I Have A 502 Efi That For Some Reason Just Kills Like The Key Is Being Turned Off. Here Is What I Have Done Already.
Replaced The Key Switch
Replaced The Boat Harness/plug
Checked All Grounds And Plugs On Everything

When It Kills You Can Start It Right Back Up?

Need Help!!!!!!

Is it a VST system or cool-fuel? I have VST system and had problem with clogged pre-filter in the VST tank. Take the VST tank off and apart and the small screen might be clogged. Mine was about 75% clogged and ran at idle, only to stall when trying to get on plane. Be careful and you can re-use all the gaskets. If that isn't the problem, perhaps pull the injectors as well as there is another filter/screen in front of each of them as well.

Another solution might be to borrow a scan tool from the local dealer and plug it in to see what's happening in the ECM?!!

Good luck and keep us posted.

Dave

I recently had this same problem but it would run fine at anything one half throttle and below. If I gave it full throttle it would go hard for a few seconds then die. It acted like it was using the fuel in the VST then dying from lack of fuel. My solution... sold the boat for a song after F'ing with it for two weeks In the meantime I switched the mechanical and the electric fuel pumps (cool fuel engines don't have the mechanical), Changed out the entire VST tank. Scan tool showed zip! I ahven't heard if the new owner sorted it out yet or not.

Had the same problem on my 35 Fountain. The Kill Switch wiring was run under the main wiring harness in the stbd gunwhale. Everytime the boat would hit a wake, wave or chop, the stbd engine would just shutdown. The boat went back to the factory for troubleshooting. They changed distributor sensor, then the distributor. Got the boat back same problem.

A local Mercury repair shop said to look at the kill switch. Sure enough once I crawled under the dash, could see it immediately. The kill switch wires were being pulled out of the back of the switch by the wiring harness.

I made a male to male jumper and jumpered the switch out to troubleshoot it.

I initially went after the fuel system as well but the fuel filters were clean.

Good luck.
Bill

I'm having similar problems with my 525 efi starboard motor. Scan tool shows no problem, but the engine dies at higher RPM and I can't get on plane. It feels like it is lack of fuel. The other problem is that it uses a lot of coolant, it is not in the bilge or in the oil.
Merc thinks that the rocker studs were not sealed properly and the coolant is being 'burned'. I'm still trying to get these guys to finish the job. Every time we try to get parts, tools, or manuals they are B/O. I hate warranty jobs, they don't seem to be a priority.

JAS III