I cant figure out this temp fluctuation

[66stingray]

Trying to get some outside perspective on this. Boat is a 05' 25ft Tracker Marine Tahoe with a I/O 350 MAG MPI, Bravo One drive. Last year I overheated due to overdue risers and manifolds. Shut it down before any damage occurred. Got new risers, and manifolds put on with a new raw water impeller and was back on the water before the season was over. Boat ran fine towards the end of the season, but started to notice that my water temp gauge was barely registering. After putting it away and starting this season, I have been chasing this rabbit hole for a while. Took the thermostat out cold and noticed it partially stuck open. Easy enough, replaced it along with a new plastic sleeve and gasket with OEM. Afterwards, temp got to normal, but would see cyclical fluctuations while underway. Normal temp, then cool down and shoot back up to normal. Changed the temp sender (for the gauge) and no change. Verified temp fluctuations with an IR temp gun on the tstat housing and its legit. I changed the tstat again and for a couple of weekends it was acting normal, that is warming up and holding normal temp. It. It now started to exhibit the same symptoms.

I have since installed VesselView and have some additional data. Water temp never goes above 158, when it fluctuates, but I have seen it go down to as low as 109, verified by IR gun on tstat housing. Disclaimer for water pressure reading: I took my sensor out (from the power steering cooler line in the back of motor), cleaned it and took some ohm readings. The sensor is not within spec from what I found online, so cant be certain that VesselView readings are accurate. Neverheless, at idle, typically it is about .07 to 1psi. At moderate crusing speed, 3,000 rpm, it never goes above 3psi. What I did notice is that my water temp fluctuations start when my pressure briefly nosedives while at crusing speeds (from 3.0 psi, goes to as low as .5psi for a few seconds, again not sure if these readings are accurate). This starts the water temp to cycle up and down along with it, but not on the overheating side, it just keeps cycling cooler to the point that I got a low reading of 109. This usually happens later in the day. My exhaust manifolds get as hot as 210 while crusing, with risers about 100-110.

I was hoping that I can finish out the season, before I pull the raw water impeller to take a look, but cant see how that would cause temp swings to low readings. I thought of 3 things: 1.) the circulation pump is freewheeling and causing the swings. 2.) my tstat housing has some corrosion on the inside lip where the tstat seals and is letting water getting through/not holding heat in the engine at higher revs. At this point I ruled out the tstat (having changed 3) and verified the temp fluctuations are indeed happening. 3.) A collapsed hose somewhere, maybe in the waterpickup because I dont see any hoses collapsing inside the engine bay. Anything else I should be looking at?

 

[LightningBob]

I’m kind of chasing an issue right now myself. I’ve done a lot of searching around online on this same subject and from what you’re describing 158 degrees is pretty cool.
I’d say the 350 has a 160 degree t-stat or it was atleast factory.
it kind of sounds like the thermostat is getting up to temp, opening, and cool water is flowing through and it’s cooling off and closing?
At 158 degrees I wouldn’t be concerned according to all of the stuff I’ve been reading.

I have an old 228R with closed cooling system and I’m about to try out a 142 degree t-stat.
some people say don’t
Other people say they’re running the 142 degree t-stat just fine

my issue was I was clearly overheating - exhaust manifolds couldn’t be touched with a 5ft stick. I changed my impeller in my outdrive recently and had water flow up through the outdrive but not past the belt driven sea water pump
I want to drop a few more degrees though because I don’t trust my gauge. 175 degrees-ish

 

[LightningBob]

I’m kind of chasing an issue right now myself. I’ve done a lot of searching around online on this same subject and from what you’re describing 158 degrees is pretty cool.
I’d say the 350 has a 160 degree t-stat or it was atleast factory.
it kind of sounds like the thermostat is getting up to temp, opening, and cool water is flowing through and it’s cooling off and closing?
At 158 degrees I wouldn’t be concerned according to all of the stuff I’ve been reading.

I have an old 228R with closed cooling system and I’m about to try out a 142 degree t-stat.
some people say don’t
Other people say they’re running the 142 degree t-stat just fine

my issue was I was clearly overheating - exhaust manifolds couldn’t be touched with a 5ft stick. I changed my impeller in my outdrive recently and had water flow up through the outdrive but not past the belt driven sea water pump
I want to drop a few more degrees though because I don’t trust my gauge. 175 degrees-ish

And just to clarify I did end up changing my sea water pump and my temp is lower.
I suspect to drop several tick marks on the temperature gauge once I put this 142 degree t-stat in

 

[achris]

At moderate cruising speed, 3,000 rpm, it never goes above 3psi. What I did notice is that my water temp fluctuations start when my pressure briefly nosedives while at cruising speeds (from 3.0 psi, goes to as low as .5psi for a few seconds, again not sure if these readings are accurate). This starts the water temp to cycle up and down along with it, but not on the overheating side, it just keeps cycling cooler to the point that I got a low reading of 109.

Think of the thermostat as an on/off tap. When the tap (thermostat) is closed the pressure builds up in the entire cooling system behind it, which is where the pressure sensor is. When the tap (thermostat) opens, the pressure drops... That explains the pressure readings fluctuating.

As for the temperature fluctuations, interesting. When you replaced the manifolds and ELBOWS (not risers), did you use the correct gaskets (between manifold and elbow) and in the correct orientation?



Chris......