While on a 3 hour run last weekend,...

[Olly]

While on a 3 hour run last weekend, I noticed my starboard engine temp was staying at 73 degrees while the port was at 159. On the return trip 2 days later, same issue. After about an hour and half (running at 3500 rpm) starboard engine went into guardian mode and the gauges read "reduce throttle" I brought both engines to idle and could not detect any issues. I throttled back up to 3500 rpm and continued on. I got the same alarm 5-6 times until I got back to my marina. The Marina looked at the boat and gave me the reply below  

"We got a tech down and scanned your engine’s with the Merc computer. You had no active alarm faults present on either engine. But in the stbd. engine’s ECM history you did have a guardian mode set, about .2 hours ago. So that was your most recent occurrence you mentioned. Other faults we found which occurred at the same time were a block pressure fault, and low oil pressure fault. As I said, they were all set .2 hrs. ago. Also as you had mentioned, at the time that this occurred your stbd. engine temp was only about 73 degrees. Way too cold. 

So, further diagnosis found your stbd. engine thermostat was stuck open. This was causing your engine to run too cold. Which also explains the rest as well. Judging by the engine’s run history. Your engine must have been running cold all day or whatnot. This caused the engine’s block water pressure to remain low, and also caused your oil to be a bit thicker and most likely not flowing like normal. Giving the engine a low oil pressure reading. Which after a bit of time the engine’s ECM would not like to see. So it set an alarm, and put the engine into guardian mode.

We checked your engines oil levels, which were fine. So no low oil pressure alarm due to actually being low on oil at all. The thick oil’s flow was reduced. The stbd. engine’s thermostat was removed, inspected, and being stuck open was confirmed. Then both engines were ran after with the Merc computer connected for 15 to 20 mins. Live ECM data in the computer was observed the whole time running. The stbd. engine was found to come up to operating temperature normally now of around 160 degrees, which is perfect. Throughout the run both engine’s oil pressure, and block pressure readings within the Merc computer remained within spec. No additional alarms were set or became active at all. You should be good to run."

I started the engines Sunday and had the same issue. Marina came back down and replaced thermostat again, same issue. They switched the sensors. The issue stayed with starboard engine. With ignition on (not running) both analog and digital gauges are reading correctly. With the engine running, the gauges drop. The alternator was disconnected to eliminate that from causing an issue, no change. Aside from finding another service department does anyone have any idea what the cause could be?

5.7 Mercruiser 220 hours 300hp

sorry for the long post. Tried to include all details

[wingless]

The last paragraph indicates an electrical issue to me.

A good first step would be to ensure all clean / solid electrical connections.

These steps are the safest way to that objective, assuming owner work instead of dealership.

1) Disconnect all battery negative terminals.
2) Sequentially inspect, disassemble, inspect, clean, reassemble, properly tighten, properly protect w/ brush on liquid insulator, all battery positive and negative terminals, from the battery, to through the contactors, distribution, engine block and back to the battery
3) Reconnect all battery negative terminals.

Duplicate the last paragraph steps, to determine if the on versus running voltage drop exists.

[Olly]

Thanks for the info Wingless. I’ll give that a try

[Olly]

Sorry I am late providing an update to the issue I have. The Marina service dept told me that after getting meter readings regarding engine temp and sensor harness, the consensus was that the engines ECM must have a short within it and replacement of the ECM is needed for repair. Before spending the money for an ECM I found a reputable mechanic to look at the boat. It turns out the engine had a fault water circulation pump. Once this was replaced, everything ran fine. Needless to say, I am done with the marina's service department

[Silverfoxx]

Had code 25 pop up under way. Raw water pump leaking.Replaced pump. Under way the following day and within a hour...code 25 again..long story,andUSboat tow back to marina...the water pressure senser was bad as well..my suggestion would be to replace senser when you have to replace raw water pump..