Mercruiser 5.0 MPI - Constant beeping

[Robertse]

Mercruiser 5.0 MPI I kindly ask for help about the constant beeping. It starts a constant solid beep as soon I start the engine on cold start. Sometimes the constant beep goes away after I stop and restart the engine, then comes back… The engine is now no longer overheating, as I pulled out the thermostat; it is in now running below 120. I recently replaced the Thermostat as the old one was rusted; however the new thermostat was causing the engine to overhead. So I will order a new one.

Now, when the engine is running, I touch the engine’s thermostat housing, exhaust manifolds, raisers, and the engine is not hot, it is warm, as stated it is now running below 120. Before I pulled out the Thermostat it was running 170 and above. I should off the exhaust manifolds and raisers to clean them and make sure the water jackets are not blocked. I have no water leaks in the engine. The Gear Lube is full and ok (however there is a small oil leak the Bravo 3 prop, I ordered a new seal and will replace it), I top up the gear lube via the bottle is it drops a bit. I the oil pressure is fine, engine oil level is good,

I have disconnected the Thermostat sensor and rear water pressure sensor (before I discounted the sensors, I clean them before them both; the water pressure sensor was slightly clogged beforehand). I also disconnected the gear lube sensor, but am still constant beeps. This is got me as I don’t know what else to check or what else to disconnect to stop the beeping. Should I replace all the sensors? Thank you in advance for your help

 

[Chris1956]

You do not give a year. The engines changed a bit over the years, especially the thermostat housing.

The temp alarm would sound if the sender wire was grounded. The sensor would ground the wire when it overheated. If the temp alarm wire fell off and touched the block or got pinched causing it to ground, the alarm will sound.

Some MerCruisers had low oil pressure alarms as well. That could also be the source of the alarm. That was in addition to the oil pressure gauge. The oil pressure sender was grounded with no oil pressure, and would sound shortly after the motor started, if the oil pressure did not push on the sender, causing it to unground. The sender was often on the back of the block and had a peppermint patty shape.

 

[Robertse]

You do not give a year. The engines changed a bit over the years, especially the thermostat housing.

The temp alarm would sound if the sender wire was grounded. The sensor would ground the wire when it overheated. If the temp alarm wire fell off and touched the block or got pinched causing it to ground, the alarm will sound.

Some MerCruisers had low oil pressure alarms as well. That could also be the source of the alarm. That was in addition to the oil pressure gauge. The oil pressure sender was grounded with no oil pressure, and would sound shortly after the motor started, if the oil pressure did not push on the sender, causing it to unground. The sender was often on the back of the block and had a peppermint patty shape.

Hi Chris, thanks for your reply. The year of the engine is 2003, I have attached a photo. From what I can see the sender was is not grounded, but I will check all the wiring again. The oil pressure gauge is workign and will goes up to mid way after running the engine for a few minutes, but I will check the wires. Thanks again.

 

[Chris1956]

Also, some of the MerCruisers had a temp sender for the temp gauge and an overheat sensor for the alarm. Make sure the wires were not switched as the overheat sender was either ungrounded (normal) or grounded (overheat). The temp sender for the temp gauge was a variable rheostat with different resistance to ground values, depending on the temp of the motor. I am not sure what would happen to the overheat alarm, if it was connected to the wrong sender.

 

[alldodge]

Looks like a 555 ECM

As Chris mentioned, your motor has a Temp sensor and Temp switch. Both of these are mounted on the thermostat housing. The sensor has one wire and it goes to the helm gauge. The other has 2 wires and it goes to the ECM

The motor has a Oil pressure sensor and switch. These should be down by the remote oil filter connections on the lower rear port side section of the block. They will be connected using a Tee fitting attached to the block. Again the sensor has one wire and the switch has two wires

 

[muc]

Agree this looks like a ECM 555.
Didn't post a serial number and I'm not smart enough for year to mean much. So this is off the top of my head.

The ECM 555 controls the warning horn, so any single wire sensors will be for analog gauges only and have nothing to do with the warning horn. There shouldn't be any temp or oil pressure switches on this engine.
Generally speaking disconnecting any sensors will set a warning horn, so probably not a good troubleshooting tool.

One of the problems with this system is that what you see on the gauges and what you feel with your hands ----- doesn't matter! What matters is what the ECM thinks it's seeing and the only way to know what the ECM thinks it's seeing is to ask it. That requires a scan tool and someone who knows how to use it.

Another funny thing about this system, because the ECM controls the warning horn it also knows if you unplug it. Not sure exactly what year it started, but those dam engineers started programing the ECM to increase guardian if the horn is unplugged. It seems they don't want to let you kill your engine by doing something stupid. It's almost like they think they know more then the average boat owner.

So stop unplugging stuff --- your just adding more faults to the system.
In a situation like this, it's usually more cost effective to get a scan tool and learn how to use it or hire someone to do it for you. If you do decide to start replacing all these sensor, do yourself a favor and save the old ones. The chances of making the problem worse by buying cheap stuff off of Amazon it pretty good.

 

[muc]