Engine won't start - no spark from coil

[MojoRisin]

Please hang with me as my verbiage won?t be technically sound as I am mechanically illiterate.

I have a 1986 Mercruiser 4.3L 205HP engine that was rebuilt this past winter. Since the rebuilds (related or not), I?ve had intermittent starting and dying issues. Early on, the engine always started and then might die while warming up at idle, under WOT (only once), or while trolling at low RPMs. The engine would die as if was **** off with no sputter or anything. It always seemed to restart right away, but as the season went on, it seemed like I needed to ?play? with some wires (related or not) to get it to go.

Lately, it hasn?t been starting and when it eventually does (hours later literally), it sometimes dies and sometimes doesn?t. To note, the engine cranks and cranks and just doesn?t turn over. I was finally able to reproduce the issue with my mechanic present as every time he was there, it worked fine. He has determined that we don?t have a spark coming from the coil. The first time he diagnosed this, the spark was not there and a few tries later he had a spark and the engine would start right up.

Early in the season, my mechanic replaced my ignition switch as this issue first started where it would crank but not get the spark signal needed. A couple of quick bursts with the key would make it start. Replacing the switch worked for that issue (related or not). My mechanic replaced the coil on Saturday and the boat started right up and we thought we were OK. Right when he was getting ready to leave, it died again and there was no spark shown from the coil once more.

The engine has a Thunderbolt ignition box and we discussed swapping the box with the one on the port engine to rule it out, but my mechanic said he has never seen one of these fail. I want the swap to rule it out, but it did not happen.

My mechanically illiterate pea brain thinks there has to be some kind of wire shorting out coming into the coil. I spent lots of time Saturday evening ?looking? at all the wires because the boat did not start again then or yesterday for that matter when I was alone.

All ideas are greatly appreciated. We?re at wits end.

Thanks,
Joe

 

[Don S]

Re: Engine won't start - no spark from coil

WHEN it doesn't work, follow these test procedures.

 

[MojoRisin]

Re: Engine won't start - no spark from coil

Thanks for the information Don.

Any ideas on what could make this an intermittent issue? This would surely be much easier to resolve if the engine never starts, but once it does, it can't really be diagnosed easily.

I know the Thunderbolt is supposed to be very reliable. Has anyone actually seen a failed one as my mechanic has said he has not and that is why he was reluctant to switch the port and starboard modules. I think if that switch was done, we could have at least ruled out the module.

Thanks again. All ideas are appreciated!

 

[MojoRisin]

Re: Engine won't start - no spark from coil

Any more ideas on how to diagnose the intermittent issue?

 

[CharlieB]

Re: Engine won't start - no spark from coil

Poor connection any number of places

Will have to test virtually all connectors both sides and shake while testing to see the intermittant failure

 

[Don S]

Re: Engine won't start - no spark from coil

Any more ideas on how to diagnose the intermittent issue?

That's why you use the chart.
When the problem occurs, have your meter handy. and start at the beginning.

With key in run position, Check for voltage at......................

That step right there will eliminate or point to a lot of different things.
If you have voltage and it won't start, well, you know it's not a problem with the ign. swtich, kill switch, main harness, etc. and you know to move on with your testing.

NOBODY can find the problem when it works.

 

[MojoRisin]

Re: Engine won't start - no spark from coil

Thanks for the replies. I understand this can't be diagnosed when working and that is what makes this frustrating becasuse out of nowhere with no rhyme or reason, we'll get a spark.

Do these Thunderbolt boxes ever fail? Should we be switching the 2 boxes to at least eliminate the issue or is it not worth the effort?

Thanks again.

 

[Don S]

Re: Engine won't start - no spark from coil

Do these Thunderbolt boxes ever fail?

Of course they fail, everything fails sooner or later. The trick is not replacing it TILL it fails.
Have you disconnecet the module and made sure all the pins were clean? Have you tried doing a wiggle test on everything while it's running to see if it causes it to stop?

 

[MojoRisin]

Re: Engine won't start - no spark from coil

Don,

I'm not 100% sure the module was disconnected but since the coil was replaced, I believe it was. Again, remember that I am mechanically illiterate and I don't know what all the connections are for. I have this thread out here hoping for a possible reply of something that my mechanic may not have tried. He is very good at what he does IMO and this being intermittent has made this very frustrating and caused me lots of cancellations of fishing trips lately. Unfortunately for me, my mechanic has lots of other customers too and I get get his overall focused attention. I sure wish this would fail for good but that has not yet happened.

I will have to ask my mechanic what form wiggle testing he has done. I know that I wiggled everything there but that was when I could not get it started when I was there without my mechanic and I was hoping to get it started.

You guys are awesome here and I print these threads and fax them to my mechanic just in case there is something that gets his attention. I really appreciate what you all do for people here.

 

[hlgarrettlee]

Re: Engine won't start - no spark from coil

Hey, I had the same problem with intermittent ignition failure. used the chart, swapped coil, still no fire then i removed the ig module from manifold to check and see if any pins might have bad connection, no pins apparently old model, wires molded into unit. anyway, went ahead and bolted back into place and just for kicks gave it a crank, Yippee! fired right off. it is my belief that the ground for the ig module although tight was not good do to oxidation of steel mounting bolt in the cast manifold, since there is a heat isolater(spacer) between ig module and manifold. anyway i had had intermitent issue for years, just out of the blue no ig, anyway although boat fired right up, new problem arose, cracked engine block, drains had been removed however blockage behind fitting did not allow water to drain, damn the luck. jb welded the 4 inch crack on port side of block about on half inch below deck, hope it holds. any thoughts appreciated