Overheating alarm - Mercruiser 7.4 MPI
 

Hi I have a 2001 Mercruiser 7.4 MPI with a Bravo 3. On the first day of the boating season I noticed an overheat alarm. The temperature on the gauge was approximately 170. I slowed the boat down and it fell quickly to around 150 and the alarm stopped. I have a MEFI gateway which shows the engine data so this alarm is being logged by the ECM.

I took it out again and noticed the alarm is going off when the temperature rises above 150. Now in the spring I had the manifold and risers replaced and after the overheating I had the sea water pump with inside impeller, thermostat and temperature sender all replaced. I am still getting the alarm.

I have looked at the engine service manual and it doesn't make clear what the maximum operating temperature is and what temperature should the alarm go off on. Also last season after a long trip I have screen shots where the temperature gauge is sitting at 170 but I didn't get any alarm. As a matter of fact prior to my first trip the ECM had no logged engine temperature events in its history yet I ran at 170 many times.

Any ideas as to what the maximum operating temp of a Mercruiser 7.4 is and what should trigger the alarm?

Any possible ideas as to what could be causing overheating (if it is really overheating) would be appreciated. Checking with an IR gun the temperature of the hoses are in the 160 range when the alarm goes off and I can hold my hand on them so it doesnt feel like its overheating. The manifold and risers were at 130.

Any ideas would be appreciated.

Thanks,

Scott

It uses a 160* TStat so seeing 170 on the gauge is fine. My guess is your alarm sender is bad. The alarm sender is separate from the gauge sender. Alarm system has tan wire with blue stripe.

Hey Griff,

Thanks for your reply. That sounds like a strong possibility. Also even though the gauge on my dashboard looks okay my mefi gateway shows engine coolant temperature (does not show engine water temp for some reason). Anyways I had taken a couple of screen shots and one of the engine coolant temperature readings was 213, but then a minute later it was 180. My mechanic replaced the water temp sensor but I just discovered that there's another sensor for engine coolant that sends info to ECM. Even though I know that the ECM shows engine coolant temperature do you know if the ECM engine overtemp alarm (MEFI3) is triggered by the engine coolant temperature? If so maybe that sensor is bad?

On a separate note the parts catalog call for a 160 thermostat. My mechanic replaced thermostat with a 140. I asked her but she said that Mercruiser recommends 140 for saltwater use which I read somewhere else. She said if anything 140 would decrease the temperature so it being a 140 thermostat wouldn't cause the alarm to go off earlier would it? (The alarm was going off with original 160 so she replaced the thermostat, water pump and temp sender).

Scott

Delete

Changing tstat temp doesn’t change alarm temp. The alarm wasn’t changed.

Okay, that's good to know thanks.

The MEFI-3 ECM sounds the alarm when the ECT switch closes. You have 2 sensors on the thermostat housing. The single wire sensor is for the gauge display and is not connected to the ECM. The 2 wire sensor is the ECT and is connected to the ECM and is what sounds the alarm

Yes there are multiple senders on that engine. The analog gauge sender looks like this. It is not connected to the ECM.
https://www.mercruiserparts.com/8064...der-water-temp

The coolant temp sensor is connected to the ECM, it looks like this
https://www.mercruiserparts.com/8052...nsor-assy-temp. You should probably replace this one if not done already.

There is also a switch for the audio alarm system, but I am not sure your engine has one. It looks like this.
https://www.mercruiserparts.com/48952-switch-assy-temp

You didn't mention if you have closed cooling

Thanks for the links, it will help me

Bravoitis