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Boat Bogging and voltage going crazy at high RPM?

J

justinweigt

New member
4.3L Merc was having voltage issues at higher Rpm's or WOT, The voltage was dropping from 13.8 to 10 or even lower and the engine would start too sputter. Once I lowered the rpms all would be normal again. I searched many forums and put a new alternator on it and replaced my coil just because it was old. The Battery seems fine reading 12.4 before any start up.

Yesterday on the water it ran better but towards the end of the day it starting bogging real bad around 2500 rpm's. Besides fuel filter, would a year old carb need to be cleaned and rebuilt already, would that cause my voltage to drop and spike? I am not sure where I should start next.

Thanks​
 
sandkicker said:
Ignition system will get flaky @ 10 Volts. I'd be looking for a crummy ground in the electrical feeds....
Click to expand...


Thank you, What is grounded besides Alternator, to the engine block and [FONT=Verdana,Arial,Tahoma,Calibri,Geneva,sans-serif]accessories? Could a battery start to go bad as you rev up the engine?[/FONT]
 
Power required by ignition system goes up as engine revs....check connection on batteries ....actually, check ALL connections.
Power to the ignition transits the dashboard.
 
sandkicker said:
Power required by ignition system goes up as engine revs....check connection on batteries ....actually, check ALL connections.
Power to the ignition transits the dashboard.
Click to expand...


Where are the Ground studs located on the engine, Im going to run all new cables just to be sure.

Thank you
 
justinweigt said:
Where are the Ground studs located on the engine, Im going to run all new cables just to be sure.

Thank you
Click to expand...

Ayuh,..... Follow the battery's ground wire to the other end,.....

Should be a ground stud on either side of the flywheel cover at it's block intersection point,.....
 
bondo said:
Ayuh,..... Follow the battery's ground wire to the other end,.....

Should be a ground stud on either side of the flywheel cover at it's block intersection point,.....
Click to expand...

Update**

So I ran all new Power and Ground Cables, I checked all other grounds and found some loose nuts on the starter and Trim Pump, I am still having voltage drop at higher RPM's and it is almost cutting the engine out completely. Could this be a Fuel pump issue or Cap issue or ignition sensor in the cap, Or a battery issue. I am just lost on why the boat runs fine up to a certain point.
 
justinweigt said:
Update**

So I ran all new Power and Ground Cables, I checked all other grounds and found some loose nuts on the starter and Trim Pump, I am still having voltage drop at higher RPM's and it is almost cutting the engine out completely. Could this be a Fuel pump issue or Cap issue or ignition sensor in the cap, Or a battery issue. I am just lost on why the boat runs fine up to a certain point.
Click to expand...

NO.... none of the above!.... except battery

How old is the battery?


Where are you measuring this voltage drop???
Check voltage when you see the problem @...
1) Battery terminals... i.e. @ the battery
2) @ the point the battery is connected to the engine, normally very near the starter
3) @ the ignition switch on the dash...
4) @ the point on the engine where +12 gets sent back to the ignition system.

When I do this type of debugging, once I get to #2 and #3 & #4 I like to keep the negative lead to my multimeter
ON the point on the block that is ground for all measurements... Usually you have to expend that meter lead with a length of wire.

The quick test for battery condition is to measure the voltage AT the battery posts when cranking the engine....if less than 10.5 V... battery dubious...or not charged fully.

Battery volts @ battery when engine OFF ?
Battery volts @ battery when engine @ fast idle?

BTW.... I don't trust dashboard V meters.
 
Last edited:
sandkicker said:
NO.... none of the above!.... except battery

How old is the battery?


Where are you measuring this voltage drop???
Check voltage when you see the problem @...
1) Battery terminals... i.e. @ the battery
2) @ the point the battery is connected to the engine, normally very near the starter
3) @ the ignition switch on the dash...
4) @ the point on the engine where +12 gets sent back to the ignition system.

When I do this type of debugging, once I get to #2 and #3 & #4 I like to keep the negative lead to my multimeter
ON the point on the block that is ground for all measurements... Usually you have to expend that meter lead with a length of wire.

The quick test for battery condition is to measure the voltage AT the battery posts when cranking the engine....if less than 10.5 V... battery dubious...or not charged fully.

Battery volts @ battery when engine OFF ?
Battery volts @ battery when engine @ fast idle?

BTW.... I don't trust dashboard V meters.
Click to expand...



I can see it at the voltmeter on the dash and a second later I can feel the motor starting to bog down, I also replaced my cigarette lighter with a digital meter and I can see the voltage drop as well. When I throttle up higher to see if it will come out of it, and it happens worse and the engine will almost die. I replaced the fuel filter just incase and the old filter was very clean.
 
sandkicker said:
NO.... none of the above!.... except battery

How old is the battery?


Where are you measuring this voltage drop???
Check voltage when you see the problem @...
1) Battery terminals... i.e. @ the battery
2) @ the point the battery is connected to the engine, normally very near the starter
3) @ the ignition switch on the dash...
4) @ the point on the engine where +12 gets sent back to the ignition system.

When I do this type of debugging, once I get to #2 and #3 & #4 I like to keep the negative lead to my multimeter
ON the point on the block that is ground for all measurements... Usually you have to expend that meter lead with a length of wire.

The quick test for battery condition is to measure the voltage AT the battery posts when cranking the engine....if less than 10.5 V... battery dubious...or not charged fully.

Battery volts @ battery when engine OFF ?
Battery volts @ battery when engine @ fast idle?

BTW.... I don't trust dashboard V meters.
Click to expand...

I see the voltage at the gauge dropping right before the engine bogs down
 
A battery with 12.4 resting volts is not healthy. You should be at 13-ish on a full battery and with engine running the meter should be at 14.4.
 
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