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Author Topic:   Mercury 150: Overheating
jdizzie posted 08-13-2007 05:41 PM ET (US)   Profile for jdizzie   Send Email to jdizzie  
My Mercury 150-HP BlackMax seems to be running hot. My 150-HP Mercury BlackMax has the [automatic oil mixing pump system disabled] I just bought the 150-HP Mercury BlackMax. The new water pump was installed two months ago. The temperature gauge [indicates that the Mercury 150-HP BlackMax] is running real hot. The motor didn't seem too warm, however the tell tale went from a steady stream to sputtering a little bit. I think I ripped through an oyster bed. Any help or thread referral would be greatly appreciated. Thanx,


rtk posted 08-13-2007 07:42 PM ET (US)     Profile for rtk  Send Email to rtk     
To free up the flow for the tell tale stream, run something up the outlet to clear the passage. Heavy monofiliment line like weed wacker line works. Sometimes a little junk gets caught and clogs the passage. I've used small tie wraps too.

Temp gauges are not terribly accurate sometimes. Did the overheat alarm go off? I'd go by an alarm before the temp gauge, as long as I was confident the alarms work.

The best way to tell is measure the temp on the engine with a melt stick. Pick up a manual for your engine- they typically go through the process of measuring engine temp with a melt stick. A handheld electronic temp gauge works too, just be sure to test both cylinder heads to see if temp is even on both sides- if it is a V6.


jimh posted 08-13-2007 08:07 PM ET (US)     Profile for jimh  Send Email to jimh     
In any outboard there are usually three or four components in the cooling system:

--a water pump;
--a temperature regulated flow device, e.g., the thermostat;
--a pressure regulated flow device, often called a poppet valve;
--a throttle regulated flow divertor, common in OMC engines.

If you have problems with overheating, I suggest checking these devices, and in the order listed above.


The fundamental source of engine cooling is the flow of water from the water pump. If there is any deficiency in the operation of the water pump there will be an increase in the tendency for the engine to overheat, particularly at high speed operation where the greatest amount of cooling is needed.

Outboard motors use raw water cooling, and because they have a literally unlimited source of relatively cold cooling water, the designers of outboard motors try to employ various methods to raise the engine operating temperature into a reasonable range, typically around 165-degree-F. At lower speeds the supply of cooling water will generally be too great, so some restriction is employed to reduce cooling water flow in order to permit the engine operating temperature to rise. However, at higher engine speeds, the restrictions in the flow of cooling water must be removed to prevent over-heating of the engine.

The modulation of the flow of cooling water is based on three parameters:

--water temperature
--water pressure
--throttle setting

The THERMOSTAT is a device in the cooling system which opens or closes in response to the temperature of the water in which it operates. Typically a thermostat is closed at lower temperatures, then when the water rises in temperature the thermostat opens. The flow of water is usually arranged so that when the thermostat opens more cooling water flow is provided. If the thermostat become inoperative and stuck in the "cold" mode, it can cause overheating in the outboard motor.

The POPPET VALVE is a device in the cooling system which opens or closes in response to the pressure in the water cooling system. Generally the pressure in the cooling system increases with increasing engine speed. The poppet valve--a name which really just describes the shape of the valve and not the fact that it is associated with a pressure controlled response in the cooling system--is usually arranged so that when the pressure rises the valve opens and permits greater flow of cooling water. If this device is stuck in the "low" pressure mode, it can cause overheating in the outboard motor.

The THROTTLE VALVE is a device which is linked to the throttle control input and senses the position of the throttle. When the throttle is advanced the flow of cooling water is increased. This is a technique used in certain OMC motors, and it may not be applicable to your Mercury 150-HP BlackMax.

There is also a possibility that there are other restrictions in the cooling system which are reducing the flow of cooling water. Engines which operate in saltwater may have deposits of salt or other minerals built up in particular places in the cooling system. This is also possible in freshwater motors, but not as common. Check your cooling system for obstructions.

Most outboard motors have an aspirator which functions to expel air from the cooling system and also to act as an indicator of flow. These are commonly called "tell tales" or confidence streams (as well as other vulgar names). Usually these aspirators include a nozzle to expel the water, and you can make some judgment about the function of the cooling system by the volume and strength of the stream of water expelled at the aspirator nozzle. However, it is very common that debris can block the nozzle path and reduce the output, and this may give a false indication of reduced flow when in fact the system is operating normally save for the restriction in the path to the aspirator nozzle.

jimh posted 08-13-2007 08:15 PM ET (US)     Profile for jimh  Send Email to jimh     
According to experienced Mercury mechanics, there is a tendency for certain Mercury motors to use a rubber vane impeller which has a tendency to have poor resiliency in the rubber compound, and these Mercury impellers need frequent replacement in order to maintain the cooling system of the motor at full capacity. It is not unusual to need annual replacement of these rubber impellers.

To monitor the operation of the cooling system of any outboard motor it is strongly suggested that a water pressure gauge is installed and monitored. In this way the cooling system of the motor can be monitored and evaluated.

Another instrument for monitoring the cooling system is a temperature gauge. The operating temperature of an outboard motor is typically quite variable, and engine temperature will change with engine speeds and load. A temperature gauge is a good way to develop an index for normal operating temperature ranges, and, when departures from the norm are noted, problems with the cooling system can be detected before an overheating condition exists.

jimh posted 08-13-2007 08:17 PM ET (US)     Profile for jimh  Send Email to jimh     
I do not believe that there is any particular link between the overheating of the motor and the disabling of the automatic oil mixing system. If you are giving the motor a fuel with the proper ratio of oil and gasoline (1:50) it should not operate any differently in terms of its temperature range than it did when the automatic oil mixing system was working.
jdizzie posted 08-13-2007 11:06 PM ET (US)     Profile for jdizzie  Send Email to jdizzie     

Thank you jimh for the wealth of information.

It was stated very clearly and to the point.

I'm gonna review it, take that cowling off the engine and see what I can see.

Supposedly, the water pump is 2 months old, I have to wait until tomorrow to receive the 2 new thermostats.

I also need to speak with the mechanic who recently serviced the engine.

Tomorrow morning I should have some more information.

I purchased a pair of ear muffs to see if I could get a stream out of the "tell tale".
Stuck a paper clip up there and no luck.

There was water pouring out of the cups however....Should this happen?

That engine seemed to be running mint to me when I ran it on saturday.... not a single hiccup.

Only the temp gauge screamin and water "sputtering" out of the tell tale.

Try again tomorrow.
Thanx again.


jimh posted 08-14-2007 08:19 AM ET (US)     Profile for jimh  Send Email to jimh     
The location in the cooling system of the aspirator varies from engine to engine and by brands. On most Mercury engines I have seen, the flow to the aspirator comes after the water has circulated through the engine. Therefore when you get water from the nozzle it is usually fairly warm, and it takes a fair amount of time for the water to appear after the engine is initially started. The engine might have to run for a full minute before any flow appears from the nozzle. The cooling system passages have to fill with water before you see any flow from the nozzle. On some other brands of engine the aspirator is located in a different position and can produce a stream from the nozzle much sooner after engine start. I had some Yamaha engines, and there would be water from their nozzles almost immediately after the engine was started.

When you run the engine on a hose adaptor ("ear muffs") it sometimes happens that the flow volume and pressure of water in the cooling system is not as great as would occur if the engine were being operated in its normal position with the water inlets submerged. The flow from the nozzle is accordingly reduced and may take even longer to appear. Instead of a spray of water from the nozzle you may only get a dribble. Exactly what you're going to see depends on the hose adapter, the volume and pressure of the water delivered, the particular engine, and how well its water pump is working.

The hose adaptor should cover the water inlets on the gear case. It is typical that not all the water delivered by the hose is taken in by the pump, and the excess water will be expelled around the loose seals of the hose adaptor. Generally these hose adaptors don't supply enough cooling to allow the engine to be run at any speed except idle. If you run the engine at higher speeds on a hose adaptor it will likely overheat due to lack of cooling. Usually when the engine is being run at high speeds the gear case is moving through the water, and the pressure of the water on the water inlets creates a very generous flow of water into the cooling system. This helps the pump deliver sufficient cooling water flow.

The amount of water that comes out the aspirator nozzle also varies with engine brands. Some engines just produce a little stream of spray, and I think Mercury is in that category. Other engines pour water out like a faucet, and my OMC V6 is in that category. You just have to get a feel for how your engine behaves, both when running on a hose adaptor and when in normal operation.

Another reason for not running the engine at any speed above idle when using a hose adaptor is the absence of any back pressure on the exhaust system. Normally the engine exhaust is under water and has to work against the pressure of water on the outlet of the exhaust (via the propeller hub). With a hose adaptor the whole exhaust system is open to the atmosphere and there is very little back pressure. This affects how the engine runs.

The temperature gauge sender or the overheat alarm sender is usually located on the top-most cylinder because these typically run the hottest; they are the farthest away from the water pump. If there is any problem in the cooling system the top cylinders will typically be the first to experience lack of cooling.

The suggestion of using a melt stick to determine the engine cylinder head temperature more accurately is a good one and a time-honored technique, however I wonder if these days mechanics don't also use an infra-red thermometer gun. I have one of these handy devices, and it will show you the cylinder head temperatures with decent accuracy, at least more accurately than the electrical temperature sender and gauge. This way you get a second opinion on the engine cylinder head temperature.

Even though you just had the water pump serviced by a mechanic, this is not a 100-percent guarantee that it is working perfectly. The water pump has several components in addition to the rubber vane impeller. These usually come in a water pump "kit" which includes seals, washers, wear plates, and even in some cases a new housing for the pump. If only the rubber vane impeller was replaced, the pump may not be working at full volume, and you may need the whole kit replaced.

SeaRayer posted 06-02-2009 02:16 PM ET (US)     Profile for SeaRayer  Send Email to SeaRayer     
My Merc 150 BM is overheating I think....Just spend a week in saltwater and flushing the motor with a water bag instead of earmuffs. Water is flowing from the tell-tale but gets extreemly hot after a couple of minutes. Is this a concern??? I dont have a temp sensor installed but can check engine temp with a laser. What is the desired temp range at idle???

Thanks for the help...


Tohsgib posted 06-02-2009 02:26 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
hardrunner posted 07-10-2009 11:25 AM ET (US)     Profile for hardrunner  Send Email to hardrunner     
I'm chasing an overheating problems too. I have a 2000 Mercury 175 EFI on a bassboat fished only in freshwater. This motor has never been anything but a runner. Every year on it's birthday,(Feb.) the water pump/impeller, lower unit lubes and new spark plugs are serviced and installed. All of a sudden my "water pressure gauge" pegs out. It use to idle around 8# to 10#, once it was up and running it use to top out around 30#. Now I'm getting an overheating alarm. Like I say, It has a 4.5 mth. old impeller. I have good water from my tell-tell hole, I put all new hoses on it. If I run it at 50 mph for a mile the alarm will go off (that's running 4200 rpms) stop and let it idle and the alarm goes off. Placement of the motor hasn't been a problem all these years. I know there are several items to consider so where must I go from here. Thanks everyone for your help.
gss036 posted 07-10-2009 12:55 PM ET (US)     Profile for gss036  Send Email to gss036     
Check or rebuild the poppet valve as that controls the watwer flow through the engine.
GolferFisherman2 posted 07-26-2009 01:47 PM ET (US)     Profile for GolferFisherman2  Send Email to GolferFisherman2     
I also have an overheating problem at very low speeds, pulling a snorkeler, with moter engaged in gear and about 700 rpm the water pressure now runs at about 1 or 2 PSI, I have been pulling snorkelers in the past all the time without any problems and the PSI would be about 6 PSI. I have a 2004 150 Mercury with 600 hours that I purchased new and changed water pump every 12 months. This over heat problem with the overheat alarm going off has just started the last two times I had it out, the temp gage also said that it is hot. I have changed all parts of the water pump including housing, new thermostats and hoses. Today I connected my raw water wash down pump hose to the back of the motor (flush connection) this increases the PSI on the gage that comes out of top or motor to 6 PSI, The motor runs cool where the temp should be and the discharge also noticeable increases in volume. Any suggestions?
msl posted 07-27-2009 08:03 AM ET (US)     Profile for msl  Send Email to msl     
Sounds like a loose pop-it valve. Pull the housing and remove pop-it valve there is a screw that may have backed out of the pop-it valve. If this screw has backed out there is no resitance, which could be why your water pressure is so low.

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