1986 Mercury 150 Overheating

[Will_Aus_08]

Hi there

I have a 1986 Mercury 150hp Black Max. At around idle it runs just over half way on the temp guage, and when towing skiers its runs at nearly full temp. The dials have always (for at least 10 years) shown the engine running very hot when running at full throttle, but only recently it is also showing hot at around 3,500 rpm when towing a skier.

I had the impeller changed last season, and the water pressure is around 10psi at idle and 20 - 25psi when operating. The water coming out of the tell tale is strong and hot to touch, and if I touch the water jackets after operation they are fairly hot to touch but are not hot enough to burn. The alarm test sequence works when I start her up, but even when the temp gauge is showing full temperature, the overheat alarm is silent.

I've talked to a few mechanics. Some have suggested a new water pump, another says try replacing temperature sender or the gauge, and another says take off the water jacket and clean out the channels as they sometimes get residue on them which effects the temperature reading.

Does anyone have any ideas where I should start?

Many thanks

Will

 

[twostroke87]

Re: 1986 Mercury 150 Overheating

Well if you had the impellor changed at a respectable shop, I probably wouldn't hop to the impellor right away.

When is the last time you had your thermostats replaced? They one could be stuck only partially open, restricting water flow heating up the engine but only one one side, not enough to kick off the alarm.

It might be the guage, hook up a new guage and see what it reads, if it looks normal then your guage is probably bad, if its the same I'd try changing out the temp sender.

To be sure, you could get a infrared thermometer, shoot it on the engine block at different speeds, idle, cruising, WOT (wide open throttle) You shouldn't really get an exterior reading of anything more than 140 Degrees fahrenheit. If it holds steady around 150-160 you might have high temp thermostats in and I would check them, any readings showing a consistent temperature of 170-180 or more you have a problem.

 

[yamamarinetech40]

Re: 1986 Mercury 150 Overheating

This engine has temp sender on each head, yes? Should sound alarm if actually hot...you say you can touch without being burned...if you can keep fingers on top of head, near thermostats for three seconds...she ain't too hot, a gauge 10 yrs. old has served it's duty for sure...the only thing wrong to me is your reading has CHANGED at 3500....you can get wax stick at parts store(cheap) that will melt at specific temp...get a 150 and a 180 and see if 180 melts...that'll tell you if gauge is lying(and i think it is)

 

[Will_Aus_08]

Re: 1986 Mercury 150 Overheating

Thanks guys for your feedback.

I ended up just taking it to the mechanic. Turns out the waterpump casing was warped (even the mechanic said he hadn't seen that before), and also the thermostats were only opening two thirds. Thankfully she is running cool as a cucumber now!

Many thanks.