How does the motor cooling process work on a 1996 20 hp Mercury outboard.

Question

I just got a 20 hp mercury outboard that when idling it will only spit water out the telltale for a while then spray a full stream for a while then only spit for a while and then repeats the cycle.

When in gear and running it sprays a full stream.

Is this normal?Is this cooling controlled by a thermostat and that is how its supposed to act?

Any info is appreciated.

Answer 1

right, bill -- I replace my impeller every two or three years as a preventive, it's the only part I replace without any indication of malfunction.

If you've been in the habit of starting your engine dry, replace it before your next outing.  Only takes about 30 seconds of dry running to damage the impeller.

Since your overtemp horn hasn't sounded, you're probably ok, but if you've never replaced the impeller then you don't know how much water it pumps with a brand new one.

A fairly simple operation but the consequences of failure are disastrous.

Answer 2

the "water pump" is a rubber impeller mounted in the prop shaft housing. it pulls water in through ports and pumps it through the motors cooling jackets. what you are describing sounds pretty normal as my 115hp johnson does the same thing and i don't have any cooling problems.just make sure that you flush the motor with a water hose and "ear muffs" every time you take the boat out of the water. this is especially important if you run in salt water. the "ear muffs" can be purchased at any marina or boating supply store(even wal-mart) for about $20.

Answer 3

Sounds like as you increase RPM's the water flow increases.  This may not be a problem. Your cooling is controlled by a thermostat, but the telltale is before the thermostat.

But if it has been a while since you have replaced the impeller in the water pump you may want to try that. The rubber could be weak and not moving as much water as it should at idle.   Not a hard thing to do and not expensive.