Help with water temp

[66stingray]

Wondering if someone can give me some guidance here. Some history on my boat:
2005 Tracker Marine Tahoe 254 with I/O Mercury 350 MAG MPI, Bravo One, open sea water cooled. Last year I was starting to overheat while over idle. I ended up getting new manifolds, risers and new water pump impeller. Prior to that I had changed the thermostat in hopes it failed, but no dice. Afterwards boat ran great but noticed the water gauge would not get to 160. Towards end of season, the gauge would barely get over the 120 mark. Didn't have a chance to look into it before putting the boat on blocks for winter.

This year, same thing with temp gauge. I misdiagnosed the water temp sender and replaced it with new Mercruiser part. No change in low temp reading. Took my fairly new thermostat out again and it was shut partially opened and rusty. Found the culrpit I thought. Ordered a new Mercuriser one with gaskets and put it in today. Gauge went right up to 160 idling and stayed there. Perfect I thought. Until I took it out. My gauge now exhibits short cyclical swings from 150-160 to about 180-185, at medium load (3000rpm). The needle doesnt stay high for long. Pushed a little more above 3000rpm, i saw my gauge dip to as low as 120! Back to 160-180. My boat never exhibited this behavior in the past, always steady with normal temps. I initially thought I might have air caught in the cooling system and tried to bleed the blue plug at the thermostat housing, but did nothing. Maybe my understanding of the open cooling system is wrong, but wouldn't any trapped air be eventually expelled out the exhaust with the water? I did not take readings yet with my IR thermometer, but doubt the gauge or sender is faulty. At this point any suggestions are welcomed.

 

[Scott Danforth]

Check with IR

Most likely its the t-stat

 

[phillyg]

Don't trust the temp gauge. I had the similar problem with a VP motor. IR thermometer showed normal temps everywhere I checked. Changed out my temp sensor/sender, no change. Finally changed out the gauge and problem fixed.

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[66stingray]

Ok, I will start there. The thing is with my stuck open thermometer I was not getting wonky readings. Any way my air in the system theory is contributing? I am more a car guy, so still learning with boats.

 

[66stingray]

I managed to get to the boat today and took some reading with my IR gun, right on top of the thermostat housing. With the temp gauge reading at its lowest I was getting about 152-3, and with the temp gauge at the highest point my IR gun was reading about 168-9 on the thermostat housing. So the temp fluctuations are confirmed and happening. The gauge needle when high is at the line in the middle of 160 and 200, so I assume its 180. I also took out the recently changed temp sender and put the old one back in but basically got the same swings. I took out the thermostat just to check and seemed fine (closed when cooled). An ohm reading of the sender, set to 2000 gave me about 122 at 160f for both (new and old), which seems a bit off since the specs say 121-147 ohms at 140f.

So any other thoughts? I am open to suggestions. I am considering replacing the new installed thermostat, just to take it out of the equation, but find it hard to believe that it could be opening and closing that quickly. As a side note, my risers are cool, but the exhaust manifolds saw a brief high reading of 200f. Again, manifolds and risers were just replaced. I have to go back and check paperwork, but 99% sure the raw water pump impeller was replaced at the time of the risers/manfolds last year. The main water pump has NOT been changed since I owned the boat. Could this cause what I am seeing? Can the raw water impeller be toast already?

As a side note, I just got done replacing my Cool Fuel module before I got involved with the temp problem. Was thinking if the 2 were somehow connected, but cannot think of anything other than maybe a plastic barb fitting piece breaking in the module cooling lines that are connected in the water lines and traveling upstream, and clogging, but why would it be cyclical?

 

[QBhoy]

Don’t suppose you have put a closed cooling 140 thermostat in it instead of the 160 for raw water cooled ?