Yesterday I started our 5.7L Merc I/O (circa 1986). I could hear a
"snapping" noise coming from the engine compartment. I popped the
engine hatch and could see the coil shorting from where the coil wire
plugs into the coil. It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.

I'm not an expert in this area. I know I must use marine-designed
starters, alternators, etc. Can I use a garden-variety coil and coil
wire, or do I need to stick with the Mercury "Thuderbolt Ignition"
components for this situation? The price difference between an
automotive coil & a Merc coil isn't all that much, so it isn't a
question of dollars. It does appear that the new high-performance
automotive coils are a little better quality.

Also, what would cause this? Is the coil wire bad? The coil, itself?
I know they can develop hairline cracks that can cause this sort of
situation.

Thanks for any explanation and advise.

Regards,

Coff

Coff wrote:
Yesterday I started our 5.7L Merc I/O (circa 1986). I could hear a
"snapping" noise coming from the engine compartment. I popped the
engine hatch and could see the coil shorting from where the coil wire
plugs into the coil. It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.

I'm not an expert in this area. I know I must use marine-designed
starters, alternators, etc. Can I use a garden-variety coil and coil
wire, or do I need to stick with the Mercury "Thuderbolt Ignition"
components for this situation? The price difference between an
automotive coil & a Merc coil isn't all that much, so it isn't a
question of dollars. It does appear that the new high-performance
automotive coils are a little better quality.

Also, what would cause this? Is the coil wire bad? The coil, itself?
I know they can develop hairline cracks that can cause this sort of
situation.

Thanks for any explanation and advise.

Regards,

Coff

Any GM HEI type coil will be fine, high performance or otherwise, but
not a "garden-variety" 12 volt coil. Your ignition is based on the GM
HEI archetecture.

Rob

In article ,
(Coff) wrote:

Yesterday I started our 5.7L Merc I/O (circa 1986). I could hear a
"snapping" noise coming from the engine compartment. I popped the
engine hatch and could see the coil shorting from where the coil wire
plugs into the coil. It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.


Did you check to see if the coil wire to the distributor was attached
correctly. My guess is you disconnected the coil while winterizing. If I
remember correctly, my thungerbolt ignition coil did the exact same thing
under those circumstances.

Bob Dimond

Your coil is fine, it doesn't need replaced. The coil wire, and the
sparkplug wires if they are the same age, need replaced.

Rich

"Coff" wrote in message
om...
Yesterday I started our 5.7L Merc I/O (circa 1986). I could hear a
"snapping" noise coming from the engine compartment. I popped the
engine hatch and could see the coil shorting from where the coil wire
plugs into the coil. It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.

I'm not an expert in this area. I know I must use marine-designed
starters, alternators, etc. Can I use a garden-variety coil and coil
wire, or do I need to stick with the Mercury "Thuderbolt Ignition"
components for this situation? The price difference between an
automotive coil & a Merc coil isn't all that much, so it isn't a
question of dollars. It does appear that the new high-performance
automotive coils are a little better quality.

Also, what would cause this? Is the coil wire bad? The coil, itself?
I know they can develop hairline cracks that can cause this sort of
situation.

Thanks for any explanation and advise.

Regards,

Coff

"Coff" wrote in message
om...


It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.



Was the rubber boot on properly?

Was there any water or anything similar splashed across the coil?

Were all the spark plug wires properly connected?

The spark should follow the path of least resistance. Normally this
requires it to jump a very small distance, the "gap" of the sparkplug. If
it jumps some place else, then it is because either the someplace else was
an easier path or the sparkplug path was not available.

Adding a film of water, dust, etc. across the coil could make that an easier
path. If that is not the case, then I would trace down the sparkplug wire
path, including the distributor cap. The fact that the engine was able to
run indicates that at least most of the cylinders were getting spark, but I
would suspect that at least one was not. You should be able to isolate
which cylinder by removing spark plug wires one at a time. If you remove
one and the engine slows down, that plug was working. If you remove the
wire and there is no change in the engine, that plug was not working.

The problem could be the plug is fouled or broke, the wire is broke, or
there is a problem in the distributor cap (it is not uncommon for the cap to
crack or actually have one terminal break off internally).

Rod

"Rod McInnis" wrote in message
...

"Coff" wrote in message
om...


It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.



Was the rubber boot on properly?

Was there any water or anything similar splashed across the coil?

Were all the spark plug wires properly connected?

The spark should follow the path of least resistance. Normally this
requires it to jump a very small distance, the "gap" of the sparkplug. If
it jumps some place else, then it is because either the someplace else was
an easier path or the sparkplug path was not available.

Adding a film of water, dust, etc. across the coil could make that an
easier
path. If that is not the case, then I would trace down the sparkplug wire
path, including the distributor cap. The fact that the engine was able to
run indicates that at least most of the cylinders were getting spark, but
I
would suspect that at least one was not. You should be able to isolate
which cylinder by removing spark plug wires one at a time. If you remove
one and the engine slows down, that plug was working. If you remove the
wire and there is no change in the engine, that plug was not working.

The problem could be the plug is fouled or broke, the wire is broke, or
there is a problem in the distributor cap (it is not uncommon for the cap
to
crack or actually have one terminal break off internally).

Rod

"Rod McInnis" wrote in message ...
"Coff" wrote in message
om...


It was arcing to BOTH the positive and negative
terminals on the top of the coil. Needless to say, with 120 gallons
of gas about 3 feet away, I shut the engine down immediately.



Was the rubber boot on properly?

Was there any water or anything similar splashed across the coil?

Were all the spark plug wires properly connected?

The spark should follow the path of least resistance. Normally this
requires it to jump a very small distance, the "gap" of the sparkplug. If
it jumps some place else, then it is because either the someplace else was
an easier path or the sparkplug path was not available.

Adding a film of water, dust, etc. across the coil could make that an easier
path. If that is not the case, then I would trace down the sparkplug wire
path, including the distributor cap. The fact that the engine was able to
run indicates that at least most of the cylinders were getting spark, but I
would suspect that at least one was not. You should be able to isolate
which cylinder by removing spark plug wires one at a time. If you remove
one and the engine slows down, that plug was working. If you remove the
wire and there is no change in the engine, that plug was not working.

The problem could be the plug is fouled or broke, the wire is broke, or
there is a problem in the distributor cap (it is not uncommon for the cap to
crack or actually have one terminal break off internally).

Rod

Thanks to everyone who responded! I picked up a new coil wire and a
set of plug wires from my local Merc dealer today. I'll install
tomorrow. I did try re-seating the coil wire, but it didn't help. No
dirt, moisture, etc. All the wires are at least 8 years old. I'll
wait on replacing the coil, itself.

Thanks again.

Coff