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Alarm goes off

eddieromo

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"I have a 1998 Bayliner with

"I have a 1998 Bayliner with a Mercruiser 5.7 with ThunderboltV ignition, when I'm on the move for about 20 minutes at around 4000 rpm I get a alarm, the temp is around 175 180, oil pressure is around 40psi and the trim reservoir is fill up to the full mark. On occasion when I reduce the speed the temp drops down to 140 then comes up to 175. At what point should the alarm go off ? over 180 ?? Its been driving me batty, I have installed a new impeller in the lower unit last summer. Any help you can give me will help."
 
high RPM over heating is a sig

high RPM over heating is a sign of clogged risers and or manifolds. Alarm was doing it's job.
 
"From what I've recently d

"From what I've recently discovered (the hard $$$ way) is that

Of the 4 switches (200 ' F set point) I tested in a pot of boiling water, 1 out of 3 actually worked! (and I was depending on them).

Jeff"
 
"Get a hand held infrared red

"Get a hand held infrared red surface thermometer... about $50 at Sears. You can get an idea of really how hot things are getting and go from there. My guess is, especially if it's fresh water cooled, the alarm is going of way too early."
 
Okay I will test the Thermosta

Okay I will test the Thermostat but at what point should the alarm go off ?? Is there a way to test the alarms.
Ed
 
The merc switch will go off at

The merc switch will go off at about 200 deg. Verify the temp is not over 200 and then replace the temp switch (not the sender)
 
The setpoint depends on if it&

The setpoint depends on if it's fresh or raw water cooled.
Fresh water - 200F'ish
Raw water - 180F'ish
 
"Okay, I'm going to get an

"Okay, I'm going to get an infrared temp gun, where should I point the gun at , exhaust , intake, or near the thermostat ??
Ed"
 
"If you want to know the tempe

"If you want to know the temperature the alarm switch is sensing, point it at where it taps into the intake manifold.
There are a couple of flow paths, and temperatures will vary depending on how hot it's running and how far open the thermostat is. The engine block and intake manifold will typicaly be the hottest, since the engine temperature is regulated by the thermostat.
The exhaust manifolds and risers/elbows should be signficantly colder than the engine... more directly fed straight from the raw water pump. They are not directly regulated by the thermostat. As the manifolds and risers age and corrode, hot spots will develop on them, indicating it's time to replace them. It's good to preiodically check them in different locations with the hand held.
A hot(overheating)engine and relatively cold manifolds/risers may indicate a stuck shut thermostat. (As the engine heats up, the thermostat should open up, directing more raw water through the engine, regulating the temperature. As more water is sent through the engine, there should be a corresponding rise in manfold/riser temperature.. more water goes through the engine, then to the manifolds, instead of bypassing the engine straight to the manifolds, such as would be the case when the engine is cold.)"
 
"Okay checked the temp the lef

"Okay checked the temp the left hand riser and exhaust manifold is about 3 degrees lower than the temp reading from the thermostat area, the right hand exhaust and riser are 30 degrees colder than the temp reading from the thermostat area."
 
"replaced the riser and manifo

"replaced the riser and manifold, the temp gauge still indicates 175 when underway, and reads 175 at 4200 rpm. Manifold still is very warm compared to the other and the flow of water out of the exhaust at the outdrive is less than the port side, which I did not replaced. What is my next step, by the way the alarm did not go off this time."
 
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