Mercury 250 Pro XB cutting out

[Stratos.250]

2002 Mercury 250 Pro XB
OT430578

This motor has developed an issue where it cuts out as if the key switch was cut off and turned right back on. You get that initial start up audible tone every time it does it. The engine cuts out but never shuts off completely. It’s most prevalent at idle but will do it at all engine speeds. It will do it under load or on the hose. Other than this the engine runs flawless. I have cleaned connections and used dielectric grease everywhere that I can. I’ve cleaned grounds . Sometimes I feel like doing that makes the issue better but I can’t be sure. What can I take loose under the cowling to isolate the issue to either console end or motor end?

Hopefully I didn’t post in the wrong place 😬

[JOE54]

notorious for cannon plug problems clean and spread pins on connector would clean all grease from connector and shine all pins , a remote key harness can be very good at isolating this problem also ,swat i use for most diagnosis with wiring problems to eliminate boat as source ....... right place

[Stratos.250]

I will give that a shot and report back. Thanks !

[EuropeanAM]

That will likely solve it- if it doesn't, I would try a separate test harness connected directly to the engine.

[Stratos.250]

That did not solve the issue. Another something I’ve noticed is that the Tach will randomly jump around a second and go back to working fine. The tach and the cutting out do not seem to coincide. Mr Don can I order a test harness from you? If not where would I find one ?

[FeFanatic]

Can you jiggle the ignition key and make it shut off?

[Stratos.250]

No I actually removed the key switch and was messing with it bending the harness etc trying to make it cut. Same thing with the harness back near the cannon plug. I can’t make the problem occur by moving anything around.

[JOE54]

clean all connections on engine pay close attn to cdms and ecu connections

[Stratos.250]

I’ve pulled and cleaned them multiple times. This engine is pretty clean in general. I’ve tried with and without dielectric grease.

It definitely has me stumped.

[Mercury49]

I sent you a PM.

[EuropeanAM]

Have you added redundant grounds (2) to the ECM/Coil Plate?

[Stratos.250]

I haven’t Lake tested it yet but a new set of plugs seems to have cleared the issue. It was really bad on the hose at an idle. Since new plugs I haven’t seen it blimp out not one time. Hope to run it on the Lake this weekend.

I did add the grounds a little while back but that didn’t seem to help any.

[EuropeanAM]

Roger that. QL77CC only.

[JustinNC]

I haven’t Lake tested it yet but a new set of plugs seems to have cleared the issue. It was really bad on the hose at an idle. Since new plugs I haven’t seen it blimp out not one time. Hope to run it on the Lake this weekend.

I did add the grounds a little while back but that didn’t seem to help any.

Just out of curiosity, what was the condition of the plugs pulled? Fouled? Gapped wrong? Tip (cant remember the proper term of the ware it arcs to) eroded down?

[Stratos.250]

The plugs looked fine visually and gap was fine. I am running QL77CC plugs. So far so good with it. They were consistent in color, I could not really tell which ones were causing an issue. Maybe all of them were cutting out, I’m not sure.