My son’s PX150/VMAX gets a High Temp Alarm while sitting at idle. Give it some throttle and it goes away till next idle.
The 150 is Peeing good all the time.
I see some mention of Poppet Valve.
Where is it?
What does it do?
How to diagnose?
Easy to replace?
Other suggestions?
I would check/replace the thermostat first. Usually a good place to start.
Water jacket cover gaskets may be ripped
Partially clogged cooling lines, increase of throttle pumps more water?? Just thinking out loud 
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author unknown
Replace water pump and t stat.
Tall Sail Marine
Water pump.
218WA Sailfish
200 Verado
The "Penn"sion Plan
Water pump is new. It is peeing good.
DRMajor
reread my last post;;;;internal cooling lines partially clogged/restricted with corrosion,mud , ect ;; most likely will have to remove some covers to verify … just sayin !!!
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author unknown
will take a look…thanks
DRMajor
Just because it is new does not mean it is working properly. That motor should pee like it wants to put a hole in the ground. Any debris sucked into the water pump could create a grove in the plate resulting in less suction.
At idle the pump is not getting enough water to the powerhead. This could be 1. Bad water pump. 2. stuck t-stat. 3. water jackets clogged. 4. all the above. 5. hole in the water pickup tube.
I have seen water pumps not installed correctly, or forgetting that rubber grommet at the water tube pickup.
If it is peeing like it should then I would take a temp gun and read each head to see where the problem would be.
Tall Sail Marine
The popet valve is a cause of overheat running at high rpms, not at idle. It relives the water pressure at high rpms, as the water pump is spinning the same rpms and the motor cannot cycle all the water it generates.
Tall Sail Marine
If the pressure relief valve is sticking open THE ENGINE WILL OVERHEAT at low rpms, and function perfectly normal at higher rpms.
if thermostat is sticking, very unlikely that it will only overheat at lower rpms.
blockage in the water passages could be an issue
poppet valve is easy to clean, also go ahead and change the t-stat, its easy and relatively cheap
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I think the PCV and Therm sound right. Will have look see at water lines.
DRMajor
Chris you are right. I didn’t think of the PRV sticking open. I see and follow that logic now.
Tall Sail Marine
drmajor
not much in the way of lines to check, but pull the t-stat cover and run with removed and do a visual inspection of the water coming out.
make sure you do this on the hose with fresh water. you don’t want to flood the motor with saltwater
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Thanks.
DRMajor