40 HP pees but still overheat alarm at speed

[airshot]

Perhaps a bad alarm ??

 

[trackerpro91]

Will replace the pump/impeller and check, first. I'm certain it needs replaced, after sitting for so long and minimal use last year.

 

[trackerpro91]

....after reading all this good advise and watching the youtuber

 

[trackerpro91]

I have read some recommendations that you put a small amount of non-petroleum based lubricant on the impeller to help seat it into the housing and "pre-lube" it for first start. Anyone know of any they like? Initially, I thought of using a small amount of olive oil.

 

[DaleH]

Water works …

 

[Pappy]

Any grease will work. Just apply a light film with your finger. It will wash away once you run the engine. Try not to cover the tiny holes in the top of the impeller housing. Those are there to purge air from the pump.

 

[trackerpro91]

Just to clarify. This engine does not depend on the thermostat much past idle. The engine is heated by the thermostat at idle to promote good idle running characteristics and keep the engine dry internally while running in colder water. The engine is not thermostatically controlled throughout the RPM range. The thermostat cannnot flow anywhere near the amount of water needeed to cool the engine on plane to WOT.

Ok- ignorance showing (again); isn’t the overheat alarm based on some sensor saying it’s too hot? Would that not be a thermostat?

 

[MrGiggles]

A 20+ year old water pump seems far more likely than a somewhat rare head gasket failure, Occam's razor and all. Some of those Mercs especially have a tiny water pump that is hardly enough when it's in good repair.

Try just seeing if the powerhead is actually hot when it's alarming, it should be hot to where it hurts to hold your hand on it for more than a few seconds, but not hot enough to burn you instantly. Can also shoot it with an IR temp gun if you have one.

While you have the lower unit off to service the water pump, remove the thermostat and flush the powerhead from the top down with a garden hose. Also remove the grates from the lower unit and make sure that passage is clear.

 

[Robert Smith]

Cold comp test, several times, yielded compression of 125 and 120 for the top and bottom cylinders, respectively. Not bad for an engine that age.

Check your temp sensor also check for sand buildup in block

 

[GYPSY400]

Ok- ignorance showing (again); isn’t the overheat alarm based on some sensor saying it’s too hot? Would that not be a thermostat?

Technically it's a temperature switch... it will either open or close an electrical circuit at a determined temperature this could possibly be the issue as it may be opening or closing at a lower temperature. I'm sure Pappy will chime in on the specifics of it.

A thermostat is used to " control " temperature... you don't have a overheat alarm to control your house furnace do you? Lol

 

[Crow]

Hello! I bought a "new to me" '91 Bass Tracker Pro 17 a few weeks ago.
QUESTION: Can anything other than a blown head gasket cause the overheat alarm to go off when the throttle is increased past "neutral +" speed?
BACKGROUND: Please bear with me, the background will help. It is a '91 Johnson 40 HP (OMC). The boat was kept in a garage and unused by a widow for over 25 years. Very little use on the motor, but it is over 30 years old (that plastic tip on the manual motor assist also broke). The first use, I took it to the river, warmed it up at slow speed for a bit and opened it up slowly. No problems. Next week, took it to Joe Wheeler with the kids. AM ride no problem. PM ride pulled my son on a tube (He is 130 wet). During the constant throttle changes, it eventually cut off for a minute. Started back fine. Would cut off if throttled too high. Next day, the overheat alarm started sounding when motor put on higher load. No alarm at slow, slow speed. Saturday hooked up motor to hose with attachment. Boat pees fine. Did not throttle high. Sunday went fishing and alarms at any speed past slow, slow. I have the SELOC manual, and it says this situation may be a blown head gasket. So, before I break this down, does anyone have any other thoughts?
Thanks in advance!

if warning horn sounds continuously at near or full throttle but engine speed is not affected and If the warning horn stops as engine rpm is reduced, a fuel restriction is indicated. Only when warning horn does not stop when rpm is reduced is an engine overheat indicated.

 

[Pappy]

Ok- ignorance showing (again); isn’t the overheat alarm based on some sensor saying it’s too hot? Would that not be a thermostat?

Nope. A thermostat is a device that opens and closes as a result of its temperature. It is a restrictor of sorts.
A sensor (in your engine anyway) is actually a sender which is a device that internally creates an electrical ground based on temperature and will complete an electrical circuit and activate an alarm. In your engine you will find the overheat sender on the cylinder head and it can be identified by the tan colored lead exiting it.

 

[Pappy]

I re-read this thread.
You mentioned the alarm beeping. Is it beeping every second or two or is it a constant alarm or is it every 20 seconds or so?
This makes a big difference so please be clear.

 

[airshot]

I have read some recommendations that you put a small amount of non-petroleum based lubricant on the impeller to help seat it into the housing and "pre-lube" it for first start. Anyone know of any they like? Initially, I thought of using a small amount of olive oil.

I keep Dow 111 vave lubricant on hand for a variety uses around my place.. It is a high grade silicone grease and is perfect for use on rubber/ neoprene and works perfect on pump impellors. I also use it as an electrical coating as it is a diaelectric grease. Make my trailer wiring connections waterproof and corrosion resistant. Just a suggestion here !!

 

[thill]

Pappy is giving you very good advice. Replace the impeller. Lightly grease it before installing. Regular marine grease or even Vaseline works well. For a Mercury, I strongly recommend you replace the gaskets around the water pump. Mercs are finicky. Take it to someone if this is out of your depth, mechanically.

This is very likely your problem and the solution.

WARNING: Don't run or play around with it too much before you change the impeller. If it gets too hot just once, you could destroy your engine. Take an overheat very seriously.

I hope it goes well for you.

 

[David Atkins]

Hello! I bought a "new to me" '91 Bass Tracker Pro 17 a few weeks ago.
QUESTION: Can anything other than a blown head gasket cause the overheat alarm to go off when the throttle is increased past "neutral +" speed?
BACKGROUND: Please bear with me, the background will help. It is a '91 Johnson 40 HP (OMC). The boat was kept in a garage and unused by a widow for over 25 years. Very little use on the motor, but it is over 30 years old (that plastic tip on the manual motor assist also broke). The first use, I took it to the river, warmed it up at slow speed for a bit and opened it up slowly. No problems. Next week, took it to Joe Wheeler with the kids. AM ride no problem. PM ride pulled my son on a tube (He is 130 wet). During the constant throttle changes, it eventually cut off for a minute. Started back fine. Would cut off if throttled too high. Next day, the overheat alarm started sounding when motor put on higher load. No alarm at slow, slow speed. Saturday hooked up motor to hose with attachment. Boat pees fine. Did not throttle high. Sunday went fishing and alarms at any speed past slow, slow. I have the SELOC manual, and it says this situation may be a blown head gasket. So, before I break this down, does anyone have any other thoughts?
Thanks in advance!

Everyone has given great advise, is the pee water hot or cool? Temp sensor may be going bad since sitting for a while but without knowing the water temperature it is unsure. If the water is hot then I would check the engine's thermostat as it may be partially blocked or not fully opening up when engine is warm. Things like this take a moment to run down the list, think of the engine as a car engine as far as the cooling water system only it pees out the water.