Intermitant Fault with mag 350 MPI

[gentlemanglobetrotter]

Hi & Help, bought 1st boat couple of months ago, Maxum 2700SCR (2003) only 60 hrs on engine, been fun up to a few weeks ago, now 74 hrs on engine

Problems-
1) Notice leaving marina at idle engine missed a beat every now and again, put on plane @ 3500 revs and seemed fine, out for a hour or so returning on plane felt it miss again, came into marina and ticked over fine
2) Went out from marina and the odd miss again, just onto plane @ 4000 revs when dies, heavy smell of fuel, tried to start would occasionally fire but then splutter to a halt, towed home, back at marina fired up and ran for 15 mins at idle no problem
3) Replaced fuel/water separator filter, on draining fuel from old filter it was very dirty – no water, changed inline fuel filter as well and again really dirty from inlet side, took boat out and tested for an hour ok at all revs, thought problem fixed
3) Went out for weekend, no issues at 1000-2000 revs to destination, stayed the night, next day headed for supplies, all well for 30 mins while waiting to dock spluttered to a halt again, wouldn’t start, 1 hour later got started, running really rough, using high revs to keep running sounded like missing on a few cylinders starting and stopping then all ok, limped back to destination between 700-1000 revs never missed a beat, fired up a couple of time during the night all well, fired up in morning for 30 mins before heading home again all well, loaded up kicked in reverse and spluttered to a halt, tried over an hour to start, would occasionally start but splutter and die, towed home again
4) After being towed back tried to start at marina and wouldn’t so hey hopefully find the problem now, an engineer looked at it and she still would not start, he checked fuel pressure - ok, removed plugs pretty dirty so cleaned and re-gapped - engine now fires up but would shut down after anything from 30 seconds to 2-3 minutes, purring away then just shut off as if you switched the key off, then he phoned back to say engine now running for last 15 mins and not shutting off- arghhh
5) Decided to get Mercruiser dealer to run diagnostic test, he did with no fault codes or issues, he checked the fuel pick up wasn’t blocked says engine running away with no faults and seems to be fine, thinks sea trials required to find fault or breakdown again etc
6) Was down last night, ran engine myself for 20 mins revving etc and purring away nicely – so frustrating

Sorry for the long story but I have lost all faith in her and had the embarrassment to be towed in twice now from the same park rangers, there is clearly something not right and I guess the more information the better so any ideas guys??

[seapuppy]

bout the only things you have are the coil...ignition module...ignition switch..a breaker...and a key.........so..if your not getting spark at certain times...one of those items may be intermitten...now one other thing to check..you should see a huge connector on top of the engine...pull that apart and clean the contacts...re-grease it with dielectric grease and re-connect.....

keep us informed...and thanks for posting back on this one..

SP

[ss3964spd]

Since no one can find the exact cause you are reduced to throwing parts at it. Can't quite tell if you actually replaced the ignition control module and coil or not. If not, do so. I've seen the ignition modules run just fine when cool, but act up when hot.

Also, if you've not done so, get a copy of the wiring schematic, so you can trace the entire ignition circuit. Definately do what Dodge mentioned above - start the engine, get it warmed, and open the engine cover - at night, the darker the better.

Finally, check your ground connections, especially the one that is probably bolted to the rear of the block.

Dan

[gentlemanglobetrotter]

Originally Posted by ss3964spd

Since no one can find the exact cause you are reduced to throwing parts at it. Can't quite tell if you actually replaced the ignition control module and coil or not. If not, do so. I've seen the ignition modules run just fine when cool, but act up when hot.

Also, if you've not done so, get a copy of the wiring schematic, so you can trace the entire ignition circuit. Definately do what Dodge mentioned above - start the engine, get it warmed, and open the engine cover - at night, the darker the better.

Finally, check your ground connections, especially the one that is probably bolted to the rear of the block.

Dan

Hi Dan

Yes i did replace ignition coil & module

Any idea where to get a copy of the wiring schematic from as i want to check every connection here

Will go down at night and have a look

Cheers

[captainron]

So here it goes..I had the same problem with one of my 350 mag mpi motors.I was at the point of ripping off the fuel injection
and going with carbs.I had the advantage of swapping parts from the good running engine.I finally swaped ECM´s and found out that the ECM was acting up when it warmed up. New ones cost to much so I installed a bilge exhaust fan and ducked the air over the ecm.Problem solved.If I forget to turn on the fan she will act up.I know I will need to replace the ecm soon.But, untill then this works.I hope this helps.

[gentlemanglobetrotter]

Originally Posted by captainron

So here it goes..I had the same problem with one of my 350 mag mpi motors.I was at the point of ripping off the fuel injection
and going with carbs.I had the advantage of swapping parts from the good running engine.I finally swaped ECM´s and found out that the ECM was acting up when it warmed up. New ones cost to much so I installed a bilge exhaust fan and ducked the air over the ecm.Problem solved.If I forget to turn on the fan she will act up.I know I will need to replace the ecm soon.But, untill then this works.I hope this helps.

Interesting Captainron as that is next on my list, i am lucky that my Merc dealer has a spare Mag 350 with a cracked block (insurance replaced) so i now have access to all the bits i need to try, i have the ecm, the HT lead for the coil also another switch for being in gear or somethingto try

As the spark is intermittent and as far as i'm aware the ecm takes the inputs from sensors ie crank sensor etc and then must output to at least the coil or ignition module to tell it when to spark then this is my next diagnosis that the ecm is breaking down and i so so so hope your right mate although if it proves to be i may try your fan idea as i see they are approx $1100 to $1400 for new, try $2500 in rip of Britain

[gentlemanglobetrotter]

Changed ECM along with start neutral safety switch & HT lead from coil, problem still exists so hoping to get crankshaft sensor and power relay to change over

I think after that its then down to a dodgy connection

[ss3964spd]

Sure must be a relief to have sorted that issue - well done!

[LRose]

Hello
I just had the same problem this week on Lake Chelan. Got towed in by the Lady of the Lake Ferry. Very embarrassing!!! Anyway.... The problem sounds the same. Running fine then dies even while running @ 3500 RPM. I had to replace the ignition control module. $385. This is the 2nd module I have had to replace. Mine is mounted on the exhaust manifold. After talking w/ a local Mechanic, trying to figure out why I had to replace the module again, he told me that the cause had to be heat or some overload in the electrical system. He also told me that if you order an ignition module w/ the wiring harness as a package the instructions say to mount the ignition control module on the hull not the exhaust manifold. So Heat will destroy the module.
L.