mercruiser varying temps under cruise

[Rolf Latham]

Hiya

previous reponses to requests for help from this site have been very fruitful......fixed an overprpopped engine and managed to get overheating fixed too.

Now I have a recurring problem that might be normal though looks suss.

On plane at 70 % throttle, the temp gauge goes from 155 ish to zero, and then goes back to 175 and then settles at 155 again, then drops to zero again. She might go a minute or 2 at 155 then it happens again.


Techo data


The temp gauge is correct as confirmed by IR thermometer

2355 Bayliner, 5 l carb merc, gen 2 drive, 15 inch prop, raw cooled

New mans, risers, and 3 inch spacers, changed 160 stat to 140,


Im about to take her into dry dock for a week and get the raw water pump looked at again, though it had a new genuine total pump fitted about 10 mths ago.



Anything else that we should check while out of the water please ?

ta
rolf

 

[CharlieB]

Re: mercruiser varying temps under cruise

You actually have two separate problems.

One the engine temp fluctuation, possible causes, poor quality T-stat, loose water pump belt, warped raw water pump base.

Intermittent temp gauge, poor contact at sender, poor contact at gauge, poor contact at main engine bayonet connector, defective sender, defective gauge.

I'm confused but your statement
"The temp gauge is correct as confirmed by IR thermometer"
Does this confirm the temp swinging from 155 to 175?

The T-stat should be full open, on full plane the raw water pump should be at a steady height relative to water line, check the belt tension.

 

[Rolf Latham]

Re: mercruiser varying temps under cruise

Thanks Charlie

I will remove the sensor and toss it into a cooking pot on the boat and compare the boat gauge temps to what the IR thermo says.

The variance under way on the gauge is 100 to 175, ...............scary yes !


Stat is brand new genuine 140 cfand works ok as tested on my stove top with IR thermo

Belt tension on the single belt is ok, double checked when I just replaced the recirc pump.

More and more I suspect a seawater pump OR related hose into boat problem, even though ALL pump bits are less than 1 year and 40 hrs old.


ta
rolf

 

[paulrfrancisco]

Re: mercruiser varying temps under cruise

Is it possable that the seawater pickup hose, the one from the pump to thruhull is collapsing? It should be a wire reinforced hose to prevent collapse...

 

[Rolf Latham]

Re: mercruiser varying temps under cruise

thanks Paul

That may be the problem

Will advise when I get her out of the water next week..........all excited to get her fixed and running properly again.........despite the price of fuel at the moment

ta
rolf