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Old 07-03-2019, 02:15 AM   #1
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Default 1998 3000SCR alarm Temp switch

Twin 5.0 Merc Cruiser carb engines alarm goes on for a second on starboard engine during high RPMs Temp gauge reads about 175 the sending unit by thermostat housing but where is the temp switch for the alarm?
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Old 07-03-2019, 12:50 PM   #2
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Twin 5.0 Merc Cruiser carb engines alarm goes on for a second on starboard engine during high RPMs Temp gauge reads about 175 the sending unit by thermostat housing but where is the temp switch for the alarm?
The alarm temp switch is in the thermostat housing. Have you checked the gear lube reservoir? Common for it to be low and cause an alarm when on plane.
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Old 07-03-2019, 08:56 PM   #3
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Mike thanks for the reply,
Yes both gear lube reservoirs are full, oil pressure gauges read on the high side, but I have not checked oil pressure with a service gauge.
I ran just the starboard engine in the slip and brought the RPM up over 3K and the alarm sounded, temp gauge reading about 175 F.
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Old 07-03-2019, 09:19 PM   #4
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Default IR photo attached

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Old 07-03-2019, 09:25 PM   #5
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This photo was not taken at higher RPM, but warmed up at idle.
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Old 07-03-2019, 09:27 PM   #6
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While 175* is about 10* high it is not high enough to trigger the temp alarm. I believe it is set to sound at 188*.

Have you changed the impellers recently ?
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Old 07-03-2019, 09:44 PM   #7
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A few seasons ago replaced impellers but not many hours on them.
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Old 07-03-2019, 10:42 PM   #8
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If by a 'few seasons' you mean 3 or more years it's time for new ones.... They take a set when the boat is not used for long periods of time and the volume of water they move is diminished.... Worst thing you can do to a boat is not use it.

May not solve your alarm issue but it's time to change them.
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Old 07-04-2019, 12:22 AM   #9
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According to the scale the red looks to be in the 160’s but white is 173 or above and there is something white on the left side of the scan, what is that?

Have you tried disconnecting the alarm temp sensor to see if that shuts the alarm off? If not the next to try is the reservoir wire as sometimes the float sticks.
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Old 07-04-2019, 12:14 PM   #10
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The white on the photo is a tube that runs from the intake manifold to the exhaust manifold. Looks like the hottest in the picture
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Old 07-04-2019, 03:11 PM   #11
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Quote:
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The white on the photo is a tube that runs from the intake manifold to the exhaust manifold. Looks like the hottest in the picture
It looks too hot, can you post a regular picture of it.
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Old 07-06-2019, 02:56 PM   #12
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Mike
Here is a photo of the tube that goes from the intake to the exhaust manifold.
It is hotter than the temperature at the thermostat housing.
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Old 07-06-2019, 03:30 PM   #13
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Looks like time for a new hose... that one is in terrible shape.
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Old 07-06-2019, 05:53 PM   #14
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Interesting, not sure I’ve seen this before nor know it’s purpose. There are no clamps on the ends if this covering another hose inside?

What is you engine serial number? I would like to do a search on it to see if the parts list shows this hose.
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Old 07-06-2019, 08:26 PM   #15
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The tube is copper with a sleeve covering it. Engine S# 0L052606 I believe the p/n is 861213 found under the water pump and front cover category
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Old 07-06-2019, 09:09 PM   #16
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I agree that seems to be it. I looked in the Mercruiser manual and cannot find it. Seems it would either by-pass the thermostat letting water out of the intake or it takes water from the exhaust manifold to the intake to preheat the carb area. Being a 1998 it should have the 8 bolt intake that only has a water passage at the front of the intake carrying coolant from the heads to thermostat housing.
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Old 07-07-2019, 10:17 AM   #17
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I've always called it a preheat tube. Mechanic I used at one point removed it and plugged both ports when the manifolds were serviced.
The 'hose' is actually a protective sleeve/ heat shield.
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Old 07-07-2019, 11:48 AM   #18
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IMO based on the thermal photo your temps are ok. I would disconnect the alarm sensor for that engine to ensure it’s what triggers the alarm.
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Old 07-08-2019, 12:06 AM   #19
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Update:
Disconnect the starboard temperature sensor and alarm stopped sounding, replaced the sensor with a new one and it went off again?
I had replaced both the temp sender and sensors on both engines, both gauges readings 175* only the starboard alarms. I purchased new impellers will change them next weekend.
Wondering if that “pre heat tube “ is causing the engines to run warmer?
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Old 07-08-2019, 02:03 AM   #20
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Quote:
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Update:
Disconnect the starboard temperature sensor and alarm stopped sounding, replaced the sensor with a new one and it went off again?
I had replaced both the temp sender and sensors on both engines, both gauges readings 175* only the starboard alarms. I purchased new impellers will change them next weekend.
Wondering if that “pre heat tube “ is causing the engines to run warmer?
Don’t both engines have the tube? Where on the engine is the sensor located, thermostat housing? If so that area read cool in your scan. Just for grins try using a jumper wire from that starboard sensor to the port alarm harness and vise verse to see what happens.
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