ctuck0659

Anyone care to take a shot at diagnosing this: Merc. 502 MPI. A few weeks ago it started cutting out with no warning when running at cruising speed. If I turned the key off & back on it would usually cycle back up, fuel pump, 2 beeps, start and run a while longer before cutting out again. A couple of times it would crank but not restart for a few minutes. Then whet I went to resart after docking for a couple hrs. it would cycle up, crank, start, die over and over. Next day it started up normally but started cutting out after it warmed up. I replaced the water seperating fuel filter- no help. Next, I replaced the ignition module on the back of the distributer. This weekend it didn't cut out entirely when running but every few minutes the tach would start jumping down and back by 1000-1500 RPMs for several seconds and then return to normal. This morning I swapped the coil with the Stbd. engine. I barely got out of my lift slip when the alarm beeped once (it hadn't done that before) and the eng. died. When I tried to cycle up, the fuel pump came on but no beeps. After I limped back into my lift, I turned the key and it beeped normally again. A few last details: I've done a code read 3 times and its always "12"- no codes. When it cuts our or starts missing only the tach jumps, not the other gauges so its not a complete voltage failure. I did notice today though a flutter in the volt meter when idling which wasn't here before. If someone has a good suggestion to try I'd greatly appreciate it. Otherwise I'm gonna have to bend over and take it to shop with a scanner I guess-

mats

As your title suggests it sounds like ignition. Probably a bad connection. I would check all the connectors around the distributor and the coil including the engine harness and the high tension wire to the distributor. Since it dies completely it points to the wiring before it is distributed to the individual plugwires.

Could also be the connector to the fuelpump.

ctuck0659

Thanks. I've checked all the connectors in the ignition and ECM system- they're all clean & rust free. I'll check the fuel pump wiring.

Turbojack

Did you replace dist. cap & rotor? This is the first thing I do when motor starts acting up.

satisfactionII

When you replace the cap and rotor replace the Ignition control module as well. It's cheap about 26-42 dollars depending on where you buy it. Good luck.

ctuck0659

Replaced the Dist, Rotor, Plugs and Contol Module already-

CcanDo

Check battery to engine ground cable ,then positive battery cable ,then with 14 0r 12 GA. jumpers eliminate the ignition switch from the circuit by going from battery to coil ,then check ground side of fuel pump .

good luck !

lsmjuno

Check the main engine harness "cannon" plug. Start the engine at idle and wiggle the connector, if engine sputters then cut the plug out and butt splice wires together. Be careful wires are not always the same color on both sides of the plug, leave enough wire to ID the color and use ohm meter to match up colors. I've had to do this on 5 mercruiser engines this year, some old, some new. O YEAH disconnect batteries before cutting through harness or sparks may fly.

ctuck0659

Thanks for the ideas- I haven't checked the fuel pump connections so I can try that but it always comes on when I turn the key off/on. I've checked all the other connectors including the main harness, and jiggled them while the eng. was running. I've also checked the battery cables even though all the gauges don't move when the tach dips which tells me its not a complete power cut-

CCstinger260

If you have spark but it still won't fire up, fuel pump is running, try checking the fuel pressure on the rail. I had the same problem with my GMC truck 454 (I know it's not the same thing) and my pressure regulator was bleeding off too much fuel pressure to run the engine. No codes, no real traceable issues, fuel pump would run, shut off for a second, then run again. I had a similar problem with a Beretta my wife had about ten years prior or I wouldn't even have thought to check it. Don't know if a boat's set up the same way, but I thought I'd throw that out there. Bill