4.3 Motor runs good for an hour then bogs down

[92celebrity]

I have a 4.3 mercruiser and the last three boat outings the engine runs great for around an hour then it loses power. It will run at about 3400 and wants to drop 200 rpm back and forth.

I have changed the plug wires, cap and rotor, checked the choke and it is working correctly. Could this be an electrical issue?

 

[89 VIP Valiant]

Re: 4.3 Motor runs good for an hour then bogs down

Is this electronic ignition? Whether it is points or electronic, I would be inclined to wonder about the coil.

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

i'm not sure i will check it is a 1992

 

[searay3]

Re: 4.3 Motor runs good for an hour then bogs down

Could be a coil. Cheap fix. If it's suspect replace it. about $30. Check the fuel tank vent for clogging. When was the last time the fuel filters/water separator were replaced. You could run off an outbaord tank plumbed to the fuel pump intake to isolate whether it is on the engine side of fuel delivery or the tank side...

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

I will check into to those things, it just seems odd that it takes an hour on the lake for the engine to start acting up!

 

[lowkee]

Re: 4.3 Motor runs good for an hour then bogs down

I second the fuel vent. If you have a large fuel tank, it could take quite a while to displace enough fuel to create a vacuum.

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

It has about a 20-22 gallon tank
where is the vent usually located?

the engine will idle fine but it just stalls out when it gets into the rpms

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

would engine overheating cause this?

 

[wca_tim]

Re: 4.3 Motor runs good for an hour then bogs down

to check it, when it starts acting up bad, slow down and take the fuel tank cap of for a couple minutes and see if it starts running right...

 

[89 VIP Valiant]

Re: 4.3 Motor runs good for an hour then bogs down

As far as it acting up after an hour or so under load, this is why I would suspect the coil. I am not an expert, so please anyone else jump in and correct me, but the coil kind of acts like a booster pack you would use to jump start your battery. As they get older, after they are put under a load for a time, they lose their ability to "boost" your ignition system spark and provide the necessary volts needed. Once it loses its ability, your spark system loses the voltage needed, and you cannot run the rpms you need to ignite the compressed gas in your engine cylinders. If anyone else can explain this better, don't be afraid to step on my toes!

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

I know the coil hasn't been replaced since i have owned the boat i will change it, is there a website that has good prices on coils?

if the timing was off would it have anything to do with my problem?

 

[Bondo]

Re: 4.3 Motor runs good for an hour then bogs down

As far as it acting up after an hour or so under load, this is why I would suspect the coil. I am not an expert, so please anyone else jump in and correct me, but the coil kind of acts like a booster pack you would use to jump start your battery. As they get older, after they are put under a load for a time, they lose their ability to "boost" your ignition system spark and provide the necessary volts needed. Once it loses its ability, your spark system loses the voltage needed, and you cannot run the rpms you need to ignite the compressed gas in your engine cylinders. If anyone else can explain this better, don't be afraid to step on my toes!

Ayuh,..... OK,.... Coils just Don't work like That.....

I know the coil hasn't been replaced since i have owned the boat i will change it, is there a website that has good prices on coils?

if the timing was off would it have anything to do with my problem?

You have a Fuel Delivery Issue....
Probably the Fuel Tank, or the Fuelpump, or maybe even the Lines...
Hook up a remote tank,+ Try it...

Bad Coils,+ Poor Timing don't exibit those symptoms....

 

[92celebrity]

Re: 4.3 Motor runs good for an hour then bogs down

i checked the vent tube and everything was clear there
fuel filter was good, carbeurator looked clean also but i should probably put a rebuild kit in it since it is apart
the carb has never been apart since 92 it still had original paint on screws

my buddy told me that when he was behind the boat tubing he could smell a lot fuel, which kinda creates a mystery

 

[mylesm260]

Re: 4.3 Motor runs good for an hour then bogs down

Okay, if he could smell fuel, that indicates you're either

A) running rich
or
B) not burning the fuel you're running.


Check you're ignition system out first so you can eliminate it as a suspect.
Remove you're coil, and test it with a multi-meter. Cross reference your resistance readings with the manual. Check both the primary and secondary side.

Tell us all what ignition system you have (thunderbolt V) for example, and a tech in here can explain what sensors are used, what trigger is used, and how you're spark advance should work. It could also be a bad hal-effects sensor, or a bad variable reluctance sensor causing a poor signal and bad ignition timing.



If you're ignition system checks out, then it's on to fuel.

When you're not getting any power, are you by chance getting popping through the intake? If so, than you're likely running lean. However, seeing as how someone behind you said they could smell fuel, I would wager you're running rich. If you're carbed, I would say bad choke linkage / Controls. If you were EFI'ed, I would say bad water temp sensor or perhaps a bad map or tps sensor. (or a bad connector to any one of those sensors)

Hope this helps.