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Mercruiser 4.3 lx alpha one

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Darren-stormin

New member
Hi

I have a suspected fuel pump issue on one on my engines.

When I turn the key to run position I can’t hear the fuel pump running although the engine does start and run for a while it cuts out. I’ve removed the fuel pipe that leads into the carb and it’s getting fuel when cranking. It just doesn’t prime.

When it is running oil pressure goes up to 60psi.

My other engine does engage fuel pump when switched to run then starts and runs lovely.

I have fitted new fuel pump to both engines now but still one doesn’t run after a while.

Any help would be greatly appreciated.
 
Firat, what is your engine other than the size? EFI or Carbed?

Once we know what you have THEN we can try to assist......

UGH!
 
I'd guess carb engine with electric pump. Uses bypass while cranking and oil pressure switch when running to power the pump.
 
Droid said:
I'd guess carb engine with electric pump. Uses bypass while cranking and oil pressure switch when running to power the pump.
Click to expand...
There is no way with key on, pumps run with engine not running.

I get a bypass but he would have said, "pushed/turned on bypass" with key in run position.

Its so dam frustrating that these people think we can see their exact sep up from our keyboard.

Wtf!
 
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There are 2 oil pressure switches, one to gauge, the other closes when oil pressure reaches operating pressure & powers the fuel pump. If oil presure drops because engine dies or oil system failure the pump suts off. Engine dies shortly there after.
The beautiful Chevy Vega used the same basic system.
For start-up, it uses a bypass similar to ballast bypass to power fuel pump during crank to fill the carburetor.

When GM stopped making 4.3 blocks with a mounting pad for the mechanical fuel pump (after EFI became standard on autos) MerCruiser had to use the electric system even though carbureted engines. Service replacement engines for earlier applications required the fuel pump upgrade.
 
One's a switch and the other is the pressure transducer (sensor)....

I think Jack's point is that with the engine OFF and the KEY to RUN, the Fuel pump should NOT be running...

My conclusion is the description is inaccurate, there are missing details, or somebody has modified the wiring...
 
When starting good engine

Turning key one click - fuel pump starts running
Second click starter motor engaged engine starts.

When starting bad engine

Turning key one click no fuel pump noise
Second click starter motor engaged engine starts runs for a couple of minutes then dies

Again because “some people” can’t read.

I’ve taken the fuel line off the CARB and the fuel pump does pump fuel when cranking.

I don’t know if there is a by pass regarding the fuel pump on good engine.

But watching start up videos of the same engine on you tube the fuel pump running on the first click of the key seems to be standard procedure.
 
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Enough of being rude.. .
Yes, when reading again I see it says carb in your troubleshooting but help us to help you. I'm not at your boat, I'm not getting paid to answer questions or pull out manuals, look at wiring diagrams or post them for you. Put the details out in plain view...
2 engines
Carburetors
Year & more importantly, serial number

None of your posts give a model year or serial number. We can only take a guess. Early 90s thru the end of carburetor engines.

Mercruiser had to switch to electric fuel pump because GM stopped making blocks with a mounting pad for a mechanical pump. And for all I know maybe the lobe was removed from the camshaft too.
I don't know the year or even more importantly the serial number of that cut over.

I don't believe your good engine fuel pump should run with the key on & engine off. No oil pressure should not allow it ro run except while cranking. So after you figure out your bad engine issue you should visit the good engine issue before you blow yourself up.

Yes Mark, one is an open/close (on/off) switch to operate the pump & the other is the sending unit for the gauge. If I recall they are in close proximity to each other. I believe your conclusion is correct & most likely all 3 issues.
 
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Darren-stormin said:
Hi

I have a suspected fuel pump issue on one on my engines.

When I turn the key to run position I can’t hear the fuel pump running although the engine does start and run for a while it cuts out. I’ve removed the fuel pipe that leads into the carb and it’s getting fuel when cranking. It just doesn’t prime.

When it is running oil pressure goes up to 60psi.

My other engine does engage fuel pump when switched to run then starts and runs lovely.

I have fitted new fuel pump to both engines now but still one doesn’t run after a while.

Any help would be greatly appreciated.
Click to expand...
Ok
Now we have some basics.

1. Electric fuel pump used with carbed engines, Cost guard regulation states the pump cannot turn on until oil pressurr 5 psi is present. Then power is supplied thru oil pressure switch to pump.

2. The way the system works,
Turn key to on. No pump

3 turn key to crank, power is supplied to pump by way of on starter solenoid by passing oil pressure switch.

3. Once engine starts and key is returned to the run position and 5 psi of oil pressure is present, the pump continues to run until key is turned off.

So. Based on above, it may be possible your oil pressure switch is bad on the engine that wont stay running.
It starts due to cranking bypass but when key is returned to run pisition, the pressure switch is not working thus no power to pump and carb runs out of gas.

your engine that pump turns on with key in run position, The wiring at the pressure switch may have been altered or jumpered to bypass oil pressure switch.

When I worked in the business and we were rigging and brand new V6 boat with carb and electruc pump. We commonly jumped (temporarily) the pressure switch when putting gas in tank and starting the boat for the very first time.

I would start with confirming wiring is to specification,
If you can put a fuel pressure gage as used for carbed engines between pump and carb fuel inlet and watch to see if the pump is maintaining pressure when key is returned to run pisition that would tell you if pump is still running and if so, your problem is elsewhere.

If pressure is lost and pump is not running then issue is with pump/oil pressure switch. If pump is still running then the issue may be fuel supply (lines/antisyphon valve/gas tank/ pick up tube)
 
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Darren-stormin said:
When starting good engine

Turning key one click - fuel pump starts running
Second click starter motor engaged engine starts.

When starting bad engine

Turning key one click no fuel pump noise
Second click starter motor engaged engine starts runs for a couple of minutes then dies

Again because “some people” can’t read.

I’ve taken the fuel line off the CARB and the fuel pump does pump fuel when cranking.

I don’t know if there is a by pass regarding the fuel pump on good engine.

But watching start up videos of the same engine on you tube the fuel pump running on the first click of the key seems to be standard procedure.
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So you know, I spend 8 hours a day reading complaints and investigation reports for medical devices and generate reports for the FDA.

If I happen to miss a few details burried in a paragrapgh of text while reading on my cell phone...... EXCUSE THE HELL OUT OF ME.

YOU, dont be so smug, its your problem, its your responibility to PROVIDE all the specifications of your boat and engine and history prior to problem so we all can suggest problem solving paths vs asking you for information. Every additional question creates more posts, loss of time and possible wrong suggestions.

We are helping for free. If you dont like the words, go to a marine business and pay what, $150.00 and hour and hope they do the right repair....maybe they will talk nicely to you as you pay them...
 
I just happened to see this thread and have a 1990 4.3 mercruiser 2barrel carb set up. I have the same thing going with my fuel pump coming on when I turn the ignition key on. I just replaced the oil pressure switch on this, and thought I had the wires in the wrong position on the sensor switch. My old switch had died, but before it died the fuel pump didn’t engage until the starter was engaged. Now with the new switch it comes on when I turn the ignition switch on… everything works fine and the new switch works after the motor starts. But I can’t figure out why the fuel pump is coming on with the ignition key turned on now. It never did that before….

With that being said, I’ve got the boat in the shop having a lower shift cable replaced, and I asked the mechanic to check out the wiring on my new oil sensor switch to make sure I didn’t have some wires hooked up wrong…. He said that everything was wired correctly, and that the fuel pump is supposed to come on with the ignition switch, but it only runs until it’s pressurized and shuts off.

This statement doesn’t make sense to me. The pump shouldn’t run unless the starter is engaged, or there is oil pressure that triggers the on switch…. My question is what the hell is going on here? Why is it running when I turn the switch on if it’s wired correctly?

Kirk
 
the connections at the switch may be ok but if the other ends of the wires are check to ensure the correct connections have been made, its hard to say.

I agree that the mechanics statement doesn't make sense - its just inaccurate.

without a serial number, its just speculation on my end....the factory service data shows a mechanical pump as original configuration so I'd say the electric pump was a retrofit...so, if it worked correctly before and you changed the switch, was the new switch correct? If it was, then the wiring isn't correct.
 
kirkll said:
I just happened to see this thread and have a 1990 4.3 mercruiser 2barrel carb set up. I have the same thing going with my fuel pump coming on when I turn the ignition key on. I just replaced the oil pressure switch on this, and thought I had the wires in the wrong position on the sensor switch. My old switch had died, but before it died the fuel pump didn’t engage until the starter was engaged. Now with the new switch it comes on when I turn the ignition switch on… everything works fine and the new switch works after the motor starts. But I can’t figure out why the fuel pump is coming on with the ignition key turned on now. It never did that before….

With that being said, I’ve got the boat in the shop having a lower shift cable replaced, and I asked the mechanic to check out the wiring on my new oil sensor switch to make sure I didn’t have some wires hooked up wrong…. He said that everything was wired correctly, and that the fuel pump is supposed to come on with the ignition switch, but it only runs until it’s pressurized and shuts off.

This statement doesn’t make sense to me. The pump shouldn’t run unless the starter is engaged, or there is oil pressure that triggers the on switch…. My question is what the hell is going on here? Why is it running when I turn the switch on if it’s wired correctly?

Kirk
Click to expand...
Do you have a aftermarket 3 prong switch ? if so one set of connections is NC, the other is NO or the switch is bad at rest there should be no continuity across the terminals on the switch that you have hooked up.

you are correct that with engine off and ignition on there should be no power to the fuel pump
 
Interestingly the original poster vanished after we hurt his feelers.

Kirk, the info we have provided is accurate. When you find the fault, please post the resolution.

It's scary but I have a feeling there are thousands of V6 time bombs on the lakes.
 
makomark said:
the connections at the switch may be ok but if the other ends of the wires are check to ensure the correct connections have been made, its hard to say.

I agree that the mechanics statement doesn't make sense - its just inaccurate.

without a serial number, its just speculation on my end....the factory service data shows a mechanical pump as original configuration so I'd say the electric pump was a retrofit...so, if it worked correctly before and you changed the switch, was the new switch correct? If it was, then the wiring isn't correct.
Click to expand...
This isn't a factory mercruiser motor. It was a 4.3 truck motor that was converted to marine stds. the wiring wasn't changed from the old oil sender switch to the new switch.

The wiring is pretty basic too. from the oil ending switch, one wire comes from the ignition wire (Purple) in my case it was connected to the automatic choke wiring connection. the other wire goes to the electric fuel pump , and the 3rd wire goes to the starter solenoid. i thought i just had the leads switched to the wrong poles on the switch.... but maybe i have the wrong switch?
 
Droid said:
Interestingly the original poster vanished after we hurt his feelers.

Kirk, the info we have provided is accurate. When you find the fault, please post the resolution.

It's scary but I have a feeling there are thousands of V6 time bombs on the lakes.
Click to expand...
No worries... I wont Ghost you.... ( no offense indented to Kghost. He's helped me a bunch over the last 5 years.)

I think most of those V6 time bombs can be defused if proper maintenance is applied on a regular basis. mines been running strong for 5 years now with only a tune up, & regular maintenance other than a new carb. i did have a shop change out the soft plugs in the block to accommodate the raw water cooling system. when i first got the boat i went through all the upgrades to marine stds with proper starter, CG approved fuel pump, and got a decent water fuel separator system installed..... i'm quite happy with my old V6 . Kirk
 
The wiring at the pressure switch should be one wire in one side (should be Ign on purple) and the other side should have two wires, one from starter solenoid and other to fuel pump

If I remember correctly.
 
The bronze shift arm, the top extends out in one direction. It MUST face the transom.

When rejoining upper to lower the bronze arm is floating in the upper and has to be manually directed to engage with the shift shaft splines and point towards transom.

Remember that the lower MUST be in forward with splined shaft turned all the way ClockWise when bronze arm is slid over splines.

It can be tricky.

I am not suggesting this is your issue but you made no mention of this and if this was your first try at doing what you did, we should have heard some bitching about it.

Also when seperating upper from lower, have a rubber tapered plug at hand and stuff it into upper gear case transfer hole so gear oil doesnt leak out
 
kghost said:
The wiring at the pressure switch should be one wire in one side (should be Ign on purple) and the other side should have two wires, one from starter solenoid and other to fuel pump

If I remember correctly.
Click to expand...
yup... that's the way its wired. My question is why is the pump coming on when i'm turning on the ignition switch? is there a proper orientation these wires need to be placed on the switch itself? If i had the ignition wire and the solenoid wire reversed it would start the pump running as soon as i turned the key, and it would continue to run after the motor started. i tried switching the wires aroun a bit, but may need to remove the switch to see which terminal is which to get it right.

i found a wiring diagram here.

1754074275811.png
 
kghost said:
The bronze shift arm, the top extends out in one direction. It MUST face the transom.

When rejoining upper to lower the bronze arm is floating in the upper and has to be manually directed to engage with the shift shaft splines and point towards transom.

Remember that the lower MUST be in forward with splined shaft turned all the way ClockWise when bronze arm is slid over splines.

It can be tricky.

I am not suggesting this is your issue but you made no mention of this and if this was your first try at doing what you did, we should have heard some bitching about it.

Also when seperating upper from lower, have a rubber tapered plug at hand and stuff it into upper gear case transfer hole so gear oil doesnt leak out
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Thanks.... That was the first time i've heard mention of orientation of a floating bonze arm. which part is it on this blow out dwg?

1754075000661.png
 
the 'shift arm' is probably the part in the upper housing that many call "the foot"...i will try to dig up a diagram.

on the Oil pressure switch, there are several different switches that can do the job...if you can get a pic of the terminals and or the switch's part number, we can figure out which wire goes where.
 
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makomark said:
the 'shift arm' is probably the part in the upper housing that many call "the foot"...i will try to dig up a diagram.

on the Oil pressure switch, there are several different switches that can do the job...if you can get a pic of the terminals and or the switch's part number, we can figure out which wire goes where.
Click to expand...
I found that shift arm or foot diagram under the bell housing exploded view, and remember having to turn the shaft to get them to line up in order to get the lower unit up and in place. I obviously didn't pay attention to it facing the transom and got it backwards. If i remember right, that stayed up in the belhousing when i dropped the lower unit. By getting that backwards, could it cause damage?
 
the upper shift shaft, in the bell housing cant be reversed...the one in the gear housing, I don't know, maybe...never saw one reversed...

I wouldn't think it would do anything other than reduce the overlap between the foot and the "shoe" that goes over it...dont have one in the garage to examine right now...
 
Well it worked fine after I was done. But maybe the less overlap caused it to bend out of shape. I don’t know… I haven’t seen it since he tore it apart. It’s a 40 mile drive to the shop, so I just asked him to fix it….thanks… Kirk
 
When/if the bronze arm get bent it causes all kinds of problems with shifting.

Those problems are caused by forcing the drive to gimbal housing to mate.....


Its like closing your eyes with one of those women you did but really didnt want to remember
 
kghost said:
When/if the bronze arm get bent it causes all kinds of problems with shifting.

Those problems are caused by forcing the drive to gimbal housing to mate.....


Its like closing your eyes with one of those women you did but really didn't want to remember
Click to expand...
That was funny....Been there, done that...Burned the t shirt. Well i let ya know after i pick the boat up next week. i asked him to save the broken parts and take some photos.... Now i know what to look out for, i'll do better next time.
 
The majority of the time, when the bronze arm get bent, by trying to force drive onto gimble housing and not having arm/control in propper fwd gear which a lot of people over look. They then force it and bend the arm.

This typically causes lack of engagement in either FWD or REV depending on cable adjustment.

I have found this arm as the shifting engagement issue many times and save good ones from bad drives.

If available they cost like $65-$70.

If you put your drive together and you had good FWD and REV engagement then you most likely did it correctly.

If it would not shift/move after sitting that may be an entirely different issue. If he found a leak that is a whole other issue.

Other than the clutch dog splines rusted causing it not to move may be what he found...but let us know what he did.
 
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