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Mercury 225 Offshore Random intermittent spark #2 cylinder.

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Esalt321

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Hello everyone. I have a mercury 225 carb that is having issues. Motor has low hours (140hrs). Boat sat for some time before I purchased it. I have ran the boat a few times and noticed that its idling rough but not terrible. Got home today and put the timing light on each plug wire and discovered the #2 coil was not firing properly. The same occurred with #5 but I couldn't get it to act up again after I went back to it. Thru out the day I noticed that it wasn't smooth at idle. With the timing light still connected I brought the rpms up to 1300 and everything smooth out. Light fires continuously but then when I bring the idle down it starts missing again. I switched the coils and the problem didn't travel. Put the timing light on the primary leads to the coil and its erratic as well. Shut the motor down for a minute and started it up again after 2 mins and couldn't get it to act up again. Double checked connection with timing light before and after. Where should I start looking for the culprit? This motor has the electronic ignition timing so that cannot be adjusted. Checked continuity for ground on each primary lead, all was good. Motor runs strong. Pushes my 25'1" Proline walk around to 46mph at 5300rpms. Could go higher but didn't feel it was necessary. Thanks for reading and happy boating.
 
Check all the ground connections since its looks like a salty motor. If it has the "D"shaped CDM modules they are bad about failing internally and there is a tech bulletin on his..
 
Thanks for the quick response. The motor is clean and free off corrosion. I will check all connections. I am not sure which ecu it has so I posted a picture. Let me know what you think. Thanks.
 
Esalt321 said:
Thanks for the quick response. The motor is clean and free off corrosion. I will check all connections. I am not sure which ecu it has so I posted a picture. Let me know what you think. Thanks.
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Picture.
 

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I apologize for my mistake. I realized my error once I posted the picture. Would like to blame the cdm modules but there was no change when I swapped them as stated. Could the cdm failing harm something in the ecu. Aside from the ecu, cps and the tps whats left on the ignition system? I will do some bench testing today and report back. Thanks
 
Ok, If you have tear drop shaped CDM's it is recommended that you change all of them. There is no problem with the ECU. Now, on the CDM is the weather connector(Plug) on top?
 
They are the new style CDM's.

The ECU will not be your problem with only one cylinder losing ignition. Chances are one of the bobbins on the stator is failing. Each bobbin on the stator powers one cylinder. You will need to check the voltage coming out of the stator bobbin for the cylinder in question.

If you find the output to be correct, you will need to look for a bad connection. That means pushing the wires out of each weather connector and looking at their condition.
 
Can this be checked at cranking speed or can it be done with the motor running? Is there a test harness required. Thanks again for the response.
 
I tested the tps and discovered that I couldnt get the voltage requested per the manual with the tps in the position allowed with the braket. I have a picture of where the tps has to be at idle in order to get a .95 volt reading.20150202_162227.jpg
 
Performed a resistance test on the stator. cyl 1-849. cyl 2-660. cyl 3-851. cyl 4-1044 cyl 5-805. cyl 6-781. So its obvious that the stator is shot I assuming. Disconnected bundle of white ground wires and checked each one separately and came up with the same readings. I am still concerned about the tps. Does this seem normal. Readings before removing tps sensor at idle were 3.45 volts. thanks
 
Couldn't find anything in the manual you provided on values for the tps. Just that if suspected to "check" it. My manual states the tps should read .95 volts at idle and 3.7 at wot. Mercury manual says the same. Timing seemed fine before discovering the misfire. No thoughts on the results for the resistance test on the stator? Thanks
 
Update. Tps is fine. I was testing it incorrectly. .95v at idle and 3.8v at wot. Removed the stator and will have the new one soon. Thanks to everyone that responded.
 
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