F150 Overheat Alarm

[Predator2016]

I have a new motor only done 115 hours and it has the overheat alarm coming on when idling for about five minutes. This has only started since the 100 hour service when the water pump impeller was replaced. The motor appears to be pumping water a steady stream from the tell tale hole. Engine Model is F150XB.
The engine temperature gauge is sitting at mid range.
As suggestions as to what could be the cause would be appreciated.

[Jason2tpa]

When you are getting the high temp alarm idling, is this idle on flush or lower unit submerged?

Gotta be 1 of 3 things....the motor is overheating, your temp gauge is inaccurate, or bad thermosensor. Let's figure out which is properly functioning...the temp gauge or thermosensors.

To determine if motor is overheating you can use a thermal gun when you get the alarm. If it is overheating, then your temp gauge is inaccurate and the therm-sensors are functioning correctly.

If it is not overheating, your temp gauge is accurate but the thermosensor is not functioning correctly. Pull the sensors and test using your service manual as a guide.

With all that said, it's quite normal to see water pissing from the engine AND have the high temp alarm activate. Just because the water pump is functioning doesn't mean water is flowing completely through the cooling system. But since the engine has only 100 hours it's unlikely its overheating due to corrosion build up (assuming you flush regularly). That coupled with a steady stream from the T/T, my guess is the cause of the high temp alarm is electrical. But I've been wrong many a times.

[Predator2016]

Thanks very much for your reply.
Firstly the alarm has come on at idle both submerged and on flush.
No alarms at at high rpm travelling over a long distance.
I checked with a thermal gun and the highest temperature I could find was 78 deg c around the thermostat housing at the top of the motor and near an injector most other areas were around the 50 deg c mark.
Where would I find the therm-sensor and I don't have a service manual so what would the test procedure.
The motor has been regularly flushed after each use using the flushing fitting and in a flushing bag.
Since this problem has started I am also having trouble getting water to flow out of the tell tale hole.
Thanks again for your help.

[99yam40]

A sticking open PRV will dump too much water at low speeds and a overheat will occur due to the block not filling up all the way

[pstephens46]

My money is on the water pump not being reassembled correctly. Something not sealed correctly. You have some flow but not enough for idle. Higher rpm is forcing enough. Take it back or remove lower and fix it yourself.