Have not had a chance to look at it yet but wanted to get some ideas to see if I need to add to my list of things to check.
Was cruising about 55mph when lost some power and got a continuous beep with no red lights on gauge. Stopped immediately and beep stopped at idle. Water pressure was 15 psi @55mph, 10 psi when moving slower and 5 to 7 at idle with good P stream. Fuel tank 1/2 full with non ethanol gas. Reserve oil tank over 1/2 full with Merc DFI oil. This was a continuous beep, not the 4 beeps like low oil. Guardian reset when I pulled back to neutral and I headed back to ramp. Ran pretty far once with no alarm so stopped to fish, next time it did it again just as I got near the ramp, only seems to do it at higher rpm.

I plan to check/clean/change fuel filter, top off reserve oil tank and check engine mounted oil tank. Another issue that I have had since I got this boat is that the voltmeter reads 12v with power off and pegs at 16v when power is on....batteries read 12.6v sitting and around 14v with motor running. I plan to check the voltage and the gauge better as I have read that over 16v will cause the Guardian to kick in even though it has not happened before....

also! Only normal 1 beep when starting....

update: removed spin on fuel filter and drained into glass jar....no water. Replaced with new filter/sensor.
checked voltage at battery, just unplugged charger - 13.6v
checked voltage at back of gauge an hour later, key off = 0v gauge reads 12v key on = 12.4v gauge pegged at 16v; ordered new gauge
will check voltage with engine running after I install new gauge.
engine oil tank full, remote tank half full, topped it off.

A continuous alarm indicates overheat. If you are sure the engine is not overheated then you might have a defective sensor. You can easily check this with a voltohm meter.

Thanks!

On the port side of engine on the adaptor plate there two nipples one brass and one white plastic nipple the brass nipple is your air dump and the plastic is your water filter screen for the cooling water for fuil rails and air compressor. I have seen these filters get clogged with pee gravel and grass and moss if it clogs it will cause air compressor to overheat. Remove port lower cowl to access nipple the hose on nipple goes to bottom of fuel rail. Remove hose from nipple and unscrew nipple and check for blockage. Hope this helps. OPTI

cool, thanks!

When mine did this, it ended up being the motor mounted oil reservoir sensor. My pump was still working, but the sensor was saying otherwise.

Thanks, a couple months ago I got the 4 beeps every 2 minutes and a red light on the oil gauge which means oil related and the tank on the motor was low but the remote tank was full. There was an air leak in the fill hose from the deck to the remote tank causing the system to lose prime, I fixed that and it has worked fine since then. This beep was a continuous beep and no red lights come on the gauge for oil or temp and it stops when I decrease rpm.
I installed new voltmeter today and readings are good, I'll check for the things in Opti's post next and if no obstructions I will probably check thermostats to see if one or both are stuck open causing the engine to run too cool.

are these the hoses that I need to remove to check for debris? I don't see a white plastic nipple, both look brass...

I am surprised no one has said anything about the half full motor oil reservoir. That will set off the alarm. You need to check why enough oil isn't being fed to the motor tank. That beat up hose has the screen for the compressor cooling hose.

Thanks, the reservoir on the motor is full, the remote tank in the boat was over half full. I'll pull that hose and check the screen!

the beat up hose has a bad end and is too short to cut back. I pulled the fitting and the large fitting next to it to clean and there was no screen anywhere? Tried to pull the hose off of the fitting going into the port fuel rail after loosening the clamp and the whole @%%#$ fitting pulled out of the rail, was only threaded in a bout 1 1/2 threads. Threads on fitting are 1/8" pipe, most of the threads in the hole look like they are much coarser. So I go to Mercury for a fitting to see what is supposed to be in there and some air pressure hose and they are NO LONGER AVAILABLE

Does anyone know what thread size is stock for the air dump barb fitting coming off of the port fuel rail on a 99 Optimax 225? Since the hose is clamped on it should be 300 psi or less and I can use 1/4" air brake line but since someone decided to replace the barb fitting with a 1/8" pipe fitting and it only screwed in a couple of threads and now pulled out and Mercury doesn't sell the fitting anymore I need to either try to tap it for 1/8" pipe and hope it holds or go to an M10 size but I am not sure of the pitch in order to find a barb fitting to use. I'm sure this has been done in shops before since Mercury doesn't offer the fitting anymore. If I can find out the correct thread in the fuel rail I could even build a high pressure crimped hose as I have a hose crimping machine...

Ok, found a new, old stock fitting to replace the incorrect 1/8" pipe fitting. Still had 7/8 of the hole threaded correctly for the new o-ring fitting so all is good there. I used 1/4" air brake hose for replacement hose. Pulled a brass filter off a hose on top off the motor going to the air compressor and blew air thru it....it built up a bit of pressure before opening and some varnish looking junk came out of it. Open one way now so hopefully it is good to go. While pulling the brass filter I noticed a hose going from the white plastic vent filter to the compressor and it was very close to the flywheel. Further inspection showed a hole in it so I am replacing it. Vent filter was clear. Hopefully all of this will fix the problem.......If not thermostats and poppet valves will be next. Original thermostat part number has been updated 3x with new covers required, not sure yet if mine have been upgraded or not.