I have a 2004 Formula 260 SS with a 496 Mag Base Engine with approx 350 hours total time.. I run is salt water. The engine has the closed coolant with sea water pump. Recent I throw and alarm and went into Guardian mode. I limped back and below is the Rinda Read out. Also I have stream coming out the port exhaust after it heats up. Recently I am losing a little coolant from the reservoir. Not a lot but it is down an inch or at mat 2 inches (in the reservoir).

I have run a few test or taken a look at the following it.
I did a pressure test on the coolant. The pressure test did well. The pressure did not lower while the engine was off. Then started the engine and kept the test going, while engine was cool and while it warmed up. Actually the pressure increased during the run test test due to expansion I suppose. Then released some pressure and it stayed steady.

I pulled off the exhaust elbow and it looked clean on the inside and flushed the elbow, no debris or rust, elbow seamed fine.
I hooked the hose up to the to the bottom of the exhaust manifold and pushed water though the exhaust manifold. It seemed clean and did not seem to be restricted.

I am the second owner. Elbows and exhaust manifold are several years old, guessing over 6 -7 years. There seems to be a good amount of water coming out the exhaust pipes. The sea pump pressure is 30 psi + at idle and more at 2000+ rpm close to 40 PSI.

Steam only comes out the port exhaust after the engine is at temp and 2200+ rpms. At the lower temp you came see the water vapor. When the engine is running the port exhaust manifold temp runs 15 – 20 degrees hotter than the starboard at 2200 rpm.

Planning to do a compression test and leakdown test.

Greatly appreciate advice before I have the heads pulled and start depleting the bank account.

The rinda numbers are below.
1. VEHICLE CONFIGURATION
Engine Make: 496
Engine Serial No: 0m683088
Model Year: 2004 Formula 260 ss
System Type: MerCruiser PCM555

2. PERFORMANCE DATA
Parameter Description Value Unit
Engine Speed.......................................... 0 RPM
Available Power..................................... 100 %
Coolant Temperature............................... 78.80 °F
Fuel Flow Rate..................................... 0.00 GPH
Port Exhst Cool Temp.............................. 77.00 °F
Sea Pump Pressure................................. 40.53 PSI
Starb Exhst Cool Temp............................. 75.20 °F
IAC Duty Cycle..................................... 0.00 %
Manifold Absolute Pressure........................ 14.83 PSI
Trim Position....................................... 186 #
Baro Pressure..................................... 14.84 PSI
Trim Volts......................................... 0.91 VDC
Manifold Air Temperature.......................... 80.60 °F
Pitot ADC............................................. 0 #
Oil Pressure....................................... 0.88 PSI
Paddle Wheel......................................... . 0 Hz
Throttle Position.................................. 0.00 %
Sensor Power....................................... 5.05 VDC
Throttle Sensor Voltage............................ 0.61 VDC
Lake/Sea Temp.................................... 431.60 °F
Spark Advance...................................... 6.00 DEG
Fuel Level......................................... 1022 #
Battery Voltage................................... 13.81 VDC

3. TROUBLE CODE STATUS
Current ECM Codes

Freeze Frame Data:
5 FUELPUMP RLY CK
--------------------------------------------------------------------
FF Baro Pressure 14.87 PSI
FF Battery Volts 12.22 VDC
FF Sea Pump Pressure 16.23 PSI
FF Boat Speed 0.00 #
FF Manifold Air Temp 98.60 °F
FF Coolant Temp 159.80 °F
FF Demand 0.00 %
FF Engine Speed 607 RPM
FF Engine State Running
FF FPC Total 41.0 Counts
FF Frequency Count 160
FF Fuel Level 0.00 %
FF Gear Position In Gear
FF Lake Temp 429.80 °F
FF Engine Load 19.27 %
FF Main Pwr Relay Requests YES
FF Manifold Air Pressure 9.98 PSI
FF Oil Level 0.00 %
FF Port Tab Pos 0.22 %
FF Available Power 2 %
FF Run Time 345.47 hrs
FF Starboard Tab Pos 0.00 %
FF TPS Percent 0.00 %
FF Trim Position 0.00 %

6 OVERSPEED
--------------------------------------------------------------------
FF Baro Pressure 14.87 PSI
FF Battery Volts 14.03 VDC
FF Sea Pump Pressure 41.89 PSI
FF Boat Speed 1.00 #
FF Manifold Air Temp 113.00 °F
FF Coolant Temp 174.20 °F
FF Demand 0.12 %
FF Engine Speed 737 RPM
FF Engine State Running
FF FPC Total 35.4 Counts
FF Frequency Count 28
FF Fuel Level 0.00 %
FF Gear Position In Gear
FF Lake Temp 431.60 °F
FF Engine Load 16.71 %
FF Main Pwr Relay Requests YES
FF Manifold Air Pressure 8.77 PSI
FF Oil Level 0.00 %
FF Port Tab Pos 0.38 %
FF Available Power 6 %
FF Run Time 345.87 hrs
FF Starboard Tab Pos 0.18 %
FF TPS Percent 0.12 %
FF Trim Position 0.00 %

7 STB EMCT OVRHT
--------------------------------------------------------------------
FF Baro Pressure 14.87 PSI
FF Battery Volts 14.03 VDC
FF Sea Pump Pressure 42.27 PSI
FF Boat Speed 1.00 #
FF Manifold Air Temp 113.00 °F
FF Coolant Temp 172.40 °F
FF Demand 8.86 %
FF Engine Speed 738 RPM
FF Engine State Running
FF FPC Total 30.6 Counts
FF Frequency Count 0
FF Fuel Level 0.00 %
FF Gear Position In Gear
FF Lake Temp 431.60 °F
FF Engine Load 13.19 %
FF Main Pwr Relay Requests YES
FF Manifold Air Pressure 8.16 PSI
FF Oil Level 0.00 %
FF Port Tab Pos 0.07 %
FF Available Power 6 %
FF Run Time 345.87 hrs
FF Starboard Tab Pos 0.19 %
FF TPS Percent 8.86 %
FF Trim Position 0.00 %

8 PRT EMCT OVRHT - same as Above

9 GUARDIAN
--------------------------------------------------------------------
FF Baro Pressure 14.87 PSI
FF Battery Volts 13.84 VDC
FF Sea Pump Pressure 41.52 PSI
FF Boat Speed 0.00 #
FF Manifold Air Temp 113.00 °F
FF Coolant Temp 174.20 °F
FF Demand 15.94 %
FF Engine Speed 797 RPM
FF Engine State Running
FF FPC Total 36.0 Counts
FF Frequency Count 3
FF Fuel Level 0.00 %
FF Gear Position In Gear
FF Lake Temp 431.60 °F
FF Engine Load 15.83 %
FF Main Pwr Relay Requests YES
FF Manifold Air Pressure 8.94 PSI
FF Oil Level 0.00 %
FF Port Tab Pos 0.16 %
FF Available Power 6 %
FF Run Time 345.87 hrs
FF Starboard Tab Pos 0.00 %
FF TPS Percent 15.94 %
FF Trim Position 0.00 %

Fault Log Data
Fault Descriptions Seconds
Guardian Strategy: 45
Low Oil Pressure Strategy: 1
MAP Sensor 1 Input High: 4
Overspeed: 27

Your water pressure numbers are way high. Your EMCT sensors are telling you they are overheating. Likely a restriction in the seawater side of the cooling system. The engine guardian and overspeed faults are a result of the EMCT faults.

Pull your heat exchanger end caps and inspect gaskets. Backflush oil and powersteering coolers. When was water pump impeller serviced last? They can come apart and become stuck in the system causing your symptoms. Saltwater can also cause "Bravoitis". Search the term and you will see.

Engine Speed.......................................... 0 RPM

Sea Pump Pressure................................. 40.53 PS

Bad sensor?