Intermittent spark on Mercruiser 350 mag mpi

Lochlomond

Lochlomond

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www.campbellsairconditioning.co.uk
In need of help have had this intermittent fault for over a year now, basically engine can miss and conk out at any rev range, will run for 3 hours or a day without issue then all of a sudden be a pain in the A.

I have traced the fault down to loss of spark from the ignition coil but can confirm that there is 12v at both the coil and module and there both brand new so i can only see it being the ecm holding off the signal to the ignition module

I have changed with good working parts the ECM, crankshaft sensor, power and fuel relays, and neutral safety start sensor/switch

Putting a diagnostic machine on shows up no faults, i have checked countless connections and to be honest their all perfect fresh water only, 100 hrs on 2003 engine

My question really is to anyone who actually knows what sensors tell the ecm not to send a signal to the ignition module for a spark

I know their can be a bad earth, loose connection, checked the lanyard kill switch, harness connection etc etc

But what I’d really like for Christmas is a full list of the sensors on the engine that can confuse the brain like its confused me and preferably not the ones that wont for example many say the IAC but that shouldn't stop a spark at 4000rpm should it

Really hope some of you guys can help I’m at my wits end, first boat as well, 2nd season and so not enjoying it as I should, cheers :)
 
Some Mercruisers usesd to have a shift interruptor switch, which cuts the ignition temporarily whilst changing gear. Not sure if they still do, but if it's present and playing up, that would cut the ignition.

Graham
 
I'd second Gingie thats its the sensor in the distributor.
When its died you need to unplug the white connector on the module under the rotor and check it with an ohm meter on the two green leads going to the induction coil, its a bit fiddly but can be done. I have changed many of these on both cars and boats, they go open circuit when hot then cool down and work again.
And there is no crank sensor on these engines so not sure what you changed there?
 
Thanks for some answers but pretty sure of the following

Their is no ignition sensor in the distributor on this engine, the plug holder on the distibutor does not have any plug connected to it, looking at all info i have read and diagrams over the last year they all say not used pointing to the plug

This engine does have a crank sensor, its crank fired!!

The shift interruptor switch if it exists may well be a problem i'll look into this certainley on wiring diagram i have from service manual 31
 
how newer mpi is it?

ie engine number. do you have the flat dizzy cap with the plug leads all inline or the old familiar cylinder type?
 
Lochlomond said:
The shift interruptor switch if it exists may well be a problem
Click to expand...
Don't take my word for it, but I'm ALMOST sure that it was there only for Alpha drives.
IIRC, the 350 Mag was available with both Alpha and Bravo, but if yours is mated to a Bravo either it should not have the shift switch at all, or it should be disabled.
 
this have shift anticipation switches now. the switch tells the ecm its in gear
 
gingie said:
how newer mpi is it?

ie engine number. do you have the flat dizzy cap with the plug leads all inline or the old familiar cylinder type?
Click to expand...

Hi

Its OM644??? its the flat dizzy, no ignition sensor, no camshaft sensor, no inteript swith as bravo 3

I think i might have found the problem last night, going for some sea trials today and catch the end of the scottish open golf if i've fixed it i'll post back to let all know as i'm sick of reading forum problems that just die and do not actually conclude, cheers
 
Problem solved

Brad thanks for the reminder i had posted in so many forums this 1 slipped the net so the good news is yes the problem is fixed

It was a faulty ignition coil plug, while down in the engine bay and engine running i was poking arround wires in this area which i had done before but this time it died as i touched the wires onto the ignition coil plug, so i started again and depending on which way i bent the wires it would miss etc (might note i had stripped back the loom tape and exposed these 3 wires, 2 were good only 1 was the problem, the 12v primary)

As this plug is part of the wiring loom i decided to solder 3 wires direct onto the 3 pins from the coil which then allowed me to cut of the faulty plug and use a bit of connector block and hey presto

Hours and hours of fun since at last now actually enjoying what boating is all about :)
 
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