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7.4 mag won't start.

9

91stingray

Contributing Member
7.4 mag, carbureted, thunderbolt 4 ign. Started the engine to day for the first time this season to warm it up and do an oil change. Engine started up easily and ran great for about 5 minutes with the throttle open slightly. Once I closer the throttle to see if it would idle it stalled out and I can't get it to start again. Let it sit a while still nothing, changed fuel filter still nothing, has fresh gas. My voltmeter isn't working now so I can't check any voltage just doesn't make sense why it would run fine and then not start. It's just turning it doesn't sound like it's trying to crank.
 
I got to start again after testing the voltage to the ignition coil. The battery and ignition coil read 14.xx volts with the key on, which seems high to me. While engine running the alternator seemed like it was getting hot and the battery voltage read 15.xx volts I noticed the temps on the dash getting close to 200 and shut it off. With my infered thermometer the alternator was over 300*f and the rubber hoses going from the exhaust manifolds to the thermostat housing were around 160*f. I just installed a brand new 140* thermostat. So obviously my engine is overheating and I think thats why it won't start once it gets hot. I wasn't overheating when I winterized it amd it had a fresh impeller last year so im not sure what the issue could be.
 
The engine is raw water cooled with pickup tube mounted at the bottom of the transom. The impeller seems to be working fine as I was using only the suction from the pickup tube with a hose in a 55 gallon drum to suck water. And plenty of water was coming out of the exhaust.
 
The temp sender is nothing but a variable resistor. It should give an accurate indication at 14 volts plus or minus.
If you're running over 15 volts, the gauge will read high.
160 sounds about right but the alternator at 300 is way high. So I believe it is over changing.
 
Droid said:
The temp sender is nothing but a variable resistor. It should give an accurate indication at 14 volts plus or minus.
If you're running over 15 volts, the gauge will read high.
160 sounds about right but the alternator at 300 is way high. So I believe it is over changing.
Click to expand...
I agree but i think we should understand what he is using to measure voltage.

14+ as he said would be battery voltage with key on only.... thats 1.5 +/- volts higher than it ever should be.

static fully charged battery at rest should be about 12.6 +/- volts only. Sone may reach 12.8/9 volts but never above that


I do agree about the Alt. And if his device is a good volt meter then that would the first place to suspect. Overcharging.
 
Droid said:
The temp sender is nothing but a variable resistor. It should give an accurate indication at 14 volts plus or minus.
If you're running over 15 volts, the gauge will read high.
160 sounds about right but the alternator at 300 is way high. So I believe it is over changing.
Click to expand...
My multimeter was giving false readings I replaced it and my voltage is 14v while running, and batteries are at 12.5v when engine is off. I'm going to borrow a amp meter from someone and see what the alternator is putting out. If it's pushing max amps I need to figure out why. im hoping it's just a dead cell in one of my batteries or a corroded ground wire. I'm not sure what else it could be besides a bad alternator.
 
Droid said:
The temp sender is nothing but a variable resistor. It should give an accurate indication at 14 volts plus or minus.
If you're running over 15 volts, the gauge will read high.
160 sounds about right but the alternator at 300 is way high. So I believe it is over changing.
Click to expand...
I have 2 sensors on my thermostat housing. Is one of them there to prevent the engine from starting if it gets too hot? I pulled the nee thermostat out and found that it was stuck when trying to push it open with a screwdriver. But ran the engine without the thermostat and it seemed to run cooler. And I was able to get it to start again after shutting it off.
 
I'm curious to know if it is possible for a bad battery to appear good and continue to charge and hold normal voltage, but cause the alternator to overheat? I know one of the batteries went for a ride last year amd was tipped over in the engine compartment after hitting a big wave. It looked like it was little wet on top, and I had to clean out some crystallized stuff that was sitting in the bottom of the battery tray last week when I reinstalled the batteries for the season. I had them on a trickle charge over the winter.
 
I've never seen a bad battery "appear good"....

What did the amp meter show?

FWIW, many good parts stores can test the alternator as can any alternator shop (if you can find one)...
 
makomark said:
I've never seen a bad battery "appear good"....

What did the amp meter show?

FWIW, many good parts stores can test the alternator as can any alternator shop (if you can find one)...
Click to expand...
The boat is not at my house currently so I have to wait until Saturday to check it with the amp meter. And im not really interested in trying to find someone that can test it for me, because i only have weekends to work on the boat and id like to resolve this issue this upcoming weekend so i can actually use this boat before the season ends, plus with my work schedule, most shops would be closed by the time I get off. But what tests can I do to rule out if I have a bad battery or not? I'd like to determine that before I start troubleshooting the wiring and alternator. All I know is both batteries are showing a fully charged voltage of 12.5-12.8 and they both read between 13.5-14.3 when the engine is running.
 
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