1998 mercury 150 overheating

Question

my 1998 saltwater 150 efi overheats.is it normal when in low speed the temperature is about 180. and then when you go faster it goes down?

Answer 1

I have a 135hp Merc and it has been running like that since new.  The temp runs somewhat hotter when trolling and goes down when running faster.  180 is not overheating, if the engine overheats an alarm will sound.   There is also an alarm for low oil in the injection system.If either ever goes off, shut the engine down immediately.  

Answer 2

you may have a bad water pump impeller-get a shop manual, it is not a problem to replace, however, if the boat is in the water, it can be a bit problematic-that would be my first look, to be safe, replace it, however, you should be aware that most of the time with this problem it might do fine at idle, and get warm at speed-obviously, check the ports to confirm there are no barnacles or other critters plugging the intake and exhaust...good luck.

Answer 3

Yes, this is common with this engine. If it gets to hot, the overheating alarm will go off.  you can put it in neutral and rev it up for a couple min. and it will cool down.  If the problem persist you might need a new water pump impeller. Good luck!

Answer 4

If your overtemp warning is sounding, you've probably already damaged the motor.  do a compression test at least.  I'd probably just pull the heads for visual inspection to be on the safe side.  Do it now, maybe just rings, wait till later, you'll have another big black anchor on your hands.