Masonm33

I have a 96 Donzi 27 ZX with a 502 MPI in it. The 502 runs fine but at approx 4500 RPM, it shuts down. The only way to describe it is like turning off the key. The boat starts right back up. No miss or stutter in it. Changed the electric and manual fuel pumps. All grounds appear to be clean and good. No codes are showing up in the computer. The only thing we can find is at cruise RPM (aka) approx 4500 RPM, the voltage spikes downward from 13.8 to 11.6 volts. Per the laptop, this happens a milli second prior to the engine shutting down. .Does the voltage drop cause the pump to quit?. It does not stutter or act like it is starving for fuel. It just shuts down. I have calls to many people and no one has any clues as to what causes this. Boat will run fine and then for no reason just shut off. If you are quick, you can throttle back and the engine will not die. If it does die, a simple hit the key and it starts right back up. Any clues as to what is causing this? HELP this is causing many gray hairs to a bunch of smart people...... Thanks as any input is greatly appreciated!!

Griff

Possibly the alternator is shorting out under high rpm or possibly a short in the purple wire coming off the alternator.

GMC271

I had a similar issue with the 502 in my 96 Formula, the motor would stumble at high rpm in rough water. Check the wiring harness that runs between the rear of the motor and the oil cooler. You may need to remove the mounting bolts for the VST and move the VST out of the way to see the problem. The main power wire, I believe that it is 10 gauge, has a non-insulated butt connector that is wrapped with cloth tape; the butt connector wears through the tape and it intermittently shorts out to the engine block. I wrapped mine with good quality electrical tape and covered the wiring harness with split wire loom to prevent the problem from occurring again. Hope this helps.

Ted G

What does the tach do? Does it drop like you shut off the key or does it wind down slowly (slower) with engine rpms?

Masonm33

Thanks for the tips Guys! The Tach drops quickly, but the other gauges are still powered up. Going to recheck all connections once again today and I agree that it most likely is an electrical breakdown at this point.

ALL_IN!

Have you checked kill switch wiring? Possibly the higher speed / vibration is aggravating a wiring issue.

Masonm33

Kill switch and ignition switch have both been checked. Funny that it happens right around 4500 to 4600 RPM...Mercury techs in Fondulac, WI are leaning toward a short also. From what they tell me, that 502 will shut down if the voltage drops below 10.5 but the lowest we found was the downward spike to 11.6. Going to run it with the laptop on it later today or tomorrow with one guy in the engine compartment and going to try to wiggle wires and harnesses. Need to do early so the water is flat.