500 efi only over heats 1/2 up the canal from a day of boating. The port mot. did this a bit ago and I checked everything and even rep. the raw water pump housing and imp. rep. the thermostate and flushed everything. Still did it. so I changed the cerc. pump and that fixed it. Now the star. mot. doing the same thing. So I went through every thing and changed the cerc. pump and still doing the same thing..
It will go from 150 to 200 deg. and drop back to 150 deg then go back to 190 deg and drop. If I rase the rpms it drops faster. Checked for air leeks on the suc. sid of the raw water pump .everything looks good W.T.F. over!!!!!!!!!!!

Have you checked the temp with a probe or a heat gun? Could be lying to you.

Im going to do that today.

check the oil cooler could be debris stuck in it

Start simple, check the thermastate. Oh and water pressure, could be impeller.

Done everything you can think of been boating in the gulf for years .
Ran out to the bay and the temp would go from 150 to 195 deg back and forth .at idle Ran 25 miles to the isands and on the way back we ran in to a fountain and ran the **** out of it. never over heated again.

di you have to run that hard to keep up see if you had a good tech there he could fix it

Im as good as anyone. But this one just was ****ed up.
I had Teeter and Big john with me and thay never seen anything like it.

I second this notion!
Do not just rely on the fact you back flushed it!

Mine had this problem several times, any running under 3500 was just fine.

Last winter I took every hose/part of the cooling system
Found major pluggage in the oil cooler holes!
Only have 5 open holes.

Took many hours and about a dozen wire coat hangers to push and prod all of the pebbles and debris out.
But low and behold it was worth it!
Saved over $500 in a pair of replacement coolers and now runs the temps it should all the way up the ROM levels

My isue was at idle ret. from running the boat only.
I just got back from running it and it is fine. the eng. with the isue is now 10deg cooler than before.
Maby I sucked up something and blew it out running back yesterday.