Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

Ah... the latest in a string of mechanical problems. (Grrrrr.)

Here's what was happening last week:

1. At higher RPMs (above 3500 or so, typical holeshot), the engine smoothly revs up, brings the boat up on plane, then dies. RPM gauge drops almost instantly from 4200 to zero. Sometimes at 3500+ RPMs, it'll caugh, start up again on its own for a second, then die.

2. It restarts easily. On the next attempt, it may or may not do the same thing again.

No problems at idle, no problems at slow speeds, no problems starting or restarting.

Some sort of kill switch with an intermittent problem?


UPDATE:
The problem was definitely worse tonight. As soon as the boat would reach about 3000 rpm and plane out, the engine would kill. At one point, it seemed to be a little better, only to kill in the midst of a half-mile run at 3800 RPM. It would still start easily and run at no-wake speeds without a problem.

THEN...

[dramatic music]

We motored at no-wake back towards the slip. When shifting into neutral prior to entering a downstream channel, the engine died and wouldn't restart. Slow cranks, clicks, etc. Clearly, there wasn't enough oomph from the battery to start the engine.

So that $20 paddle on eBay came in real handy -- had to PADDLE THE FREAKING THING about 200 yards to the slip.

Any ideas? Would a bad alternator explain all this? I'm hoping that would explain the lack of battery power to start the engine, but also perhaps why it was dying at higher speeds. Did I kick a dog in my last life?

Help!

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

Two paddles on our new-to-us-boat: One with the blade broken in half, lengthwise, and the other doesn't look a whole lot better. ISTM what would be really handy would be strong aluminum paddles with collapsing handles.

It would certainly explain the dead battery, but I don't see it explaining the engine dying at 3000 RPM--esp. as the battery still had plenty of juice to re-start the engine.

Jim

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

I was thinking along the lines of other electronics in the system (fuel pump?) that might not be getting the voltage they need. Or am I revealing my total ignorance?

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

That occurred to me, but I figured me taking WAGs wasn't going to do you much good :).

Jim

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

I'll take anything at this point!

When the engine was dying, it felt very much like running out of gas... sort of a sputter, then nothing. At lower speeds, it was OK. So, my total noob theory is that a dying/dead alternator isn't ultimately supplying enough voltage to the fuel pump. At lower speeds, it's fine, but at higher speeds, the pump can't keep up with the engine's demand for fuel.

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

I suppose it's possible. The thing that throws me off, tho, is, after dying, you had enough juice to restart. That takes a lot more current than running a fuel pump.

Either way: You know you have an alternator problem for sure. So fix that, put the battery on a charger to get it back up to snuff, put the battery back in the boat and see what you have.

Jim

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

I totally agree, but my thought was that the starter motor only requires a short burst of power (relatively speaking) with a warm engine. In contrast, the fuel pump may work well enough at lower voltage and thus present no problems at idle or no-wake speed, but when the engine wants a larger stream of fuel, the increase voltage demand (by the fuel pump) can't be fulfilled by the failing alternator.

The alternator IS the original, so it's going on 18 years. I figure replacing that--even if the problem is something else entirely--is probably a good idea regardless.

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

Also... for what it's worth, the boat's voltage gauge is reading just shy of 11 volts. (I know, I know, it's not the most accurate thing in the world. I'll borrow a tester.) But still.

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

I don't know how accurate they are (they're relatively worthless if they're not accurate, btw), but 11VDC is definitely well below healthy for a 12VDC lead-acid battery. That battery is effectively dead. But you already knew that :).

It just occurred to me: These boats have no "idiot light" to warn you that the alternator isn't doing what it should. I regularly scan oil pressure and water temperature, but, in all honesty, pay little attention to that voltmeter. I think I'll add that to my list of gauges to scan regularly whilst under way.

I wonder what those battery idiot light circuits consist of...?

Jim

Re: Row Row Row My Boat (4.3L Mercruiser, Bad Alternator?)
 

Here's my plan to replace the alternator myself. I'm SOMEWHAT mechanically inclined.

1. Disconnect battery entirely (obviously).
2. Disconnect the two wires coming from the back of the original alternator.
3. Remove the original alternator by loosening/removing the two bolts.
4. Reconnect the wires to the new alternator
5. Put new alternator into place, making sure belt tension is appropriate. (1/4" to 1/2" with moderate pressure).
6. Reconnect battery.


Am I missing anything here?