4.3 fuel pump continuously priming

[kanteen]

Hey Everyone,
It seems that when I turn my key to the on (not crank/start, just on) position my fuel pump primes and does not shut off. Normally it would prime for a couple seconds and then shut off. I inspected the cabling and everything appears intact, but I have been working around the engine lately so I don't rule out a wiring problem. Flipping the ignition kill switch to stop will shut the pump off. Disconnecting either wire from the oil pressure switch will also shut the pump down. This leads me to believe It almost has to be the oil pressure switch. With all that being said, I guess my question is, is it possible for an oil pressure switch to fail in this manner? I've always heard of them failing the opposite, not allowing the fuel pump to run at all, so this has me stumped.

Any suggestions for testing or confirm it's the switch? Would there be a relay or something else that could produce this? I don't know what bypassing the switch would do since the pump appears to be working?


Also, I do have a seloc book with the correct wiring diagram, it all seems correct.


Thanks for any help!


P.S. Serial number: OK147411

 

[Bondo]

Ayuh,.....Welcome Aboard,.... If power is passin' through the oil pressure switch, without oil pressure, the switch is Bad, or by-passed,...

The fuel pump shouldn't run at All, unless the starter is engaged, or the motor is runnin',....

 

[kanteen]

Ayuh,.....Welcome Aboard,.... If power is passin' through the oil pressure switch, without oil pressure, the switch is Bad, or by-passed,...

The fuel pump shouldn't run at All, unless the starter is engaged, or the motor is runnin',....

Thanks Bondo! Sorry to make you repeat yourself, but I just want to double check i'm hearing you right. When you said the pump shouldn't run at ALL unless the starter is engaged or motor running, I take that as meaning a 3-4 second prime when the key is in the on position (not cranking or starting) is not expected, correct? I'm almost certain it did just that in the past, what would cause this (relay somewhere??)

I'm just trying to figure out what to expect when I replace the switch... Basically, I shouldn't hear anything when the key is "on", and the boat should start and run, indicating the switch works.

 

[kanteen]

Thanks for the help! It is definitely the switch. Hit it with a multimeter with the key in the on position, had 12v on both sides, indicating the switch was stuck closed. Gave it a few good whacks with my pliers and the N.O. side (purple/yellow wire to pump/starter) is no longer pushing 12v while the key is in the on position. Now the pump does NOT come on while the key is in the on position, only run/crank. Time to replace the oil pressure switch.

So for anyone else who encounters a constantly priming fuel pump I guess they can fail closed and provide constant power to the pump while the key is in the "on" position. Never knew that was possible. They usually fail open with no power to the pump and no fuel.


Thanks again Bondo!

 

[Bondo]

Thanks Bondo! Sorry to make you repeat yourself, but I just want to double check i'm hearing you right. When you said the pump shouldn't run at ALL unless the starter is engaged or motor running, I take that as meaning a 3-4 second prime when the key is in the on position (not cranking or starting) is not expected, correct? I'm almost certain it did just that in the past, what would cause this (relay somewhere??)

I'm just trying to figure out what to expect when I replace the switch... Basically, I shouldn't hear anything when the key is "on", and the boat should start and run, indicating the switch works.

Ayuh,.... Correct,....

 

[kanteen]

Sorry for taking so long to post the fix. It ended up being the Oil Pressure Fuel Shut Off Switch. Usually they fail and the fuel supply is cut off, but it in my case it was the opposite. For my 97 4.3LX I used Mercruiser part 87-864252A01.

 

[philbullet]

Good mornimg to all.
Reading this i realized on my eng. swop and rebuild to the 4.3 ,when i installed the 4.3 and also when was testing in previous boat (the eng.) that on my set up the after market Holly fuel pump STAYED running while key was in ON position.After rebuild and installed into my boat it's the samething.Now i havn't ran eng. yet,still waitng on a few parts.I asked my brother ( auto mechanic) about the holly ele. fuel pump running with key in on position and he said they do that,run and bypass after preasure amount is reached (forgot how many lbs. he said) .
From what the wire diaghram show's i guess i will have to do some back tracking now.LOL
I my also have a bad oil preasure switch?

 

[Bt Doctur]

1 do you have a carb or injected motor? carbed motor use a pressure switch , the power to the pump during cranking comes from the solenoid, power to the pump when running comes from the purple wir(ign. side)

 

[thumpar]

In an auto that would be fine but this is a boat. The fuel pump running when the engine is not running is a safety hazard.

 

[Bondo]

Sorry for taking so long to post the fix. It ended up being the Oil Pressure Fuel Shut Off Switch. Usually they fail and the fuel supply is cut off, but it in my case it was the opposite. For my 97 4.3LX I used Mercruiser part 87-864252A01.

Ayuh,..... Atleast ya came back to report,...... Thank you,....

 

[Bondo]

Good mornimg to all.
Reading this i realized on my eng. swop and rebuild to the 4.3 ,when i installed the 4.3 and also when was testing in previous boat (the eng.) that on my set up the after market Holly fuel pump STAYED running while key was in ON position.After rebuild and installed into my boat it's the samething.Now i havn't ran eng. yet,still waitng on a few parts.I asked my brother ( auto mechanic) about the holly ele. fuel pump running with key in on position and he said they do that,run and bypass after preasure amount is reached (forgot how many lbs. he said) .
From what the wire diaghram show's i guess i will have to do some back tracking now.LOL
I my also have a bad oil preasure switch?

Ayuh,.... Yer Bro is Right,.... for an Automobile,...

For yer Boat, wire it as We've been talkin' 'bout in this thread,....
The fuel pump should Not run with the key "On", unless the starter is Startin', or the motor is Runnin',....

The brand of pump don't matter, the Marine Safety factor is in how it's wired,.....

 

[philbullet]

Thanks Bondo and other's,glad I came across this post, now I have one more thing to look at and didnt realize I had a problem.
Another helpful thread.thanks guys.