1997 mercruiser 4.3 starts right up, runs for about 30 seconds, idles down and dies repeatedly. Oil gauge doesn't peg at all. No oil pressure alarm.

[1997capri]

New boat owner here. I have a 97 bayliner capri 2050 with a 4.3 mercruiser. I just got the boat this season. Test ride had all the gauges working. When I go to actually pick it up, the previous owner told me that when he took it out the previous day, the boat wouldn't run, and he ended up replacing the "oil sensor" I believe. Anyways when I get the boat, everything runs fine except for the oil gauge doesn't work, and the boat just takes some time to get the engine to not idle down, repeated starts and just giving it some gas before the boat runs uninterrupted without stalling out. Put probably 20 hours on it myself with this routine.

Now, the boat just starts up, runs for about 30 seconds, idles down, and dies. I can start it right back up, it'll run for 30 seconds, and it dies again. There's gas in the tank. The boat fires right up, but it dies after half a minute. The oil gauge does not move at all still. There's also no oil pressure alarm that sounds off, even though the gauge doesn't move at all, it just dies. What could this be? I'd like to do the work on this myself and learn about this boat.

 

[jonny rotten]

Someone smarter than me will chime in but it could be a carb float issue. I have the same motor. If it's a 2 barrel Rochester carb they are real easy to rebuild, Youtube videos to guide you through. I never did it before and now have done it twice. Rebuild kits are everywhere online

 

[1997capri]

Someone smarter than me will chime in but it could be a carb float issue. I have the same motor. If it's a 2 barrel Rochester carb they are real easy to rebuild, Youtube videos to guide you through. I never did it before and now have done it twice. Rebuild kits are everywhere online

So do you think that the problem I'm having with the engine dying is just unrelated to the oil pressure gauge no working? I'm just wondering if it's a safety feature to kill the engine if the oil pressure is not detected, if that makes sense

 

[dubs283]

I'm just wondering if it's a safety feature to kill the engine if the oil pressure is not detected

Oil pressure would only come into play if your engine is carbureted with an electric fuel pump.

If equipped with an electric pump it'd be appropriate to check fuel pressure and tune up conditions

 

[Scott06]

New boat owner here. I have a 97 bayliner capri 2050 with a 4.3 mercruiser. I just got the boat this season. Test ride had all the gauges working. When I go to actually pick it up, the previous owner told me that when he took it out the previous day, the boat wouldn't run, and he ended up replacing the "oil sensor" I believe. Anyways when I get the boat, everything runs fine except for the oil gauge doesn't work, and the boat just takes some time to get the engine to not idle down, repeated starts and just giving it some gas before the boat runs uninterrupted without stalling out. Put probably 20 hours on it myself with this routine.

Now, the boat just starts up, runs for about 30 seconds, idles down, and dies. I can start it right back up, it'll run for 30 seconds, and it dies again. There's gas in the tank. The boat fires right up, but it dies after half a minute. The oil gauge does not move at all still. There's also no oil pressure alarm that sounds off, even though the gauge doesn't move at all, it just dies. What could this be? I'd like to do the work on this myself and learn about this boat.

Jumper around the oil pressure switch that supplies power to fuel pump and test it.fuel pump is fed power off starter during cranking, then off ignition circuit via oil pressure switch when in run. Seems like fuel bowl is getting filled during cranking then runs til bowl is empty.
verify oil pressure with mechanical gauge. Likely gauge or sender is bad. Take wire off oil pressure sender and ground it gauge should peg if it is good

 

[havoc_squad]

Oil pressure would only come into play if your engine is carbureted with an electric fuel pump.

If equipped with an electric pump it'd be appropriate to check fuel pressure and tune up conditions

To the OP, the first thing you should be doing for mercruiser when asking for help is provide the full engine/drive name, the model, and the serial number on the identification plate of the engine.

Example:

Mercruiser 4.3LX Gen+ Alpha One Gen 2
Serial number: OF803000

If that is a balanced shaft GM 4.3L V6 engine made after 1992 that is installed and not an older non-balance shaft 4.3L V6 (which I highly doubt), that engine does NOT have a location to mount a mechanical fuel pump.

It would certainly be an electric fuel pump and the oil pressure switch is a dead man's switch that kills the power to the fuel pump when oil pressure on engine drops below a certain threshold.

You need to rule out oil pressure issues on your engine first. Track down and make sure the wiring is correct and not damaged. Then buy an oil pressure testing gauge and verify the engine is putting out the specified oil pressure.

The fuel pump is powered upon engine cranking by the voltage supplied when holding the key in the start position and stops power when when released. Oil pressure is required to keep the fuel pump running when ignition key is on.

With a sufficiently large enough wire and an extra helper, you could temporarily connect positive battery voltage to the positive side of the electric fuel pump just long enough to verify the engine remains running.

If fuel pump works well and engine runs fine, move on to diagnosing the oil pressure switch not powering the fuel pump.

If the engine dying issue goes away, its either a bad oil pressure switch or the PO screwed something up on it. I hope the previous owner didn't do a hack job and bypass the oil pressure switch to have the fuel pump continuously run with key on engine off.

If you plan on doing any DIY work on your boat, do yourself a favor and buy Mercruiser service manual #18 for your engine and #14 for the Alpha One Gen 2 outdrive servicing.

Yours is likely Thunderbolt IV or Thunderbolt V without knock sensor wiring diagram.

See the section in the factory service manual #18 section 4D-11 and 4D-12 for buzzer testing procedures to verify the buzzer is working correctly. If that is working correctly then you know you need to trace down the issue at fault through the wiring diagram and checking the oil pressure sensor.

 

[QBhoy]

Sure sounds like the oil pressure sensor is the issue. This is what gives the signal for the fuel pump to run when cranking, after all.
Other than that…can only be a fuel issue

 

[1997capri]

To the OP, the first thing you should be doing for mercruiser when asking for help is provide the full engine/drive name, the model, and the serial number on the identification plate of the engine.

Example:

Mercruiser 4.3LX Gen+ Alpha One Gen 2
Serial number: OF803000

If that is a balanced shaft GM 4.3L V6 engine made after 1992 that is installed and not an older non-balance shaft 4.3L V6 (which I highly doubt), that engine does NOT have a location to mount a mechanical fuel pump.

It would certainly be an electric fuel pump and the oil pressure switch is a dead man's switch that kills the power to the fuel pump when oil pressure on engine drops below a certain threshold.

You need to rule out oil pressure issues on your engine first. Track down and make sure the wiring is correct and not damaged. Then buy an oil pressure testing gauge and verify the engine is putting out the specified oil pressure.

The fuel pump is powered upon engine cranking by the voltage supplied when holding the key in the start position and stops power when when released. Oil pressure is required to keep the fuel pump running when ignition key is on.

With a sufficiently large enough wire and an extra helper, you could temporarily connect positive battery voltage to the positive side of the electric fuel pump just long enough to verify the engine remains running.

If fuel pump works well and engine runs fine, move on to diagnosing the oil pressure switch not powering the fuel pump.

If the engine dying issue goes away, its either a bad oil pressure switch or the PO screwed something up on it. I hope the previous owner didn't do a hack job and bypass the oil pressure switch to have the fuel pump continuously run with key on engine off.

If you plan on doing any DIY work on your boat, do yourself a favor and buy Mercruiser service manual #18 for your engine and #14 for the Alpha One Gen 2 outdrive servicing.

Yours is likely Thunderbolt IV or Thunderbolt V without knock sensor wiring diagram.

See the section in the factory service manual #18 section 4D-11 and 4D-12 for buzzer testing procedures to verify the buzzer is working correctly. If that is working correctly then you know you need to trace down the issue at fault through the wiring diagram and checking the oil pressure sensor.

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Thank you so much for this comprehensive response! I'll be looking into these possible causes on my next day off! You're a great resource.

 

[nola mike]

Sure sounds like the oil pressure sensor is the issue. This is what gives the signal for the fuel pump to run when cranking, after all.
Other than that…can only be a fuel issue

Not quite. The oil pressure switch has nothing to do with the oil pressure sender (gauge).

 

[QBhoy]

Not quite. The oil pressure switch has nothing to do with the oil pressure sender (gauge).

Apologies. Was of course the switch I would be referring to.