1993 force 120HP over heat at wot

mouse4x4

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Nov 23, 2014
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Ok so I have this new to me 20ft trophy CC the engine is in good shape not covered in corrosion it has been used in fresh water most its life then stored for around 5-6 years. I got it installed new impeller an housing out the gate as doing a look over I noticed the buzzer was unplgged under the dash thought maybe I bumped it installing new steering so I hooked it up. took boat out idles runs out at wot around 5000 rpm if I run over 4500 for any exteneded time I get the buzzer going off for over heat. I can hold my hand on the thermostat housing an not burn my self I have since installed a new thermostat an grommet an gasket and took the plastic plug out top of block an installed a tell tail. Take boat back out have awesome water pressure same thing if I run in higher rpms alarm will sound but I can hold my hand in the pee stream of water and touch the thermostat housing for a while at least 10 seconds before hot. no burning paint or anything like that. I pulled the plugs out they have no trace of water everything is good. so I took the thermostat out installed a stanless washer with a 1/2 hole to restrict watere flow a bit at idle to make sure block is full, I can run wot all day long now no problems or alarms are the temp sensors known to fail or get overly sensative or should I look else where. kinda stumped here. I just don't understand how it works fine with out the stat I thought that is what the spring was for that holds the stat seated was at wot the water pressure pushes the stat out against the spring to allow full flow.
 

jerryjerry05

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May 7, 2008
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You test the thermo?
Drop it in a cup of boiling hot water.
It was Sunday and some don't get on here on Sunday.
Get a heat temp gun.
 

mouse4x4

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Right thermostat am I correct on the way the cooling system works I am going to try testing the thermo switch just didn't know if it is something that is known to go bad or get overly sensitive
 

pnwboat

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Oct 8, 2007
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What temperature is your thermostat rated to open at? There are a couple of different temperature ratings on the thermostats. I always check the temperature rating stamp on the thermostat itself. I've received the wrong one on a couple of occasions even though I've ordered the correct part number.

If the thermostat is not the issue, then I would do like you said and check the temperature sensor on the head. I don't know what temp. the sensor triggers the buzzer, however, I seen mention on the Forum here at about 180 degrees F. The temp. sensor is not something that is known for a high failure rate. They're pretty reliable.

The other thing to check is the timing. If the ignition timing is off, usually too far advanced, you may experience overheat at WOT. This can also melt a piston.

If the carburetors are set too lean, that can also possibly cause overheat at WOT. Idle mixture screws should be set to around 1 to 1 1/2 turns out from lightly seated. Exact setting is depends on your specific motor. Check that your whole fuel system is in good working order. Fuel pump, carbs, fuel lines etc.

If that doesn't do it, then I maybe would suspect a restriction in the exhaust cover water jacket. The water passages are very small in some places and it's not uncommon for sand and small pebbles to restrict the water flow.
 
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mouse4x4

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ok will check all that when i get home the fuel system is in good shape all new lines new fuel pump good filters never messed with fuel screws as everything idles good will check timing havent looked at that either.
 

mouse4x4

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ok so i checked the temp of 2 sensors i had they both go off around 160 degrees +- 5 degrees and i had 2 thermostats they both open around 133 degrees so what next i really would like to put a thermostat back in this engine. i just dont know what to do next the water pump was new at beginning of season complete pump it pees a very hard stream out the tell tail i installed in top of the head. what is next.
 

mouse4x4

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Just did a compression check all cylinders are 150psi across the board engine cold only 35 degrees outside so that is good carbs screws set at 1 1/4 turns out I am stumped here I don't run shallow water an it is brackish but flushed the inside of head is clean what is he next step
 

foodfisher

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Check for rubbed wire insulation from the sensor to the buzzer. Heat may expand it just enough to short it out.
 

mouse4x4

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Nov 23, 2014
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checked it out rerouted the wire today this motor looks like it was kept in a time capsule everything is clean no 2 stroke residue anywhere looks like a new motor this is blowing my mind i am gooing fishing tommorow with my washer in place an letting it eat not going to let it ruin my time as long as it goes when it stops i will bolt another motor on i guess
 

mouse4x4

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Nov 23, 2014
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i hate to run it this way but i am at the point where i am just going to i know it is getting good water and has a good head gasket i have yet to have a problem with plugs but carry a extra set with me i use quicksilver oil
 

pnwboat

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Restriction in the water jacket would be my next guess. Exhaust cover water jacket would be the most likely place for a restriction due to the size of the water passages there.
 

Jiggz

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Agree with PNW and in addition, it could also be scale build up in the water jackets. Since the problem goes away with the T-stat removed, it indicates it requires more water flow to keep it running cool. Since the motor was stored for more than 5 years it would not be surprising to see scale build up even when used in fresh water. You can either open and inspect the heads and water jackets for scale or restrictions or continue to use the boat without the t-stat for awhile until the scales are flushed. Or flushed the unit using a small sub pump and radiator flush solution. Personally, I'll do the open and inspect first to see if there is debris restriction or scaling and then take corrective action appropriately.
 

mouse4x4

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what kind od can of worms am I getting into by removing the exhaust cover I assume broken bolts galore inside the thermo housing is clean as can be I wouldn't think it would be scale is there any where that is normal for restriction between the water pump and the block
 

Jiggz

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Besides the t-stat there is no designed restrictions in the water flow except for the narrower passages in the head's water jacket. On other models, the exhaust plate has a water pressure relief valve which opens when the t-stat is closed but closes as soon as the t-stat opens. If you do not believe scale is a problem then the only thing I can think of causing the overheating at WOT is a semi blockage or semi-restriction in the cooling water circuit. Unfortunately, it requires a tear down of the head and exhaust plate to investigate the actual cause. If you find it really challenging to do a tear down then maybe you should just try operating it for awhile without a t-stat and hope this will clear the cooling circuit.
 

mouse4x4

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So here is my game plan since I am done with boat for winter I am going to pull lower off t stat housing off reverse water flow with garden hose into a bucket an see if anything comes out. Then install another water pump kit. Then run it if it still does the alarm thing exhaust cover will come off. I checked timing it is 31 degrees at wot carb settings are set at 1 1/8 turn out. I just don't want to get into a bunch of broken bolts on the exhaust cover
 

mouse4x4

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If need be is there a good way to get the exhaust cover bolts out with out breaking them off. I hope a little back flush will get anything out that is there. I went to a merc dealer an was talking with them about my problem he said there is some grommet on the water intake tube the can corrode up bad at the bottom of block is this true.
 

Jiggz

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That is correct. There is a grommet at the upper end of the water tube which keeps it up there held by the water line/tube bracket. Technically, if the grommet is totally deteriorated, the best indication will be a loose water tube that easily sways. Do not try pulling on the tube for if the grommet is really loose it would drop and it is a PITA to put it back on. To verify the grommet is still intact, drop the LU, find an appropriate sized tube and insert it to the water tube and hold it with a hose clamp or zip ties. Connect the other end of the tube to an adapter to connect it to a garden hose. Slowly open the water spigot enough to have water flowing into the cooling system. DO NOT OPEN ALL THE WAY! Using a flashlight look into the upper end of the water tube. If there is water flowing out of the water bracket, the grommet is basically loose or badly deteriorated. Remember, water will eventually ran back down into the exhaust plenum so this is better done with two people, one monitoring the tube while the other is slowing opening the water spigot. You do not have to keep the water constantly running. Bottom line, if the water tube feels loose, it is very likely the grommet is deteriorated and leaking water badly, thus greatly reducing cooling water flow into the system.
 
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