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Old 27-05-2021, 17:22   #1
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Overheating alarm - Mercruiser 7.4 MPI

Hi I have a 2001 Mercruiser 7.4 MPI with 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
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Old 28-05-2021, 10:10   #2
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Re: Overheating alarm - Mercruiser 7.4 MPI

The alarm is a different sensor than the gauge.

The alarm comes to the main round engine connector on the tan / blue wire. The sensor wire is blue / tan.

Not seeing a temperature alarm switch on that engine, just a low oil pressure alarm switch and a drive unit oil level alarm switch.

Why not locate where those two blue / tan wires connect to the tan / blue wire, then sequentially disconnect each switch from that harness to see which is unhappy?
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Old 28-05-2021, 11:10   #3
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Re: Overheating alarm - Mercruiser 7.4 MPI

Quote:
Originally Posted by wingless View Post
The alarm is a different sensor than the gauge.

The alarm comes to the main round engine connector on the tan / blue wire. The sensor wire is blue / tan.

Not seeing a temperature alarm switch on that engine, just a low oil pressure alarm switch and a drive unit oil level alarm switch.

Why not locate where those two blue / tan wires connect to the tan / blue wire, then sequentially disconnect each switch from that harness to see which is unhappy?
Thanks for your reply I will try to locate the wire. It definitely has an engine temperature alarm, it shows the on Mefi3 gateway as active and I'm getting a high pitched alarm when it occurs.

That's a good idea about isolating the wire. I also just found out that there's two sensors, one for the gauge and the other for the ECM. They call the first one the water temp sender which was replaced the second one they call the engine coolant sensor and that sends the info to the ECM. In the parts catalog it's listed as 805218T

Do you happen to know if the alarm is connected to this sensor (you said it's not connected to the other)?

If this sensor is the problem it makes sense because there seems to be a discrepancy between the two. The gauge in the helm doesn't seem to go much past 170 usually but the engine gateway showed a temp of 213 so maybe that is the issue

Parts catalog for this engine that shows temp sensors:

https://www.perfprotech.com/mercury-marine-parts-catalog/senders-and-harness-bracket/assembly/2705/90?umodel=22376

Thanks

Scott
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Old 28-05-2021, 11:23   #4
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Re: Overheating alarm - Mercruiser 7.4 MPI

Yes the gateway showing 213F while the helm gauge shows 170F is likely why the gateway is issuing an over temperature alarm.

My suspicion would be a connection / wiring problem, especially given the measured component readings.

On my Mercruiser 454 MAG MPI Horizon engines, the manufacturer used right angle push on connectors for many of the sensors and switches. Some of those went bad and I snipped them off, replacing w/ properly sized and properly crimped ring tongue connectors, retained w/ properly sized nuts and washers, onto the existing threaded sensor posts.
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Old 28-05-2021, 11:54   #5
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Re: Overheating alarm - Mercruiser 7.4 MPI

Quote:
Originally Posted by wingless View Post
Yes the gateway showing 213F while the helm gauge shows 170F is likely why the gateway is issuing an over temperature alarm.

My suspicion would be a connection / wiring problem, especially given the measured component readings.

On my Mercruiser 454 MAG MPI Horizon engines, the manufacturer used right angle push on connectors for many of the sensors and switches. Some of those went bad and I snipped them off, replacing w/ properly sized and properly crimped ring tongue connectors, retained w/ properly sized nuts and washers, onto the existing threaded sensor posts.
Okay there has been talk about it being maybe the wires between engine and ECM or the engine harness if it's shortening out under load. Is that a big job if it is that?
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Old 02-06-2021, 19:25   #6
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Re: Update on Overheating alarm - Mercruiser 7.4 MPI

It turns out that the flushing port on the engine was open and that supposed to suck air from the engine.* It had overheated right before discovering that it was open and once it was closed and ran again it did not overheat.

The impeller may need to be changed but the hose being disconnected as exasperating the problem.**

Thanks for all of your help,

​​​​​​​Scott
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