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TOPIC: 5.7 merc overheat alarm once then runs ok

5.7 merc overheat alarm once then runs ok 18 Aug 2017 05:59 #1

  • Big Ben
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Ok so I have a strange one. First off I have a 2004 5.7 merc, carb, full closed cooling with San Juan kit.

And I'll get this out of the way - its a long story (over 9 years now so not new) the gauge on the dash reads 20-25 F hotter than the engine. Gauge always reads 175F and I run a 140F t stat so the engines true running temp is 155 F or so but dash says 175 F.

So I has always run perfect at 175f (on the gauge) till the other day. Now every time I start it, it will once climb up to 200F on the gauge and alarm goes off and then it quickly cools down to 175 and stays. I can be a low idle at the dock, hi idle at the dock or going 5 knots down the channel out from my slip. I don't have to pull back on the throttle or change anything just watch it heat up, alarm and then cool off and run ok. The other day I had 750 lbs of men on board, we started off down the channel, alarm went off, then cooled down, I then worked it hard on plane for the next 20 minutes out to our fishing spot and it ran a perfect 175F. It alarmed once when I restarted the main engine to come back in again after it warmed up.

So the gauge sensor and alarm sensor are different which tells me this is probably a mechanical thing not electrical. They are both 2" to the right of the t stat housing in a fitting that has a heater hose for the hot water heater and cabin heater take off.

Using my temp gun at low idle at the dock
T stat - 155F perfect
2" left of t stat - 155 perfect
2" right of t stat - 180-185 f when overheating then cools to 155-160 (remember my gauge reads 20 higher)
exhaust manifolds - 160 F - perfect
heat exchanger inlet - 160 F perfect
heat exchanger outlet - 90 F - perfect.

Every temp is right on except the R/H side. I thought maybe I had a lazy t stat the way it climbs and then drops and runs good.

Done so far - change t stat to a new 140F, change dash gauge (just because), change alarm sensor, pull raw water pump (perfect but put a new impeller in anyways), back flush p/s cooler, scrub and feel leg water intake with my hand in the water, bled and bled the system trying to find any air -none. No difference. With the heater take off out and t stat out I ran my fingers between the 2 and had wires shoving in and out looking for blockages - none.

So do I have a "hot" bank on the RH side ? Why does it climb then drop and run good every time I start ? I only run for 20-30 at a time to get out to my fishing grounds so I haven't done lots of testing with it other start, alarm, cool off and work good for the next 20 or so. Even at low idle at the dock it will alarm and then just come down and settle at proper temp. At no point can I find the t stat housing climbing in temp - just beside it where the heater/alarm/gauge sensor take off is the only spot I can get a hot reading

Anyone else ever have this ?

I attached 2 pics - one showing the heat take off with the sensors beside the t stat. The second shows my take off - coolant in the bottom and out the top. Alarm sensor on the left and gauge on the right

Thanks,
Dave

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Dave
Nanaimo, BC

2005 242
5.7/Bravo II
Last Edit: by Big Ben.

5.7 merc overheat alarm once then runs ok 18 Aug 2017 16:19 #2

  • Pcpete
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I had a similar problem lots of years ago and it seems to me that it was a bubble or void in the sensor "tower". I think I ended up replumbing the heater takeoff/return but I don't remember how. Sort 'bout that part.
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P/C Pete
Edmonds Yacht Club (Commodore 1993)
1988 3818 "GLAUBEN"
1980 Encounter Sunbridge "Misty Blue" (Sold)
MMSI 367770440
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Boating on the Salish Sea since 1948

5.7 merc overheat alarm once then runs ok 20 Aug 2017 05:47 #3

  • Big Ben
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Thanks Pcpete, I'm hoping its something like that. I bought some different fittings but they are to tall and hit my heat exchanger so I will regroup and try something else. I will let you know if/when I get it figured !

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Dave
Nanaimo, BC

2005 242
5.7/Bravo II
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