Sorry in advance for the long winded post, just trying to give as much backstory/info as possible...
Engine is a 2006 Optimax 90 HP with 415 +/- hours. Engine has ran flawlessly it's first 9 years of life (was purchased as a showroom model/leftover in 2011). Midway through last season it began giving a very intermittent fault in the 1400-2000k RPM range. Constant tone (beeeeeeeeeep) and restricted RPM. Typically happened when getting up on plane slowly. If I punched the throttle and got above that 2k RPM range it would run fine all day. Sometimes it would go days, weeks even, without issue. Other times it would happen repeatedly and could be replicated as many times as I wanted, provided I eased on the throttle. There has never been a reduction in water flow out of the tell-tale (I know this is downstream of the compressor and is not a good indication of water flow, etc.) I have replaced, checked, and checked again the strainer for the water intake (always clean.) Replaced water pump last season (entire unit, impeller, gaskets, etc.) Replaced a thermostat last season, checked poppet valve (seems to be functioning fine.) Couldn't narrow down the root of the alarm through the end of last season and it was happening so intermittently I ran as is and lived with it, with the intention of having it serviced in the off season. Fast forward to this March... I left it in the hands of the local mercury service center for a spring tune up, made aware of the issue and told to go through and fix whatever needed fixing. Mechanic replaced serpentine belt, and felt the cause of the alarm was an intermittent failure of the neutral safety switch. Replaced the switch, he also replaced the thermostat "to be sure" and inspected the poppet valve. He thought everything was kosher. Put the boat in the water come mid may, ran beautifully without alarms 8-10 times out. Then suddenly, "beeeeeeeep". It was back exactly as before. Came home and checked the strainer (clean) thermostats and poppet (clean and opening as they should per a water temp test). At this stage in the season marina is now 3-4 weeks out, trying to troubleshoot on my own. Invested in the VesselView Mobile module which has thus far been helpful in isolating things. Got a fault for the Air Compressor temp sensor, upon pulling back the protective black jacketing on the harness noticed a wire had chaffed quite a bit. Replaced with new OEM part. Also throwing a fault for the "block pressure sensor" which interestingly enough also shows up 60 something times in the Freeze Frame report on the diagnostic printout the mechanic gave when the boat was serviced. I'm thinking these are my mystery alarms I'd been getting. When the alarm is activated temps never seem to be an issue (120 degrees to 155 -/+). Initially last week when I first installed the VVM module I was getting coolant PSI anywhere from 3 psi at idle to around 15 underway (which as I understand is normal and within specs.) It then dropped to 0 and stayed there, all the while coolant temps remained fine. Disconnected the grey tubing attached to the block and sensor, checked for obstructions and saw none. Air and water both flow freely through the lines Here's where I'm confused: The original block/coolant temp sensor in the engine is marked "881879-12". I purchased an OEM coolant sensor from the local dealer I was told would replace it. The part number on the package is 881879-11. I plugged it in expecting it to function fine and solve all my problems, however it only reads 0.0 on the VVM app. Also ordered a used sensor on ebay just to try, part number 881879-11 as well. This too reads "0.0" however when disconnecting it for a split second just as it comes disconnected the PSI will jump to 1 or 2, then drop again. Checked the wiring harness for abnormalities, saw none. Stumped, I tried the original part (881879-12) again today after again inspecting the lines and water pump this morning, pressure suddenly read 40 psi with the engine simply in the on position, not running. Fired up, it read 15 while running in a barrel/muffs. Took it to the harbor and did a sea trial. PSI started around 15 at idle, then as I made my way through the no wake zone slowly dropped once again to 0.0 where it remained. After 2 or 3 minutes of this, when trying to accelerate, once again the alarm would sound and guardian mode activated, VVM showing fault "Block Coolant Pressure value too low".
The original block sensor has a white dot stamped on it, while the new OEM one has a blue ring and blue dot stamped onto it and reads 8M6000623 on the brass portion of the sensor. Could it be I got a bad sensor from the dealer? Or the difference in part numbers? Their price tag shows a date of 2016 on it (6 years old at this point.)
I know a constant tone can also stem from oil issues, but I do not believe this to be the culprit. Oil consumption seems normal, and I checked the tank float to ensure it's working properly which it is. Other sources for a constant tone are engine overheat (temps are within tolerances) or a sensor out of range (block sensor?).
To summarize, I/The mechanic has replaced and/or inspected:
Water Pump
Thermostat (2x)
Poppet Valve
Compressor Strainer
Block Pressure Coolant sensor tubing
Air Compresser Temp Sensor
Block Pressure Sensor (One used, one new OEM, both showing part number 88187911, or 8M6000623 on the sensor itself.)
I am at a loss here and am told to have it worked on will be a 3-4 week period without my boat. Here in Maine, that's a good chunk of our season.... What's more frustrating is I left if back in March to avoid this entire situation. I have full printouts of the diagnostics and Freeze Frame history if that helps!
Thanks much in advance. I am fairly mechanically inclined and really getting to know this motor however I am now at a loss. Looking forward to some insight from those that know these Opti's inside and out.
-Matt
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