deboatmon

I have twin 502EFI's that have had substantial upgrades done. They have performed great for over two years but recently I've had a miss come and go that I'm having trouble finding. It seems electrical in nature as I can go through rough water and the problem occurs, and then will go away. Definitely a loose connection on something.

Typical cruise for me is 3500 RPM and when the miss starts I lose about 200 RPM. Seems like one cylinder to me. You can hear the difference in sound and feel the difference as well. And, while missing, the difference becomes much greater with a lot more throttle. It is surely a loose connection somewhere, but where ? I've replaced the distributor cap, plugs, and checked the wire ends thoroughly. Isn't there an electrical connection to the injectors as well ? Could this be the problem ? If so where is that connection ?

Any suggestions would be appreciated.

Outdrive1

I've recently had some of the same symptoms. I have a 502mag efi motor. I just had Az Speed and marine put a vortech supercharger on it. During their test run they had a miss fire. The did a compression check and bench tested all my injectors as well as checking my fuel pressure. They thought that the test equipment they were using (basically a spacer in between my header and the head with o2 sensors in it) might have been touching one of the spark plug wires. They welded a bung on to the header itself and they said it ran fine. I've had it out three times since then, this last time at about 5200 rpm it felt like I was starting to miss and I lost some rpm, it did it intermittently and it also did it once while I was at idle. I was going to change my plug wires, but if you just went through all the wires etc maybe thats a waste of time? I you find anything else out let me know, I have about 170 hours on my motor. How about you? Maybe there is a tsb on these Mercury ignition systems. Some one should know.

maxum

Had a miss on a 500hp, procharged rpms would drop then kick back in ended up being the ignition sensor in the distributor under the cap, U may want to give it a try if it doesn't work you'll have one as a spare as these things tend to go good luck

Griff

Check/Clean all connections to everything on the ignition system. There are lots of them. Module connections, dist connections, module, coil, Any one of them could cause this. Intermittent crap is the hardest to find. Eventually it will get worse and become easier to find.

Also, if don't mind getting a shock, run the engine on a hose and run your fingers across all the plug wires from the cap to the plug. If you get shocked, you've got a couple plug wires arcing.

GLH

Something of that nature happened to me on 502 Mag MPI last year. It ended up being a ground happening. We diagnosed it running with the Merc diagnostic tool plugged in and the tool would shut off at the same time the miss would happen.

Outdrive1

As much work as they did installing the blower they could have easilly disconnected something or I could have a bad plug wire. I think I'll put the hose on it and try the shock therapy and go over my connections. 2 weeks until the poker run in Havasu and theres no way I'm going to miss the last run of the year with my boat sitting in Az Speed & Marine's parking lot for three weeks.