Help...Temperature Gauge 20° swing

[Sarges12]

Some history first...
This past winter I did a complete cooling system maintenance overhaul on my 2001 Merc 5.0 EFI engines with Bravo 3’s. I replaced risers, spacers, manifolds, raw water pump (kits), circulating pump and thermostats. The boat was launched 3 weeks ago. On the trip to the slip I noticed the temp gauge on the starboard motor would climb slow to about 180° then go back to 160°. It did this constantly for the 20 min ride. This has never done this before. It still does it while idling in the slip but everything feels cool. I used an infrared gun shooting all areas of the block, thermostat housing and circ pump. Nothing I find alarming.

I’m concerned with the fluctuating temp gauge. So I started back tracking. I tried a new thermostat and temp sending unit. No difference. I pulled the raw water pump and inspected that and reinstalled. Tried swapping thermostat housing from the port motor as someone said it could be the housing. Nothing changed. I even bought a new temp gauge...still fluctuating.

I’m beyond done trying to figure out what the heck is going on.
At this point do I try another new circulating pump or even change the raw water pump kit again? It was even suggested doing a short haul and pull the drive back off and inspect the water gasket on the drive.

Has anyone ever had this issue? Suggestions before I turn it over to the marina?

 

[Brite Idea]

I'm guessing RWC? Did you use Merc thermostats? It sounds like a water flow problem. I've seen FWC systems with air trapped in them do it and also the after market thermostats. Also under the thermostat on some models there's a o-ring if yours has it is it there? You have that plastic slotted insert in your thermostat housing is it in correctly? Do you pickup water through the drive or a thru hull? Your water pick up thru the transom assembly could be corroded and not allowing enough water thru. Just throwing out some ideas because over the years I've seen one of each. Also I've seen the wrong circulating pump used spinning the wrong direction and not moving water.

 

[Sarges12]

Sorry, yes RWC. Merc thermostats.. When I removed the thermostats the cork gasket under it wasn’t there. I put one in which it called for. The plastic sleeve can only go in one way. Like I said I swapped the entire housing with the port motor which is running fine.
The drive is new last year and the water is picked up through the drive. When I inspected the raw water pump I had to disconnect the hose coming in through the transom assembly, I’d like to believe there’s plenty of water available. Thought I was going to sink the boat while trying to plug the hose. LOL.

I bought the circulating pump from Michigan motorz which looked exactly like my old one. From what I’m told they have a brass impeller inside which will circulate water in either direction depending on which way the motor turns. I may just try both the raw water pump and circulating pump again. If nothing else, I’ll have spares.

I just hate dumping money with trial and error.

 

[Express 390]

Sorry, yes RWC. Merc thermostats.. When I removed the thermostats the cork gasket under it wasn’t there. I put one in which it called for. The plastic sleeve can only go in one way. Like I said I swapped the entire housing with the port motor which is running fine.
The drive is new last year and the water is picked up through the drive. When I inspected the raw water pump I had to disconnect the hose coming in through the transom assembly, I’d like to believe there’s plenty of water available. Thought I was going to sink the boat while trying to plug the hose. LOL.

I bought the circulating pump from Michigan motorz which looked exactly like my old one. From what I’m told they have a brass impeller inside which will circulate water in either direction depending on which way the motor turns. I may just try both the raw water pump and circulating pump again. If nothing else, I’ll have spares.

I just hate dumping money with trial and error.

I agree with Brite Idea in that air is trapped !

 

[Sarges12]

But it’s raw water cooled. Not a closed system. I thought water just circulated through this system.

 

[Brite Idea]

It could be sucking in air somewhere. Broken hose or warped wear plate in water pump or heavily scored pump housing or wear plate. The simple test is to install a clear pick up hose and run the motor look for bubbles. If not move up the path of water flow.

 

[liv2ryde100]

My guess would have been air in system also, but rwc shouldn’t be an isssue. Second guess is the pickup tube is squeezing down on itself not allowing water to pass through, but I would assume it would happen at higher rpm instead of lower so that’s out too.

 

[liv2ryde100]

Maybe another hose somewhere is collapsing?