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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10708
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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My Mercruiser 5.7efi's fuel pump works intermittently. When

Customer Question

My Mercruiser 5.7efi's fuel pump works intermittently. When I turn the key the fuel pump runs for a few seconds and then I start the boat as usual. The boat will run at idle with no issues, but when bringing the rpms above 1000 in neutral or in gear the motor quits. When I turn the key off and then back to the on position the voltage on my dash and gps drops between 10-11volts and the pump does not run, but the motor will turn over at a normal speed. If I engage the starter several times the fuel pump will turn on allowing me to fire the engine on the next try. A couple things worth mentioning is that I have replaced the starting battery yesterday and after using the boat all weekend the boat just quit Sunday evening after a 45 minute run and would not restart (same problem the fuel pump would power up, plus low voltage). Lastly, when I tried to start the boat Monday the pump ran without issue, and the motor ran great at idle. I did not bring the rpms above idle. It appears to be an electrical issue, but I'm not sure where to start (oil pressure switch, wire harness, relay, etc). Any advice would be greatly appreciated. Thanks!
JA: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Soon. I'm trying to find the right Marine Mechanic for you right now. As soon as they are ready, I will let you know. Is there anything else the Marine Mechanic should be aware of?
Customer: The motor has approximately 20 hours, but is 15 years old.
Submitted: 8 months ago.
Category: Boat
Expert:  Jason replied 8 months ago.

Hello my name is ***** ***** am I speaking with?

All Mercurys are cataloged by serial number, can I get that number so I can run it and see which engine you have.

Customer: replied 8 months ago.
Hi, my name is***** serial number is *****
Expert:  Jason replied 8 months ago.

Hi Tim,

I ran the serial number. This comes up as a carbureted engine, but you said the engine is EFI. Is your engine EFI or Carb? Is the serial number you gave me correct?

Customer: replied 8 months ago.
It is definitely a fuel injected motor. The model is MCM 5.78L EFI. I have seen people refer to the this motor as a TBI.
Expert:  Jason replied 8 months ago.

Can you double check your serial, because it is coming up as a carb.

Customer: replied 8 months ago.
I've attached a picture showing the engine specs and serial number.
Expert:  Tan replied 8 months ago.

Hello. My name is Tan.

That does come up as EFI the number is ***** OM045431.

We need to check voltage at the fuel pump when engine acts up. Does it loose all voltage?

The fuel pump gets it voltage from the ECM. You could have an ECM issue or a relay issue.

The wire we want to check voltage on is the dark green/white going to the fuel pump relay.

Customer: replied 8 months ago.
can i test the green/white wire voltage at the ECM? If not, I'm not sure where to test. Also, the engine compartment on this boat is incredibly tight and I if I need access to the fuel pump I will need to remove the port manifold.
Customer: replied 8 months ago.
Is the fuel relay on top of the motor next to the ECM?
Expert:  Tan replied 8 months ago.

Test the wire coming from the ECM at the fuel pump relay. It will be next to the ECM.

Customer: replied 8 months ago.
I have 5.37 volts with the ignition open, and less than 1 volt when engaging the starter. I also got 12.68 volts when testing the positive lead at the solenoid mounted on top of the intake manifold.The fuel pump is now running, and the time it takes for it to cycle off after the ignition is open varies greatly. Also the injectors sprayed fuel a few times and the motor will not start now. I think it may be flooded, if that is even possible on an efi.
Expert:  Tan replied 8 months ago.

Are you testing voltage on the green/white wire coming from the ECM?

For the engine to flood you would have to have no spark or to much fuel pressure.

Have you tested fuel pressure to the Throttle body?

Have you checked for spark?

Customer: replied 8 months ago.
I tested the green/white wire coming out of the ecm and a green/white wire attached to the fuel relay and both read 5.37 volts. When I opened the ignition this time the fuel pump never stopped running, and the injectors sporadically sprayed fuel. I have not checked for spark or fuel pressure yet, but the engine was starting and running great at idle 2 hours ago. Where and how do I check the fuel pressure on this engine?
Expert:  Tan replied 8 months ago.

It takes a special fitting. We know you have fuel pressure as the injectors are squirting fuel. So far as you know the fuel pump hasn't quit yes but we can't tell till you have the engine running. We aren't going to know anything on the fuel pump till the engine acts up. What we are looking for is the fuel pump to quit. When it quits is when we need to check voltage on the green/white.

Customer: replied 8 months ago.
I'll check the spark and get back to you. Are my voltages "normal" on the green/white wire?
Expert:  Tan replied 8 months ago.

Yes that is enough voltage to turn the relay on. All that wire does is turn the realy on the pump gets voltage from a fused 12 volt source elsewhere.

Customer: replied 8 months ago.
I tested the spark with a spark plug test tool and saw the light bulb glow on 2-3 times over several crankings. I confirmed the tool was working on my portable generator. The pump is now running continuously and never cycles off. I'm not sure what to troubleshoot next.
Expert:  Tan replied 8 months ago.

Is the engine running?

The pump is supposed to run continuously when engine is running.

Customer: replied 8 months ago.
The engine started and ran at idle for 10-15 seconds then stalled out. The voltage on the green/white wire was .79 volts. I couldn't restart it after that. Yes, the pump is running continuously even when the motor is not running.
Expert:  Tan replied 8 months ago.

Is the pump running when the voltage on the green/white drops to .79 volts?

Customer: replied 8 months ago.
I am not sure. I wasn't able to hear it over the engine, and I was too slow at pulling off the flame arrester.
Expert:  Tan replied 8 months ago.

Ok since the engine quit I'm going to say no. So what we have is a ECM problem. The ECM has an internal malfunction causing the pump to run erratically.

Customer: replied 8 months ago.
Can a shop bench test and ECM? I noticed they cost about $1500 to replace.
Expert:  Tan replied 8 months ago.

No we know it doesn't work as the fuel pump keeps shutting off. Do this run a jumper wire to the green/white on the relay now run engine does it quit?

Customer: replied 8 months ago.
The fuel pump runs continuously when the ignition is open, and the motor will not start at this time. Should the jumper be a 12volt (+) supply to the green/white wire, and if that is correct can I possibly damage the ECM in the event it is okay?
Expert:  Tan replied 8 months ago.

You will want to cut the green/white wire for this test. This way you don't send voltage to the ECM. I just want to verify the fuel pump is the issue.

Customer: replied 8 months ago.
Just to verify, I should cut the green/white wire to the relay, and then connect 12v to the relay.
Expert:  Tan replied 8 months ago.

Correct. If engine runs normally then the ECM is bad.

Customer: replied 8 months ago.
I ran a jumper 12v (+) to the green/white wire of the relay and the pump did not run. Once I touched the 12v(+) probe of my voltmeter to the green/white wire and the 12v(-) probe to ground the pump began to ran.
Expert:  Tan replied 8 months ago.

Try another relay.

Customer: replied 8 months ago.
I switched the two relays with identical part numbers, and this time I could not get the fuel pump to run at all. At this point I'm going to call it a day. Once I can pick up a new relay I will resume troubleshooting.Do you think the other relay may be the problem, and not fuel pump relay? Just a thought, I've only had this engine for 3 months so I'm still learning.
Expert:  Tan replied 8 months ago.

Here is a wiring diagram.

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