1994 4.3 Mercruiser fuel pump priming issue

[Greekman98]

Here’s the issue- My fuel pump used to prime for a few seconds and start just fine. My boat no longer primes and won’t start but sounds like it turns over and just dies immediately. Here’s all that I’ve tried:

-Checked engine kill switch (lanyard)
-Checked neutral shift kill switch
-Checked for voltage to fuel pump during cranking (gets 12V)
-Replaced both the oil pressure sending unit and switch (tried tying switch wires together and didn’t start either)
-Traced positive and ground wire and both get power only during running/cranking

If I give the fuel pump direct power before starting up it runs just fine (so doubtful it’s the pump itself) but I don’t want to wire a switch if I don’t have to given it ran fine before. Not sure what could have happened but I feel like I’ve tried everything at this point, not sure where to go from here. Thanks in advance!

 

[Bondo]

Ayuh,..... Welcome Aboard,....... Which 4.3l,..??
Efi,..?? Mpi,..?? Carbed,..??

 

[Greekman98]

Thanks! It’s carbed, it’s the 4.3L Alpha One (262 CID) GenII

 

[Bondo]

My fuel pump used to prime for a few seconds and start just fine.

Ayuh,..... Yer motor can't, 'n never could have done this,......

Only the electronic injected motors have such circuitry,....

Is yer carb a 2bbl, or 4bbl,..??

 

[nola mike]

Here’s the issue- My fuel pump used to prime for a few seconds and start just fine. My boat no longer primes and won’t start but sounds like it turns over and just dies immediately. Here’s all that I’ve tried:

So it catches, then dies? But if you prime it before, then it runs (that would sound like the pressure switch)? Did you confirm that the pump does NOT get 12v once it catches? Curious as to whether you have factory wiring, wondering if someone replaced your mechanical fuel pump with electric at some point. Like bondo said, factory wiring won't let the pump run when you turn the key to "run".

 

[Greekman98]

For sure not factory wiring as it has been replaced by an electric fuel pump (ran great for the past year though and I didn’t touch anything before it stopped working). But just tested the wires and confirmed that it only gets power when cranking so I agree that it sounds like a switch issue. I replaced the switch as that was my original thought but no dice. I also tried jumping the wires to the switch and still nothing.
I tried wiring the purple/yellow wire to the switch straight to the battery and it ran though so maybe a bad/corroded ground with the purple? Not sure where it goes though.

 

[nola mike]

Sounds like it might have been miswired. Are you getting 12v to the switch with the ignition on? If the pump is getting 12v while cranking (and it runs), and you're getting 12v switched power to the op switch, then either the switch is faulty or you have the wires on the wrong terminals.

 

[achris]

Your problem is fully covered in this thread.

4.3 Mercruiser cold start issues

I seem to be having a hard time starting my boat after it sits for a day or more. The boat starts right up if it been running. I pump the throttle between a half a dozen to a dozen times but it still takes a while to start. Towards the end of last year I even had to start using starter fluid to...

forums.iboats.com

 

[Greekman98]

I don’t think it’s wired wrong as it’s ran fine in the past and I haven’t changed anything. I’m convinced it’s a grounding/wire shorting out. I’ve also tried hooking each wire up to each terminal and nothing. I traced the purple/yellow wire to the starter and it gets voltage when cranking (makes sense since I see power going to the fuel pump when cranking). However, the other purple wire going to the switch never gives off voltage so I’m assuming that’s the culprit. It goes into the wiring harness though so it’s hard to trace. Any idea where it should be getting power from? Thanks for the help!

 

[nola mike]

Yeah, that's the main switched power from the ignition. It should also go to your choke (if electric) and the positive side of your coil. If you're not getting power to the op switch when the ignition switched on, the pump will only run during cranking.

 

[Greekman98]

Ended up being a short in the purple wire, boat started right up after fixing and fuel pump primes now! Thanks again, should be out on the lake this weekend

 

[nola mike]

Ended up being a short in the purple wire, boat started right up after fixing and fuel pump primes now! Thanks again, should be out on the lake this weekend

Glad it's running. What exactly do you mean by "prime"? If the pump is running when the ignition is on and the motor isn't running, that's not correct and is a safety issue.