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Wide water temperature swings at idle?

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Wide water temperature swings at idle?

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Old 07-12-2018, 07:13 PM
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Default Wide water temperature swings at idle?

1991 Cigarette 35 Cafe Racer.
Twin 540s built by White's in TN.
I bought this boat earlier this month, with a survey and sea trial, all seemed fine.
Raw water cooled (brackish in my area).
Since I've been running it, I've noticed that at idle (such as in my canal or the channels leading to/from it), the water temp gauges will fluctuate from 120-130 all the way up to 190-200 or so, which worries me. Oil temps never get above 150-160 or so at idle.
When running on plane the water temp gauges are at 100.

I just put in 2 new raw water pumps and impellers. No change in the condition.

Is this indicative of a specific condition? Is it likely an instrumentation problem, or a real fluctuation in water temps? The changes are fast - you can watch the needle move around much faster than I'd typically expect to see... and both motors behave the same way.

Bravo drives, by the way.

Today I pulled the hose from the transom assembly to the sea strainer, put the muffs on the drives, and ran the hose. No water came out of the hose leading to the sea strainers, which struck me as odd. Backflushing from the sea strainers to the drives resulted in water flowing as expected out of the water pickups on the drives. Is this a clue?

Any thoughts appreciated!

Last edited by ancho; 07-12-2018 at 07:17 PM.
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Old 07-12-2018, 08:10 PM
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I would check to see if to much water is bypassing the engine during idle. (I don't know your setup) I have made flow restrictors for my bypass. What is your water pressure at idle? Also, the way I have solved the issue you describe is to put in new 140°F thermostats with 3-4 3/16" holes in them.

Water pressure gauges are your friend...
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Old 07-12-2018, 09:55 PM
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Water pressure gauges show 0 at idle.

Currently no thermostats - restrictor plates only with 1/2 inch hole in them.
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Old 07-12-2018, 10:10 PM
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Originally Posted by ancho View Post
Water pressure gauges show 0 at idle.

Currently no thermostats - restrictor plates only with 1/2 inch hole in them.
Im no expert, but with zero pressure at idle it sounds like the water is slipping through there to fast to do much good. Radiators hold ~15# or so of pressure otherwise they wouldn’t cool much at all.

Its also possible that the pumps are weak and once the boat gets moving the speed shoves water through there. The pumps really don’t do much once you’re moving.

Last edited by Baja Rooster; 07-12-2018 at 10:12 PM.
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Old 07-12-2018, 10:12 PM
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Crossover ? Which one ?

Or factory t-stat housing ? Which one ? Where is restrictor placed ?
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Old 07-12-2018, 10:29 PM
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Surely not a water circ pump installed here > but a cross over is used here ? correct ? Whats the set up ?

Large temp swings with an ENGINE THAT HAS THE WATER CIRC PUMP INSTALLED ends up being the water circ pump is bad. But also if one is by passing to much volume of cooling water that can be the issue as well, Possible too much water pressure and its by passing alot ?

Possible for not seeing water at the sea strainer / water inlet hose --- Too much water pressure at the muffs can cause water not to flow upward and or inward. The water pressure pushes the muffs away. Try different settings of water pressure for the water hose / muff combo. Just saying as a possibility.

Will add it can be hard to get water to travel upward with the hose and muffs especially without any suction ie : engine off no impeller working. Air and gravity to over come

This is so true when running Volvo pentas latest drives on the water hose / muff combo. The water pressure at the water hose has to be just so for it to pump water . Just saying as another example what I see for running engines / drives on the water hose / muff combo . .

Last edited by BUP; 07-12-2018 at 10:52 PM.
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Old 07-12-2018, 10:39 PM
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Originally Posted by Baja Rooster View Post


Im no expert, but with zero pressure at idle it sounds like the water is slipping through there to fast to do much good. Radiators hold ~15# or so of pressure otherwise they wouldn’t cool much at all.

Its also possible that the pumps are weak and once the boat gets moving the speed shoves water through there. The pumps really don’t do much once you’re moving.
the boat does not have a radiator as it is fresh water cooled.pressure in the radiator is to prevent the coolant from boiling.simple fact is the more cool

water you move through the system the cooler the engine will be.the fresh water cooling system works by the transfer of heat into the water and then overboard..
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Old 07-12-2018, 10:55 PM
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Our boat normally shows zero at idle and bumps to 12-15 at cruise. Seems normal to me.
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Old 07-13-2018, 09:51 AM
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I just had a similar problem and turned out to be the impellor in the water circulating pump impellor broke in half! not sure how that happened but made gauges do all sorts of weird things one day.
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Old 07-13-2018, 11:58 AM
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Had a similar problem and it turned out to be a brand new Hardin marine crossover had a casting flaw and the hole inside the crossover was only about 3/8" as opposed to 1" ID it's supposed to be. They replaced it and sent a new one...engine is cold now....
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