Engine temperature change

We have twin 7.4. All was well until I changed the oil and transmission coolers on both engines. I got both main heat exchangers serviced. When I put it all back together the Starboard engine worked as before. The Port engine ran a low temperature at start. Gauge would not move then it would go to 200. and drop back to 170 then to 120 then to 200. The reservoir bottle would fill beyond full. I replaced the thermostat and things did not change. I bought a new main heat exchanger. The engine reads 120 then all at once it goes to 180 the reservoir does not over flow. The engine does not use antifreeze. If I speed up the engine temp will drop to 120 then go back to 180.
Any idea what is the cause of the temp fluctuation. I have used a heat gun and same components on both engines read the same all the time. I have aimed my temp gun at the top of the temperature gauged and it reeds as per the gauge on the dash.

I have not had this happen with my 7.4's, but have had the same thing in several cars that have similar waterpump/thermostat setups. Have you made sure to bleed the system thoroughly and burp ALL air pockets? This sounds very much like you have air trapped in the system. Pretty common on closed loop cooling systems when either flushing or replacing major components. It can take a while to remedy, but your bouncing temperature readings tell me to start there. Open your pressure cap, start the motor, and let it run until you can consistently NOT see any bubbles coming from the coolant. Keep an eye on the temp gauge while doing this to ensure you don't run too hot. If not under load and only idling at the dock it should not be an issue.

Thanks 67
I did notice air bubbles going into the expansion bottle but they stopped after a few minutes. Will try running engine with rad cap off. I did both engines the same way so assumed it was parts I put in. I got all the heat exchangers from mr cool they seem good quality and arrived on date promised. The one one replaced was original to the boat.

Took rad cap off and let engine heat up. Got a few bubbles. When engine was at temp 170 or so shut it down a gallon antifreeze blew out the open rad cap area. No it was like a volcano. It seemed to be all liquid no air
Has any one ever seen the impeller in the antifreeze pump wear out. Engine has 2950 hours on it.
Temperature still sits at 120 then goes instantly to 180+ the back to 170 and sit there. If I speed up it goes to 120 the back to 180+ then back to 170.

The very quick, almost instantaneous, needle movement does sound an awful lot like a air bubble. You've certainly got a hot pocket in there. There could be an issue with your circ pump creating turbulence, but it still sounds like an air pocket. Is there a plug in the t-stat housing that you can remove to atleast make sure you have solid AF coming out?

But just for good meaure, I'd also take a look into your t-stat housing real closely to see if there isn't a piece of something stuck in there - which can creat turbulence, as well. I wonder something got inside during the recent work? Did you do anything with the t-stat recently?

I replaced the heat exchanger with a new one from mr cool. Now gauge sits at 120 then after a few minutes it goes to 190 then instantly to 170 to 180. It will stay there until I alter RPM up or down and the engine will go to 120 then to 190 then to 170 to 180. I had a shower after running the engine for an hour and the water was not hot like it was before I changed all the coolers.
I put in new thermostat and it did not help.
Based on temperature gun both engines are running the same all over. The top of the temperature transmitter reads what the dash gauge does IE 120=120 180=180 and any place between.
Should I look at replacing the antifreeze pump. Raw water pump is new last year.

If you are certain all air is out of the system in would look to the seawater pump that was replaced.. was tge impeller intact on the old one . Having chunks of imoeller flisting around blocking passages can play hell with the seawater side .The risers are one of the places this can get stuck in. Another issue can be a stuck shutter valve but your syptoms dont seem that is the problem . I hope you find the problem and it's easy to clear up before damage occurs. Another thought I just did my shutters exhaust tubes because I had a lower tube on my portside develop a pinhole leak . Really glad I did because it ripped apart at the seam with little effort. Also the shutter on that one was showing signs of heat damage. I decided to replace my hoses because inspection showed some scale and also softness in what I'm sure were all original hoses . Anyways after changing all the hoses my portside riser is cool as a cuke just like the starboard one .
Good luck

The sea water pump came out without any damage to it. We put 100 hours on the boat and we still get the 120 then 190 then constant run. I expect it could be a defective thermostat that sticks. Does not use antifreeze or the antifreeze over flow does not boil over. No white or any colour smoke when the engine is started. Just changed oil. No water in it and in the 100 hours it had not gone down. Engine has 3,100 hours on it but we run at 1200 to 1400 RPM most of the time. Due to bad weather we had to run at 2000RPM for several hours this trip.