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Alarm Going Off for Mercruiser 350 Mag MPI


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I have an odd thing happening with my boat. About once everytime out, my one beep constant alarm goes off. I stop the engine and restart and the alarm doesn't go off again. I have had this happen a dozen times now. All gauges read normal when the alarm goes off (temp, oil pressure, rpms, voltage, etc.) 

Most of the time, it is within the first 15 minutes of my initial engine start when this happens, and all but one time it was when I was fully underway at about 20-25 mph. Twice it was when I had a  wakeboarder behind the boat (meaning usually it is just when we are heading out for the day when it happens.)

When the alarm fires, the boat decreases in speed. I usually stop the boat immediately and cut it off. Then restart, and all seems to be good. Anybody with any guesses here? Taking it to a mechanic next week, but would like to here some thoughts. Here are things I have been thinking about:

* Gear lube alarm. This seems like the logical one because I see a lot of things online about this giving false positives because of a stuck float. However, the engine seems to slow when we get the alarm and I believe that doesn't happen for gear lube based on my reading. 

* Injector not firing. Perhaps, this is it. The rpms are normal, but I am cruising pretty slowly (20-25 mph) when the alarm goes off. Maybe the rpms would stay low on V8 when you are cruising at that speed?

* Some sensor not working properly. Not sure which one, but this seems to be logical as well.

Anyway, any assistance would be appreciated. Thanks.

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The traditional issue would be a bad oil pressure switch.  There are 2 oil pressure senders, one is an analog for the dash gauge and the other is a simple on/off that the ECU uses to determine if there is a low pressure issue.

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Without using a diagnostic computer it will be extremely difficult to determine what is causing the alarm and guardian power reduction.

The most common cause that I often see cause the guardian mode to engage above 3000 RPM is from a sea water flow issue.  This is shown on the diagnostics as a "BLK PSI LO" code.  There is a pressure sensor mounted either on the sea pump, the power steering cooler, or transmission cooler that reads sea pump flow pressure.  If the sensor gets clogged with debris, fails, or the flow pressure is low when above 3000 RPM, then there will be a constant alarm and guardian mode will reduce engine power until the engine is run below 3000 RPM.  The alarm will continue to sound two short beeps once a minute until the key is turned off.

A disconnected knock sensor on the side of the engine block can cause a very similar alarm and running issue, but the diagnostic code will be "KNK SNSR1" or "KNK SNSR2".

There are several other faults that can cause the running issue you described, so without proper diagnosis you may have to rely on trial and error or luck to get it fixed.

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