]

Re: Overheating

Overheating could be caused by any or a few of the following.

1) thermostat
2) blocked exhaust manifold/elbow
3) faulty engine circulation pump
4) bad water pump impeller (in the leg)
5) bad gasket/base in the leg water pump
6) faulty water pocket cover
7) incorrect ignition timing
8) incorrect valve timing
9) 'X' dimension too high
10) blocked water intakes

You need to go through each of these and verify that they are ok.

Chris..........

 

[imastevesmith]

Re: Overheating

I hope I won't be sounding like a idiot here but...
I am fairly good with auto mechanics and a few of the boat issues correlate... but not this cooling issue. I appreciate the list and I will check thru them one by one. Can you give me any instructions or quick tests to rule out some of them.

 

[Bondo]

Re: Overheating

Ayuh,....

Anytime there's an Overheat Issue,...

Replacing the Impeller is the 1st thing to do.....

 

[JustJason]

Re: Overheating

an easy way to check things is to go to home depot and get yourself some clear vynil hose and splice it onto the end of the cold water hose that comes off the transom plate.
with the engine cold and in the water (not on muffs!!) leave the hose disconnected and start the engine up. Check for good consistant flow of water. it should blow water pretty good. only run the engine for 15-20 seconds. if your not getting water or weak water then the impeller is definitly shot. if your getting good water flow hook up the line you spliced in and run the boat. while the boats going have somone check for air bubbles in the line, you shouldn't have any, and if you do that will cause an overheat.

 

[jamowers]

Re: Overheating

I just went through the same issue. Mine did that with a brand new impeller. Though you can check it with the clear hose trick. Mine was a blockage in the 4 holes in the manifold and riser tube. There are also 2 holes at the end of the riser where water flows out to the outdrive. Check those as well. I did not puill the manifold but did pull the end cap (near tstat housing) and it is old but looked ok. After I cleared the blockage it runs really cool, but I am ordering a new riser as mine is flaking on the inside and it is a matter of time till it clogs again. While you have the riser off check the shutter valve in the exhast tube down in the rear of engine compartment while your in there. I pulled the drive off and replaced the impeller as well as thermostat only to find a block!!! Very frustrating. List above are the best things to check.

120 1982 merc

 

[BMsuper]

Re: Overheating

I have the Mercuriser 2.5 (4cyl. GM) with an alpha outdrive that is also starting to overheat. I have not started to investigate this problem yet as I have been reading all the responses on overheating. I just bought the boat about two months ago and have used it about a dozen times. This last time the engine stalled when going to low speed. When I pulled the air filter the carb was spewing gas down the carb throat and the engine compartment was very hot. I just completely rebuilt the carb so I don't think that is the problem.

Is it possible the engine was so hot the gas was boiling in the fuel bowl into the engine?

The temp. gauge did not show a high temp?

I am going away for a week, when I get back my first job will be to replace the impellor in the outdrive.

I have read that there is a "poppet" valve in outboards that keeps the water in the block to allow cooling of the block before it expels out the weep hole. Is there such a valve in the Mercruiser inboard/outboard?

As you can tell, I am new to boating, but I'm learning fast.

 

[Bondo]

Re: Overheating

I have read that there is a "poppet" valve in outboards that keeps the water in the block to allow cooling of the block before it expels out the weep hole. Is there such a valve in the Mercruiser inboard/outboard?

Nope,.... That's the effect of the Risers.....

 

[achris]

Re: Overheating

I have the Mercuriser 2.5 (4cyl. GM) with an alpha outdrive that is also starting to overheat. I have not started to investigate this problem yet as I have been reading all the responses on overheating. I just bought the boat about two months ago and have used it about a dozen times. This last time the engine stalled when going to low speed. When I pulled the air filter the carb was spewing gas down the carb throat and the engine compartment was very hot. I just completely rebuilt the carb so I don't think that is the problem.

Is it possible the engine was so hot the gas was boiling in the fuel bowl into the engine?

The temp. gauge did not show a high temp?

I am going away for a week, when I get back my first job will be to replace the impellor in the outdrive.

I have read that there is a "poppet" valve in outboards that keeps the water in the block to allow cooling of the block before it expels out the weep hole. Is there such a valve in the Mercruiser inboard/outboard?

As you can tell, I am new to boating, but I'm learning fast.

Please don't hi-jack someone else's thread... Start your own....

 

[APPALOOSA2]

Re: Overheating

I think both of you should start with impellers "lower unit pump" and change your thermostats. Both easy jobs and not costly $50.00 for both.

BmSuper if your guage is not working at all I would also change the sending unit $15.00 and located on the thermostat housing on the 120 2.5. You will need to know the serial # of the engine for the correct sending unit some were 3/8 thread and newer where 1/2 thread. Serial # on starboard side of engine on a plate near the rear next to your trim pump.

 

[achris]

Re: Overheating

Being Mercruiser 120's, my first check would be the very narrow water passages at the stern end of the cast iron manifold and the the just as narrow passages in the cast iron exhaust elbow....

If they are more than about 5 years old.... trash-can time....

But as I stated... 2 people...2 problems (although they look the same the cause could be very different).... 2 separate threads please....

Chris............