DT-40C Overheat Mystery

[hunkojunk1]

Have a 1987 DT-40C, Oil injection, No trim.
Motor starts & runs like a champ.
At idle & low RPM overheat warning sounds & excessive white smoke.
water flow at port looks ok.
Have changed thermostat & impeller looks good.

will run at high rpm all day with no problems.
I do not have any specialty tools so not sure what to check next?
Does anyone know the proper weight & brand of oil?

[boats.net]

I would put a new water pump impeller in it and inspect water pump housing closely to see if melted or distorted.

[JUST-IN-TIME]

that or water pump grommet

also clean the poppet valve