2003 90hp Johnson 4 stroke fuel issue

[Fcw1550]

Hello Everyone,
Hoping for a little experience here. I've got a devil of a problem that has just cropped in my 2003 90hp Johnson 4 stroke motor. I'm not a marine mechanic but I am a Mechanical Engineer and I work with a lot of Power Generation equipment so I'm quite used to troubleshooting and failure analysis. In this case I'm fighting a problem with the electric fuel pump. The pump is actually operating fine, it's the fuel control circuit that is giving the motor problems. During the start up sequence, the control provides voltage to the fuel pump for a short period (I've read 6 savings) to pressurize the fuel rail. On my motor, that voltage sometimes appears immediately after the diagnostic start sequence, some times within 10 or 15 seconds after the diagnostic sequence, some times as long as 30 seconds, and needless to say, sometimes not at all. If the voltage appears (whenever it feels like it) the motor will start and run great. If the voltage does not appear in the start sequence, the motor will not start and I need to turn the key all the way off, wait a few seconds and try again. Usually after two or three failed start sequences, the voltage will appear, pump spins up and I'm ready to go. Please note, the pump responds every time there is voltage applied to it, something in the control logic seems to be randomly bypassing the fuel pump start up. So the 1500 dollar question here, is this likely the ECM, or are there other sensors that feed into this specific portion of the fuel control.
Any help would be greatly appreciated,
Fred

 

[daselbee]

OK Fred...you have a misconception of the way it is supposed to work. When you key ON, +12v is always present at one of the two terminals of the fuel pump. The other wire at the fuel pump goes back to the EMM. The EMM then provides the GROUND for that circuit for about 10 seconds, and the pump runs to pressurize the system.
That is the way it is supposed to work.

Are you saying that you have metered the +12v line going to the fuel pump (gray wire) and find that it sometimes only has 12v intermittently?
Or are you saying you are metering the other wire (ground wire) and see +12v sometimes and GROUND other times?

If you think about it, you will see that when the GROUND is supplied internally by the EMM, the metering techniques need to change.

If you are getting intermittent +12v on the gray wire at the fuel pump, then you will ALSO be getting intermittent +12v at the gray wires feeding the ignition coils. The two circuits are the same. If so, you will have no fuel pump, and no spark. Have you checked spark too?

At any rate, if you have intermittent +12v on the gray wire, you have a main relay problem. Main relay will click pnce when key goes to ON, and is located inside the plastic electrical holder box.

If you have solid +12v on the gray wires feeding the coils and the fuel pump, and you have spark when cranking, then you probably have a bad EMM; an EMM that is failing to provide that internal ground to the fuel pump when called for. (Or possible wire intermittency in the harness going back to the EMM.)

 

[Fcw1550]

Daselbee

Thanks for the explanation. I have only checked the voltage at the connector to the fuel pump. Therefore I do not know if it's the voltage supply or the ground that 's missing. My boat is in Delaware and I'm in PA so it will be Friday before I can confirm which leg is faulty. I know that I have spark as the motor would pop with ether when I first started down the troubleshooting path.
Thanks for the info, I never would have guessed that the control was on the ground side.
I'll report back what I find after the weekend ,
Thanks again,
Fred

 

[daselbee]

If you want to exchange phone numbers, you can call me when you are at the boat.....

 

[Fcw1550]

Thanks for your offer. I've sent you my cell number through PM. Let me know if you do not receive it and I'll attempt to send again.
Thanks,
Fred