impeller has been changed out 3 times. never start dry. is on 19 ft 2001 trophy. has always seemed to have had this problem. bought it new.
is ''peeing'' while running good steady stream.oil is automatically mixed with fuel.will the motor shut off when reaches a certain ''overheated'' temp? or blow up? what happens?

there is a seal on the tube from the impeller housing that may be the cause- or possibly a blocked intake or passage- there should be water coming out the exhaust or discharge port while running- also possibly the thermostat

it could be too little oil

No experience with outboards, but 2-cycle engines overheat when not lubricated enough, i.e not enough oil in the gas mix. Any lubrication has to come from the fuel, which is a mixture of gas and oil.

Are u mixing the oil with the gas in the right ratio? Some outboard motors have water that run through them that dont run them right, if there,s a spot on the side where the water is suppose to come out & its not coming out with the right frequency. Also r the prop blades balanced or have they been bent, this can cause an engine to over heat. And also the fact u have changed the impeller three times (you forgot to say how long u have had the engine) might show u the problem, what is wearing the impeller out. Maybe not the right size. If u live anywhere near wilmington California theres a place called minmar where the ladies there can fix any out board motor( on anaheim near santa fe) Only other thing l can think of is ur boat has to much weight with the equipment in it or something for ur motor.

No it only telling you something is wrong> First off you need to know if the engine overheating or just the alarm going off> see if mercury had a problem with them>Pull out the thermostats and see if the defective>

l dont know what year you have but this is the cause for over heating
1st water impellor[you said you changed that]
2nd thermostats you have 2 of them
3rd popit valve
but l would say change your thermostats

the pee stream only indicates that the water pump is pumping - it does not tell u that water is circulating the powerhead.

When the overtemp alarm sounds, the motor is too hot right now. Piston meltdown could occur within a minute or two if not sooner. Shut it down within 5 or 10 seconds, u avoid damage. Otherwise..well, a compression test will tell the story.

Apart from water pump, thermostat & poppet valves, other causes are:
water inlet screens blocked (motor wo not pee).
water tube blocked (motor wo not pee).
blockage in water jacket.
inadequate oiling.
lean mixture (more air, less fuel - partially clogged or marred hi-speed jets if carbureted)
timing incorrectly set.

you've been overheating for 7 years & never took it in for warranty service?

it may verywell be the cap on top of the oil injection tank. Check this cap for cracks or any other obvious flaws. It is pretty common for these caps to crack and sound an alarm l had an 02 with the same problem.