Problem with motor shutting off

[mark350]

Hello,

Hopefully someone can help me solve this mystery.....

I'm working on my 1998 5.7 liter mercruiser (in a 22.5' Sting Ray) and I am running into a problem. I've had this problem before but it went away - now it's back to haunt me.

Here's what happens. I start the motor... it will run for about 1 minute then shut off... if I give it throttle, it will die sooner. However, I am able to start it back up every time it stalls with no problem - then it will run for another minute before stalling again (I can do this over & over). It's acting like it's burning off all the fuel in the carburetor bowl and not getting anymore to refill the bowl (until I start it again) - just a thought.

I notice when the key is in the "start" position, the electric fuel pump is pumping fuel to the carburetor... however, when the key is in the 'run' position, the electric fuel pump is off (would this be correct)?

*I just rebuilt the carburetor and that did not fix the problem. Any suggestions would be much appreciated. Thank you.

 

[JustJason]

Re: Problem with motor shutting off

the electric fuel pump should be getting its power from the oil pressure switch.
You have some backwards wiring there by friend.

 

[fishingman220]

Re: Problem with motor shutting off

I am going through a similar problem right now, but i am not sure but i dont think your pump should cut off untill you turn the key off. i would test your fuel pressure near the carb to see what you have feeding the carb. i dont really know why it is cutting off but i think that might be your prob.

 

[mark350]

Re: Problem with motor shutting off

JustJason & fishingman220,

Thanks for your replies.....

The fuel pump is definatley not pumping any gas while the motor is running (only when the key is in the 'start' position).... I didn't think that was normal.

JustJason: the electric fuel pump should be getting its power from the oil pressure switch. You have some backwards wiring there by friend.

So you're saying I should find the oil pressure switch and see if it's getting power to it? *There are only 2 wires coming off the fuel pump... and the positive only shows power when the key is in the "start" position.

Thanks for helping...

 

[JustJason]

Re: Problem with motor shutting off

Don's has posted the schematic a million times for it. It's here some where if you want to search for it.
He'll probably jump in and post it again for ya just because he's just a nice guy..... when he's not being a grumpball

 

[Don S]

Re: Problem with motor shutting off

The oil pressure switch is a Normally Open switch, when the oil pressure build up to 5 psi, it closes and allows power to go the fuel pump. It's a safety feature. To keep the pump from running if the engine isn't.
The switch is normally on a tee above the block mounted oil filter.
Here is the wiring diagram.

 

[Don S]

Re: Problem with motor shutting off

when he's not being a grumpball

Hey!!!!!!!!! I resemble that remark.

Where's my smite button when I need it.

 

[mark350]

Re: Problem with motor shutting off

Thanks for the help... I guess I'll replace the oil pressure switch (check it first with the volt meter).

 

[JustJason]

Re: Problem with motor shutting off

I don't know about you but my smite key is in between my Scr Lk and by Sys Rq buttons It's the 1 shaped like a lighning bolt.... hehe

Mark350.... don't just go replacing the switch just yet. Double check the wiring from the fuel pump to where it goes into the harness.... look the best you can for home made splices.
What's the backstory on this rig? Is this a new to you boat? Have you owned it for years and all of a sudden this just started happening? Has it ever run correctly for you before?
Whats the dilly?

 

[Gary H NC]

Re: Problem with motor shutting off

Hey!!!!!!!!! I resemble that remark.

Where's my smite button when I need it.

Didn't you mean "resent"..?

 

[mark350]

Re: Problem with motor shutting off

I don't know about you but my smite key is in between my Scr Lk and by Sys Rq buttons It's the 1 shaped like a lighning bolt.... hehe

Mark350.... don't just go replacing the switch just yet. Double check the wiring from the fuel pump to where it goes into the harness.... look the best you can for home made splices.
What's the backstory on this rig? Is this a new to you boat? Have you owned it for years and all of a sudden this just started happening? Has it ever run correctly for you before?
Whats the dilly?

We bought the boat new... it has, however, had a lot of work done with the wiring... here are some examples:

a. depth finders installed & removed
b. CB radio installed & removed
c. The tube in the back of the speedometer popped off and corroded the fuse panel so that was replaced.
d. bilge pump rigged

I notice the oil pressure gauge does work... but I assume the rely that connects it to the fuel pump may be bad.

The two wires coming off the fuel pump go into a harness and then snake back to God knows where... I do, however, see two wires coming off the oil pressure gauge - and one is color the same as the wire that's connected to the fuel pump. Should I check the voltage to that wire when the engine is running (should it be live when the motor is running)? What course of action do you suggest?

Thanks for your help.

 

[JustJason]

Re: Problem with motor shutting off

Get the engine running however you can and check for power on the purple wire going into the switch then for power on the purple/yellow coming out of it. If you have power on both lines then the p/y is not making its way to the fuel pump someplace along the lines.

The oil pressure gauge is on a different sender. Actually, the gauge uses what they call an oil pressure sending unit, the fuel pump an oil pressure switch. You want to be looking at the switch on the side of the motor, not the sender up top.

The work you described done is all usually done by someone who is 1 step up from a lot boy... an apprentice, or a rigger. Usually not a mechanic (but hey... sometimes we do when it's slow )
If the work was recently done, honestly I would bring it right back to whoever did it and tell them your fuel pump worked before they touched it. If the work was done a long time ago then your on your own.

The key here is there's always power on the purple/yel wire. Pull the wire off your choke and see if you get power there when the engine is running, its on the same circuit.

There's also the posibility that your purple/yel wire shorted at 1 point and melted the wire internally. In which case your better off cutting it out, capping the ends off, and wiring around it the way the schematic describes to. Put your meter on resistance and ground out your black leak and check for a path to ground through the purp/yell. Shouldn't have one.

And oh yeah... there's no fuel pump relays.

 

[Limited-Time]

Re: Problem with motor shutting off

We bought the boat new...

I notice the oil pressure gauge does work... but I assume the rely that connects it to the fuel pump may be bad.........................

Thanks for your help.

The oil pressure sending unit, and oil pressure switch are two different components. The sending unit sends a variable resistance signal to the oil pressure gauge. The Oil pressure switch is an open/closed circuit (off/on) device.

 

[mark350]

Re: Problem with motor shutting off

JustJason,

Thanks again for your help. The oil pressure switch was definitely the problem. I started the boat, then pulled the wires from the oil pressure switch and crossed them together. The boat ran perfectly and did not stall.

So I guess that solves it - I need a new oil pressure switch (I notice the switch was very corroded). Thanks again for your help!!

 

[JustJason]

Re: Problem with motor shutting off

sure thing.... always best to test first though instead of stabbing at it. Glad you found your problem!