Why does my outboard eat coils?

[MooseheadDoc]

DISCLAIMER: some details in this post may make no sense due to my mechanical ignorance.

Engine: Johnson 175hp 2-stroke J175PXSTM

At the start of the season the engine did not have spark in all cylinders. Boat was new to me at end of last season. This was my mechanic’s first real look at it. He replaced some wiring and contacts, checked the optical sensor, new plugs, new aftermarket coils. Boat ran great.

I tied up one night after running the boat all day with no issues. The next morning went out to fish- lost 1000rpm’s.

Mechanic replaced the coils with used OEM set (new OEM coils not readily available). Boat ran great.

Used the hell out of it all weekend. Tied up last night without issues. Went out to fish this morning… lost 1000 rpms. My assumption is the coils again. Waiting for mechanic to have a look. The only good news is that I’m leaving on vacation tomorrow and won’t need the boat for a few weeks.

I have a good mechanic that I trust (a necessity for the mechanically ignorant). When he put in the OEM coils he said that he also checked the “trigger voltage” which was appropriate.

Aside from the possibility of bad luck- getting 2 sets of defective coils- is there anything that could be causing my engine to eat coils, while otherwise running well? And why would the coils fail AFTER use instead of DURING use?

I‘ll update when I get more info from the mechanic.

JB

 

[Doc Stressor]

The most common source of ignition coil failure is a bad sparkplug wire. As these wires age, the resistance goes up and the coil overheats. Did your mechanic replace all of the sparkplug wires?

 

[wspitler]

Look at fuel issues, your focus may be misplaced. Probability of two sets of coils failing is remote. 1000 RPM loss may be related to a single cylinder issue or air/fuel issue.

 

[MooseheadDoc]

True. At this point I am only assuming it is the same problem since it is behaving the same. I’ll update once I hear more from the mechanic. Last time the coils appear to have been the right answer since there was no spark to the cylinder(s) and replacing the coils solved the problem.

 

[MooseheadDoc]

@Doc Stressor

will check on wires. I like simple solutions.

also, neighbor mentioned “stator”. Would bad stator cause coil failure?

 

[seasick]

Have you tried using one of the removed 'bad' coils. Maybe it's not coils. In any case a simple spark tester should tell you if you have spark to a specific cylinder. It is very odd that the coil would go bad the next day after heavy use as opposed to go bad when being used. I don't know what the issue is but I am not 100% convinced it is coils.
Now if you use a spark tester and find a suspect coil, swap it with a coil for a know working cylinder and test again to see if you now have spake ( in that case, I guess the coil went bad)

 

[MooseheadDoc]

Belated update, for what it’s worth…
in hindsight it appears the root of the problem was ‘bad coils’ that failed in turn. The prior owner had done some electrical work which included replacing coils with some non-OEM part. One coil failed and was replaced, then the second coil failed. Parts acquisition is an issue, so my mechanic initially replaced the with used coils scavenged from a parts engine until we could find new coils. Engine has been running fine since.

 

[seasick]

Thanks for the follow up. I would not have guessed premature failure on multiple coils. One to add to my debugging list

 

[ROBERTH]

Problems like this I had many many years ago with my OMC Seadrive which is basically a Johnson motor was that my wiring across the motor was bad. I had found a ground wire that crossed over the top of the block, somewhat buried that had over 6 spots that were swollen with corrosion, so the ground was not carrying the load, thus why I kept throwing parts that kept going bad. Stator, coils, etc. I replaced the wiring on the motor and never had the issue again.
One other issue I had was that I had installed one of the coils backwards. It would ground out on the motor cover latch. I figured it out when I was at the dock, boat on trailer in gear with a load. Motor missing, pulled cover and then it ran like a bat out of you know where! Was easy on my motor to install backwards. Fixed that and no more issues for over a year when I finally sold that boat.

 

[MooseheadDoc]

Problems like this I had many many years ago with my OMC Seadrive which is basically a Johnson motor was that my wiring across the motor was bad. I had found a ground wire that crossed over the top of the block, somewhat buried that had over 6 spots that were swollen with corrosion, so the ground was not carrying the load, thus why I kept throwing parts that kept going bad. Stator, coils, etc. I replaced the wiring on the motor and never had the issue again.
One other issue I had was that I had installed one of the coils backwards. It would ground out on the motor cover latch. I figured it out when I was at the dock, boat on trailer in gear with a load. Motor missing, pulled cover and then it ran like a bat out of you know where! Was easy on my motor to install backwards. Fixed that and no more issues for over a year when I finally sold that boat.

 

[MooseheadDoc]

Thanks for the info. I’m not convinced there isn’t a load issue that cooked the old coils but the new coils are tolerating… so far! If there is my yard guy couldn’t find it. Hopefully there won’t be more to the story….

 

[ROBERTH]

Now that I think more on this, when I pulled the stator off the motor, looking down I could see the damage to the black wire. I did have my engine in the shop first time or two and the mechanic never did find this and I saw that it would not be easy to diagnose or see unless he did a resistance test to all the wiring.
Your parts might be good for now, but don't expect long life if you have this issue. I would get partial season out of them before one would go, etc. Got tired of limping in or getting towed.