Alarms

Rob, it has been my experience that lots of engines just don't pick up water very well from the clamp on muffs you mentioned. If the water pump is weak, it may perform OK for a while in the lake but not produce enough pressure when it's trying to capture water from a hose and muffs.

 

Rob,
Its very likely a water issue as Larry has noted.
Though something similar happed with my 2001 Merc 90. It was the low oil alarm in the tank. Which is a magnetic proximity switch, a float/magnet switch inside the tank. The float separated from the magnet and resulted in a false positive alarm condition.
The float can be superglued back on with some effort or replace the tank. I glued the switch rather than pay $150 for a new tank. A search on the Internet should find some diagnostic and repair examples.
Vic

 

Kerz, interesting you mention the magnetic sensor problem- I had a 2013, 115, Merc PRO XS. The worst engine I was ever associated with. When it didn't have bugs it ran super- Bought it new and had a lot of crazy problems with it including the sensor falling off the oil tank float mechanism. Also Within the first month the Oil tank dipstick, made of plastic, broke off and fell to the bottom of the tank. I didn't replace that since you don't really need it anyway. I had to have three (3) full sets of electronic coils replaced to finally a get the engine to run smoothly. Parts build in Mexico. Air Pump belt came from the factory installed wrong- tore that up and had to have it replaced-- all under warranty but I really never knew if I was going to make it off the water or not for a full year and dealing with all the trips to the shop and wait time was very frustrating. After the first year all was good. I never read or saw where anyone else had issues like I had. I was really glad that the new Merc 4 strokes have a simple fuel/air system. From what I've read that has been an issue with lots of owners. The four's seem to be much more reliable long term. I had always like the simplicity of the 2 cycle engines but the air pump system didn't seem reliable long term. I've certainly been happy with the new four now. Other than one bad alternator, replaced under warranty, all has been good. Two years in on the Z-18 now.