Outboard Running Cool

[Sabre]

Hi Guys,

I have a 97/98 model Johnson "Ocean Runner" fitted to my Bowrider. My question is, it seems to run very cool....When just idling and not underway it sits at around mid temp (about 50 - 60 degrees), yet when underway it drops right down to the same temp as the water. It had a new impellor put in about 30 hrs ago, and would have one of the strongest telltales I've had on any motor.

Logic would say it's better to run cooler, but I've been told it's actually a bad thing.....Anyone have any thoughts?

[BaitThrower]

Is there a thermostat actually in the motor? It may have been removed which would cause it to run cool and allowing water to always circulate around the heads...

It is of benefit to have an outboard running within a specific temperature range because combustion is most efficient within this range, and this is what the thermostat controls. Too cool and it is not operating efficiently and might result in poorer performance or more fuel usage etc.

[nigelr]

Too cool will lead to improper fuel burn and carbon formation, 2 strokes like to work pretty hard.
Could be a faulty temp. sender unit, or gauge. Might be worth fitting another sender unit it you can get one, and see if the gauge reads the same.
Try not to be too gentle on the motor, it can do more harm than good.
Cheers.

[PinHead]

why is running a motor too "cool" harmful to it ????

[nigelr]

Improper oil/fuel burn leads to carbon formation, usually burnt off during usage at normal operating temps.
Cheers.

[PinHead]

at cooler temps the air is more dense hence more air for combustion..therefore that will lead to more efficient fuel burn at lower temps.

[nigelr]

Just going on my own observations and experience.
Cheers.

[GBC]

I had an ocean runner the same age as yours - Its is three boats ago now and the memory is a bit fuzzy but I think it used to do the same. Was never an issue.

[Noha]

running to cool will also glaze the bore from what i have been told if the thermostat has been removed

[BaitThrower]

at cooler temps the air is more dense hence more air for combustion..therefore that will lead to more efficient fuel burn at lower temps.

Dont think it works that way entirely... otherwise why would outboard manufacturers even bother to put in thermostats to begin with?

"The two things that need regular maintenance or may cause problems are the impeller and thermostat. The impeller will be the cause of 95% of outboard overheat situations. It is rare that a thermostat goes bad, but it does occasionally happen. If you find that a thermostat is bad, do not simply decide that you can discard it and operate regardless, like some would with an automotive application. The thermostat of an inboard plays a very crucial role in warming the engine to a satisfactory operating temperature. If an outboard is operated without a thermostat installed, it will surely suffer in some way from poor performance all the way to causing internal damage. An outboard is designed to operate at specific engine temperatures, and if it does not obtain that temperature, it will run very sluggishly, and also cause fuel/oil mixture to not totally burn, which causes excessive carbon buildup. Carbon buildup in an outboard is one of its worst enemies. This condition usually most often causes problems with the piston rings becoming stuck in the piston ring grooves. If this happens, you can pretty well kiss your powerhead goodbye, because it most certainly will allow the piston to touch the cylinder wall, and immediate scoring will result. Once a piston/s is scored, it is only a matter of time before the engine will self destruct unless it is caught in time to perform an overhaul."

From http://www.boatpartstore.com/obcooling.asp

I have read this in many other texts as well.

[Spaniard_King]

why is running a motor too "cool" harmful to it ????

Pinhead,

the modern outboard is made with stringent tollerances in mind, if the engine never reaches its correct operating temp then the correct tollerances will never be acheived (primarily piston to bore clearance) this will cause excessive wear due to the slop being allowed. Also correct combustion temperature will not be acheived and soot/carbon build up will be detrimental to the engine in some form or another. I had a honda recently which had carbon fouled spark plugs, this was due to a combination of poor fuel and a cold runing engine!

Hopefully its all good now! (hey Dale ;) )

[Sabre]

Thanks for the input. I'll look into it further and see what turns up.......