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Discussion Starter · #1 · (Edited)
I have a 200 Mercury Saltwater and replaced the thermostats recemtly becuase the engine alrm was going off. Went to go out today motored out through the No Wake Zone, No problem. Once I got up on plane the alrm started going off again after about 100 yars. I noticed that I was getting a steady stream of water too. Any suggestions?
 

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You sure it's the overheat alarm and not the low oil alarm? I had an old 150 Merc that would have trouble filling the under cowling oil tank when the weather was really cold. The screen on the pickup was partially clogged and the cold would cause the oil to get thick. Those two things combined would prevent adequate oil from flowing from the external tank to the under cowling one. You might want to check that out as well. Once I cleared the screen, all was well.

Do you have a temp and/or water pressure gauges?
 

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This is from the Mercury manual. Hope it helps.

Warning control of LOW-OIL, WATER SEPARATOR
and OVER-HEAT conditions.


Warning is provided
through activation of a horn and indicator lamps. A
LOW-OIL condition exists when switch in enginemounted
oil tank is shorted to ground (CLOSED). A
WATER SEPARATOR condition exists when excessive
water accumulates in the bottom of the separator
to short out the sensor. In either case, 30 seconds
after switch is closed, thewarning lampwill illuminate
and the warning horn will be activated. The horn will
beep 4 times in 1 second intervals followed by a 2
minute off-period. It will then repeat its beep sequence.
Continuous lamp illumination and horn beep
sequence will occur until the key switch is turned off.
If there is no LOW-OIL condition then the WATER
SEPARATOR must be checked. An OVER-HEAT
condition occurs when the coolant temperature rises
above 200°F (93.3°C). The warning lamp will illuminate
and the over-heat horn will sound continuously.
The ECM will retard the ignition timing until a maximum
RPM of approximately 3000 is obtained. The
ECMwillmaintain this RPMuntil engine temperature
drops to 190°F (87.8°C).
 

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Even though you are getting a flow of water out the indicator does not mean the head is getting adequate water flow. If that impellor hasn't been changed in the last3-4 years the vanes take a set and while it will move some water, it's not enough. Other options which may be checked first are the water hoses, across the top of the two heads and possibly a restricted water jacket full of salt/corrosion (saltx may help some). I'd bet on an old hard impellor, OR a sling tube that is not seated in the impellor housing. The sling tube is snug in the housing seated into a rubber bushing. It slides over the motor housings water tube and is much larger in ID than the motors water/cooling tube. Either way you'll need to drop the lower unit. PS, impellor rotation is clockwise as viewed from above, a little dish soap helps with install and dont forget the shaft key. I always put a thin coat of synthetic grease over the entire shaft to prevent corrosion and damage to water pump housing seals.
 

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I have an older 75hp and have had my oil alarm sound off. After many attempts to figure out why and with no results in stopping it, my conclusion was that it was something within the wiring plug that is on the side of the motor. This plug has about 8 wires going into it with about 8 wires coming out of it. As near as I can figure, this plug only facilitates disassembly in some way. Bottom line is that it seems that corrosion had/has gotten into the plug causing a bad connection which in turn causes the alarm to sound.

Now I just keep the wire for the alarm disconnected from the plug and watch that my oil is still being mixed by noticing that amount of blue smoke and the level of the oil reservoir.

Of course, this may not be your problem, but corrosion on electrical connections or their host are usually the culprit, especially since you say it happens when you "open it up", which creates more vibration which in turn might be causing the connection break.

Just my 2¢

Good luck

Ron
 

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Discussion Starter · #7 ·
Thanks

Thanks for everyones replies. The alarm is constant while on plane, full throttle. I can bring it down, turn off the motor let it sit for about 30 sec start back up and as long as I don't throttle up I can limp in without an alarm.
 

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Poppet

Its your poppet valve or water pump. Replace the water pump impeller and the entire poppet valve assembly (including the valves seat) and your problem will be fixed. If you want, do the poppet first, I'll bet that does it.
 

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Its your poppet valve or water pump. Replace the water pump impeller and the entire poppet valve assembly (including the valves seat) and your problem will be fixed. If you want, do the poppet first, I'll bet that does it.
:thumbup: :thumbup: :thumbup:

Completely forgot about the poppet!
 

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Is the motor actually overheating? Can you touch the powerhead w/o getting burned? If you can hold your fingers on the cylinder head for a few seconds it's not overheating.

==============================
Jim
23' Grady White Gulfstream "Get Reel"
Ocean Pines, MD
 

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I had the same engine on my last boat. Mercury recommends changing the impeller every season & I found that failure to do so causes the problem you are having. Being lazy, I would just take it to the dealer & have it serviced. They would change the thermostat, impeller, & the poppit valve. The bill was under $275.00 & I considered it money well spent. Never knew there was a water seperator in that engine. I had a Racor on mine mounted in the fuel line down by the oil tank.
 
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