Temperature Drop at WOT on 150HP V6

[hkeiner]

I recently installed a temp guage on my boat. I connected the existing temp sender (which was already installed on the motor) to a new Faria gauge that displays a range of 100-250 degrees. After running the motor a few times, I noticed that the Faria gauge displays approximately 150 degrees at idle and then drops to about 125 degrees at WOT. I checked the accuracy of the Faria gauge at idle running on a hose using a handheld infra-red meter and it displayed pretty close to 150 degrees too. I have not tried confirming the displayed temperature at WOT because it is a bit impractical and unpleasant to hold the handheld gauge steady on the back side of the motor when running at WOT on the water. I am not the original owner of the motor and so do not know it's full history. On the other hand, the motor runs well and shown no signs of ever overheating (paint discoloration and such). I have read several posts on temperature problems and solutions, but they all seem to deal with termperatures that run too high, rather than too low.

Anyways, I am thinking that a drop in temperature at WOT is not normal behavior and that I may be running the motor too cool at WOT, which may not be a good thing over the long term. Would these symptoms indicate that the poppet valves or thermostat are stuck open or have been removed from the motor? If so, which of these two components are most likely suspect and should be checked first? I also have a pressure gauge installed on the motor. Is there anything I should look for on this gauge as a clue. Any other thoughts on this?

Thanks in advance for any responses.

 

[big_b16]

Re: Temperature Drop at WOT on 150HP V6

You do have a lot more water flowing through the engine while at WOT. Not sure if it's enough to lower the temp below the idle temp. Does your temperature gauge raise, then at a certain point drop indicating the t-stat opening?

 

[gss036]

Re: Temperature Drop at WOT on 150HP V6

I don't think there is anything wrong with the temperature. The poppit opens about 3000 rpm and dumps excess water out through the exhaust. I think it is pretty normal for the temp to drop WOT.

 

[j_martin]

Re: Temperature Drop at WOT on 150HP V6

I've discussed this elsewhere.
It would seem to me that when you figure you have about a 50,000 btu/hr fire over a v6 piston at idle, and about 400,000 btu/hour at WOT, and you consider heat transfer through aluminum and steel, It would have to run considerably cooler water in the jacket to be anywhere near the same inside the cylinder.

In other words,
1. It's normal
2. Mercury knows what they're doing.

hope it helps
John