90hp overheating, plz help

[mitch_robbo87]

hi guys. just wondering if anyone has any knowledge of overheating issues . I have a 90hp 2 stroke oil injected motor details( Yamaha 90AETO 6H1 L 1009809 P )think it may be a 1991?
the alarm sounds when at idle after a minute or so, pumps plenty of water and will never happen if driving or sitting with revs up, I have had the impeller , thermostat & heat sensor replaced. have also run "salt away" through the motor , have also run with thermostat out of motor but still happens. I can drive all day with no issues but as soon as I pull up at a spot and am sitting there idling alarm will sound and I have to pick the revs up for 30 seconds or so while in safety mode for the alarm to stop. could really use some input as I am out of ideas... also if im flushing the motor after a day out in the ocean with fresh water with the earmuffs it can idle and it wont have any issues. any input would be greatly appreciated. thanks heaps
robbo.

[Biminibrad]

Over heat

First thing to do is verify that it is overheating. Get a digital heat gun and scan power head when alarm goes off.

[99yam40]

Have a look at the PRV
if stuck open it will dump too much water at low RPMs and cause overheat

[boscoe99]

Another vote for checking the pressure control valve.

Change the valve, the compression spring and the rubber valve seat.

[mitch_robbo87]

any chance you could post me a link to that service manual boscoe cheers for your help guys

[robert graham]

The PRV is that spring operated black plastic plunger thing with the black rubber grommet that sits under the same cover as the thermostat....calcium/mineral deposits can cause them to stick. You can remove it to clean with a brush and mild acid and it should be fine. The rubber grommet may need to be removed and cleaned also....good luck!

[mitch_robbo87]

hi mate yeah I have often pulled that out and cleaned it but it seems to move freely and doesn't stick, did it again just then to be sure... but still doesn't seem to be the problem . would it pay to pull the side off the water jacket and clean in there or it wont be anything to do with that?

[robert graham]

Well, pulling that head/cover off and cleaning/scraping the scale/calcium deposits out may help your cooling problems but there's always a chance of snapping off some of those bolts that are stuck with corrosion. And you'll need a set of new gaskets, and torque all those bolts in order and to specification....so it's no small job and has it's potential problems....How is your water pump body, impeller, gaskets, grommets, water tube up to the motor?...those would be easier to check before opening up the head...good luck!..

[99yam40]

either the pump is not pumping enough water at low RPM or it is leaking somewhere before getting to head.
have you replaced the pump housing, cup, plate, and all seals/gaskets?

[mitch_robbo87]

Cheers mate all fixed now the lower pump housing had a small crack and was loosing pressure... Thanks heaps!

[robert graham]

That's great!...find the problem and fix it!...glad you didn't pull the head off, since it could have been a real "bucket of worms"....good luck!