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Mercruiser 120 over heat


FishingTheFL

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List,new parts-

thermostat

Impeller housing (upper and lower)

Gaskets

Circulation pump

 

What else is there to change?? Won't over heat on muffs, put on the water take it out for less then 5 mins and she gets red hot... Removed the large house that goes to the water pump to check flow... Gushing out... I put back flow threw intake house... Still over heats... Motor doesn't feel hot to the touch but gauge goes nuts... Thanks for any help... Tired of trying to get this junk to run... Keep putting effort into it because fear of the same crap happening even if I get a different boat...

 

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You say its not hot to the touch, maybe your gauge and or wiring to it along with sensor have issues. A cheap harbor freight heat gun would not be a bad investment. You could also remove the thermo as new ones can be bad ,unless you checked that on your stove. That will allow full flow and at least take that out of the potential problem list.

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Ill test the thermostat. I cleaned all the old impeller pieces out, put water threw the intake hose to flush anything back threw. I even checked to make sure it was pumping up to the water pump... Water coming out like a garden hose... Could bad timing cause heat up issue? Seems it won't go full throttle and I have a feeling that's timing but I swear I had it running better, rebuilt the crab and it seems I've had nothing but problems since the carb was rebuilt. Also had a new cam and gear put in it, think the guy did that wrong. I have another cam with the gear on it from another 120. Might swap them out today.

I'm pretty sure the gauge works fine. Maybe not but when the valve cover warms up the gouges go up.

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The flapper valve is in the exhaust tube to keep water from entering the engine in following seas if you have good exhaust flow. The valve is not the issue. Is there any contamination in your exhaust water ? Catch some and look at it Also is your cooling system open or closed ?





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I didn't see anything in the exhaust water. I took the thermostat out and did a boil check, it opened but didn't close when cooled off till the next day. I'll do a bucket check of the exhaust tomorrow after I put another thermostat in... I know we did it last year. Maybe I was mistaken and we didn't.

 

I did put it on the muffs and start it up for a 10 sec video that I will try to upload soon as I figure out how. Water is coming up but I don't think its no 30gpm at idle. Possibly but I don't think so. I don't think my hose provides that much water... Could the impeller burn out testing timing on muffs? No high rpms just enough to see if it backfires through the carb.

 

I forgot to switch props from one outdrive to the other. Wonder if this is a possibility why it won't get to plane. Seems like the power is there just won't plane after thinking about it a bit

 

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Here's another thought. There is a key on the impeller. It may be sheared. Pumps water at idle. But impeller slips at higher rpm


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i had the same thing happen to me . i put a new impeller kit in a alpha 1 . it ran and pump water on the muffs . when in the lake the thermostat gauge climbed pass where i knew something wasn't right ,back to trailer it went before overheat . when i took the water pump apart again found i used a wrong gasket witch closed off half a two part water port . once the right gasket was installed all was fine . the impeller kit came with multiply gaskets for different applications 

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i had the same thing happen to me . i put a new impeller kit in a alpha 1 . it ran and pump water on the muffs . when in the lake the thermostat gauge climbed pass where i knew something wasn't right ,back to trailer it went before overheat . when i took the water pump apart again found i used a wrong gasket witch closed off half a two part water port . once the right gasket was installed all was fine . the impeller kit came with multiply gaskets for different applications 



Did the impeller kit twice, first time I didn't change the base, second time I changed the base... Only,thing I can think off is the base had some washers under it... I don't think they belong there but they were there when I removed... I'll try the thermostat. If its not that I'll take the dirge apart again and make sure all gaskets are right. I know I have no passages blocked.

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Water tube was made out of copper, don't think it could crack. I'm pretty sure the tstat is the problem. It didn't close when it cooled down. My understanding is they close till they hit 170 then close at 150 then open again... The one I took out popped open, but even after 20 mins of being in open air it didn't close. Maybe half way. Hour later it closed most the way with maybe 1/8" gap.

Either way, flexplate went in my truck so I am stuck working on that for now... Murphy Monday

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I had the same issue with my old boat, I took it to 3 different shops, they couldn't get it working. Would work on the muffs but you put in the water it would overheat in no time. 

 

Even had the mechanics out on the water with me to show them....that's why she's my old boat....nobody could figure it out after a lot of money went in it...said screw it got a new boat. 

 

 

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5 minutes ago, Ryno23 said:

I had the same issue with my old boat, I took it to 3 different shops, they couldn't get it working. Would work on the muffs but you put in the water it would overheat in no time. 

 

Even had the mechanics out on the water with me to show them....that's why she's my old boat....nobody could figure it out after a lot of money went in it...said screw it got a new boat. 

 

 

you've gotta to know when to hold'em when to fold'em  :thinking:...

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Yeah, well just got into boating about year and half ago. Got stiff on two boats... Can't get a third till I sell the brown Betty I got or rebuild the motor. Was quoted $1500 to $1700 to rebuild. Personally, I thought about buying a 15hp with controls and put it on my ez steer... Keeps the main motor from being used much. Got the idea from a local here in the flx

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i had the same issue with an older 120 and seen the malkings for the same problem on my inboard 454. the impeller had come apart in my 120 and a chunk of it had made its way up the intake hose to the elbow that goes into the side of the engine i took the intake hose off quick as the engine was running to check for water flow and the rubber peices were jammed in the elbow. my 454 had the impeller come apart with the previous owner and the chucks were stuck in the tranny cooler inlets side.  hope this helps.  good luck

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