Got the boat three days ago 1998 Sonic Prowler 502 MAG MPI, and already having problems. Boat ran strong and great on a 45 min sea trial. Got the boat home and ran a good weekend until I hit some pretty good ways giving the boat a good workout. After hitting the first few rollers the boat started to miss. after about 20 to 30 minutes of working the throttle I finally got the boat up and running without missing, this was in calm water. Ran for the rest of the day no problems, but was calm water. Today get the boat out and run a good 10 minutes calm water, no problems, then pass a 50 footer throwing some good rollers and as soon as I hit them engine starts missing. When I say missing I mean it will not get over 3000 to 3500 RPM, and you hear the motor "choking" out. Strong smell of gas out of exhaust. As long as the water is calm no problems but as soon as I hit some waves the motor starts choking out. PLEASE HELP!!! point me in the correct direction. Check plug wires, GOOD, checked fuel/water separator, GOOD. It is just crazy that it only does it when you hit rough water.....

it really does sound like water contaminated fuel, on a smooth ride the water stays at the bottom.

when you say you checked the separator, did you pour it out? replace it?

Kind of sounds like water in the fuel, but once the filter fills up with water, it usually keeps running like crap until you dump the filter.

My bet is on a short somewhere. I would put a scan tool on it and check for codes. Then I would check the main electrical harness connection first and all the rest of the connections that plug together.

I just visually checked it and looked good, but with water you need to replace it so I will do that first thing this morning. It will be a week before I can get back home to get to the computer to check the codes, so I'll check all the connections and hope for the best.....Anything else?

Check all your grounds also.

So changed the fuel/water filter and check the plug and plug wires. Ran good and then out of no where smooth water start doing it again, this time it is doing it at lower RPM'S as well as higher RPM's. I know I need to hook it up to a computer but what stuff can I check til then. I'm thinking about replacing the plug wires in the morning. ANY HELP!!!!

Ditto, sounds like a bad ground. Had the same thing happen on a Rinker I owned about 14 years ago. Seems like it was a ground behind the dash that tied into the ignition or kill switch.

My 502 Mag MPI acted the same way last year. I found a lose ground wire om the ammeter guage. Fixed that.... no more sputtering. After going thru some ruff stuff mine would crap out at 1200RPM eventually go back to working correctly, bit more ruff stuff and crap out again. It drove me nutz because it was the 4th of July weekend and there is absolutely no calm water on GLOC except for a few coves that weekend. Good luck.

Check battery connections, and grounds.

same thing happened to me on the 4th of july weekend. Everytime I hit waves it would completely cut out and then start back again. You probably have a loose ground. I thought I blew my motor when everything shut off immediately at 50mph.