Wont Stay Running - MerCruiser 4.3LX (GEN +) (2 BBL.) GM 262 V-6

0ldog42

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1997 MerCruiser 4.3LX (GEN +) (2 BBL.) GM 262 V-6

Where does +12v come from once engine is running? Generator, right?

Symptoms: Engine starts, runs for a minute then quits. Wait a minute or two and it starts and runs 10 or 15 seconds then quits.

The way I understand the system... You turn key which gets +12v from the battery and sends it to the fuel pump runs, (I can both hear and feel it running.) Continue to rotate the key and the engine starts. At this point when the key is released the +12v now comes thru the oil pressure switch (that is a normally closed, if you loose pressure the switch opens shutting the power to the fuel pump stopping engine.

If the switch was bad the engine would run till the float bowl emptied and quit, just like my symptoms. If you pulled the OP switch leads and jumped them simulating a good switch the engine should stay running if that's the problem... well it doesn't. Even replaced switch, of course to no avail.

An online tech suggested the carb needed cleaning because it was flooding the engine and stalling it. Rebuilt carb with the exception of removing the "T" keeper, spring and ball. Put everything back together and you guessed it... still the same. No water in separator or carb when puller.

What's the chances of the Generator going bad so there is no +12v at the OP switch?

Thanks for help, laughter not welcome at this point.
 

harleyman1975

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May 12, 2003
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After engine dies is there still fuel in the carb? is the fuel pump still PUMPING FUEL? (not just running) I'm thinking that there may be a restriction in the fuel line. This boat is 20 yrs old. Rubber fuel line between the tank and pump could be collapsing, anti syphon valve could be dirty/rusted/frozen, fuel pickup in tank could be plugged or even defective.
 

Rick Stephens

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Power to ignition comes from the battery through the ignition switch direct to coil. Generator charges the battery, the battery supplies power.

Power to electric fuel pump comes from 2 sources:

1) the 'primer' source is off the outside lug of the starter. This powers the pump during cranking.

2) second source is off an oil pressure switch. That should be switched from the ignition switch, through the oil pressure switch, to a solenoid that is fused.

Failure points are the pressure switch, the solenoid and the fuse as well as al the connectors/contacts in that chain. Testing can include turning ignition switch on and testing that power comes on at the oil pressure switch. If it has power then you can short across the two terminals on the pressure switch and if the rest of the system is working this should immediately power your fuel pump. If you have power at the switch and shorting across it does not power the pump then you need to check your way between there and the pump.

If turning your ignition switch to run position turns on your fuel pump then the system is unsafe.

Rick


Edit, looks like no solenoid on factory wiring. Direct power to oil pressure switch which directly power the pump.
 
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tlewis1

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Oct 9, 2012
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I'd agree with leaning towards a fuel issue...I think your barking up the wrong tree chasing the electrical circuits....I'd first see if you can keep the engine running with a little carb cleaner down the throat of the carb (flame arrestor off) when the engine starts to die. I'd also look at running a separate tank with fresh fuel; as harleyman mentioned could be issues with the fuel tank or line on the boat side.
 

0ldog42

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Thanks for the quick responses: First, the line into the carb was pulled and run into a jug, the fuel pump was giving us maybe 1 1/2 GPM. The anti syphon valve was replaced three years ago when we purchased the boat. Second, I believe the ignition switch operates as such... turn key, lights and gages energized; pushing key in puts +12v to fuel pump; rotate switch engages starter then when you release the key after start it rotates back disengaging starter and pops out removing +12v from pump at which time the +12v should now run thru the oil pressure switch and you have a float bowl's worth of gas to build up oil pressure or the engine quits.

HARLEYMAN... when you ask is there still fuel in the carb, do you mean in the float bowl?

RICK STEPHENS... Jumpering the OP switch was what we did to check the FP, We had hooked 12v and grd to the pump prior to make sure it worked.

This problem happened all of a sudden. I would expect bad fuel or poor delivery lines would show up gradually. It'll take me a while to check out the hand fuel pump, but I'll get back to you. Thanks again.
 

alldodge

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Engine starts, runs for a minute then quits. Wait a minute or two and it starts and runs 10 or 15 seconds then quits.

I to see it as running out of fuel and the pump is not running once its started. Runs for minute, then just seconds

pushing key in puts +12v to fuel pump

Has the key switch been replaced? The wiring should be that there is 12V on the purple wire going to the OP switch. When starting 12V is supplied to the fuel pump by the Purple/Yellow wire coming from the starter.
Starter wiring.jpg

when you ask is there still fuel in the carb, do you mean in the float bowl?

Yes when the motor died was their still fuel in the bowl? Push the throttle forward and do you see gas squirt in.

See this as either a bad OP switch, a bad connection feeding the OP switch or feeding the fuel pump
 

Mercruiser420

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Nov 24, 2015
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Did you check for spark when it stalls? does your coil get hot? What's your alternator charging at?
 

Rick Stephens

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pushing key in puts +12v to fuel pump;

I thought a primer function on the ignition switch was only used in some outboards... I've never seen that in any wiring diagram for a Mercruiser sterndrive. I'm no where near as experienced as AllDodge, so maybe he can clear this up, but seems to me if your ignition switch does that then it was modified from stock. Every system I have seen the fuel primer circuit runs off the purple/yellow wire attached to the starter and powers the pump only when starter is engaged.
 

NHGuy

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Why not put a test light on the fuel pump lead. Start the engine, if it lights during cranking but not during run, chase the electrics back to their source. I think from your description that this is a likely scenario. And if I'm right, your fused fuel pump lead isn't live where it comes into the oil pressure switch or it could be wired wrong.
I noticed you replaced that switch. When you did that you should have put the lead to the pump on terminal "P", the one from the starter goes to "S" and the normally-on wire from the ignition system goes to "I".
"I" should be purple, "S" and "P" should be purple with yellow stripes for most years.
 
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NHGuy

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Rick I think you and I just found a project. That outboard type, priming ignition switch would make our sterndrive carbed engines fire off like fuel injection.
 

Rick Stephens

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Rick I think you and I just found a project. That outboard type, priming ignition switch would make our sterndrive carbed engines fire off like fuel injection.

When I leave the boat sitting for a month or more, I short the starter to prime the carb. Starts up in half revolution. Beats cranking for a minute.
 

0ldog42

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Just started it and kept spraying gas from a bottle down carb and it kept running. Hooked up line to carb input but didn't run, does gas have to be under pressure to carb?
 

alldodge

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For a carb it does not have to be under pressure but the line feeding the carb does. This way as the carb uses the gas, more comes in.

Since the motor will run while spraying fuel down it, you have a fuel delivery issue. I'm still guessing its the electrical connection to the pump. Need to find out if your getting power to it and that the pump has a good ground
 

NHGuy

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My fuel tank is about 15" from my pump, so I can crank for 10 seconds and get ignition. I still like a quick start though, so sometimes I give it a drink of carb cleaner or gasoline down the primary bores.
 

0ldog42

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The engine will keep running if raw gas is sprayed down carb. with fuel pump disconnected. But it looks like it's flooding out if fuel is run through the carb. This was after rebuilding the carb. Pulled the carb again and looked inside. Looking at the fuel pumped into a glass jar you can see a distinct separation between two liquids, both of amber color. Testing the bottom liquid shows that it's not water because it will burn. Plan on draining the gas tank, flushing that and the lines this weekend.
 

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0ldog42

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" Hooked up line to carb input but didn't run, does gas have to be under pressure to carb?" When I made this statement I failed to mention we connected a line to the carb input which was connected to a funnel held over the carb, which should have kept the bowl full. The picture is of the fuel pumped from the tank on the left of course and new gas on the right.
 

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alldodge

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The gas has phased separated, been there a long time. After pumping it out and starting fresh you should be good.

It doesn't need much pressure but does need some and the funnel should have worked. The needle valve may be sticking a bit
 
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