288 intermittent alarm
 

Having an issue with our 03 288 w/ 496 HO.
Alarm went off so checked gear lube. It was a little low. Added lube. No alarm.
Later that day alarm goes off again., gear lube was down a hair. Thought it was a gear lube leak.
Had it pressure tested by boat shop. No issue. Thought reservoir float was bad. Replaced reservoir.

Next time out alarm goes off. Check gear lube and it's full.
Start up no alarm. It happens again.
Notice temp gauge is slightly higher than normal. Normal is 190, it was a hair above that..
So replace sea water impeller.
Go out yesterday and alarm goes off again. So frustrated.
Temp is up so prettty sure it is the over heating alarm.
Shop owner said the closed system antifreeze never needs changed.
I just read in the general forum on 496 overheating saying the pink AF can get dark and metallic and will not do it's job after 7 years or so.

But all 496's are not the same so hoping for some insights on what the heck might be going on. I want to boat.
Advise please..

Somebody needs to do some testing, need to put a scanner on it. The solid alarm can be ECT, oil pressure. lube, low water pressure, and if a sensor looses ground. The most common failure is the water pressure sensor

Are you getting any power reduction from the ECM when the alarm goes off? My guess if your like most , soon as it goes off the throttle is pulled back

Thanks for your reply AllDodge.
I had not noticed a loss of power until yesterday before the alarm went off I did feel the loss of power. It seemed like I might have spun the prop. It was the first I had this since I stop at first sign of the alarm. After sitting it would cool down and no alarm for a while. Obviously it has gotten worse. I could cruise 5 to 10 miles at 3500 rpm no issue. Then next time I would cruise a few more miles and it goes off. Stop, raise hatch check things out, by then I guess it cooled enough so it would not go off when I started up and cruised a little more.

Did notice steam coming out of thru hull exhaust when running quiet with captains call.
Also found this:
Constant tone alarms are for:
1.) engine coolant temp overheat,
2.) low engine oil pressure,
3.) engine overspeed,
4.) exhaust manifold cooling temp overheat
5.) sea pump PSI low.

I do think I have an engine coolant overheat issue since sea pump was just replaced. Old sea pump did not show signs of wear, and was completely intact so no loose pieces to get stuck anywhere. But ...that is my uneducated guess.

Maybe use an IR temp gun to measure temp at the thermostat housing. The gauge may or may not be correct but in any case, the ECM uses another sensor on the thermostat to tell it if its overheating.

Assuming B1 and hope it wasn't a coupler

I hope not also. I 'think' it was due to the alarm. Fingers crossed.

Thanks. Will do.

Ran into this on my old Dominator. Would run cool enough at idle but as soon as the boat was planed out, it would start to warm up. Like Crown said, backflush. And, when you think you've done it enough, do it again.lol

You need a new shop/mechanic.... 5yrs/1k hrs on engine coolant whichever comes first. Right out of Merc 496 FSM.

190*+ is not normal operating temp for a 496. That is too hot. Stock they should be 160-180* range. At 190* + ECT temps you are probably close or tripping the EMCT sensors. These trip at 212*.

As said, backflush complete seawater side of cooling system. Pull heat exchanger endcaps, flush internally, replace endcap gaskets. Endcap gaskets deform and will block water flow. Also could possibly be the "Bravoitis" tube corroded.

A techmate/scantool scan will pinpoint exactly what fault is being thrown.

Backflush and if that does not do it check to see if the brass sea pump housing has grooves in the housing. Has it every had a new sea pump housing?