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Bill Sheffield
 
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Default mercruiser backfiring, destroying distributor caps

Some of these ignition systems use a resistance wire in the "run" circuit to
limit coil current. Some use a resistance coil. Make sure you have the right
components for your ignition setup.

Bill
"dave" wrote in message
om...
Dave,
The tach jumping around probably indicates a problem with the primary
ignition - the 12 volt parts. That would include battery, key switch,
coil, distributer pickup and electronics, and wires in between.
The cap and rotor corrosion may indicate either moisture or other
contaminates in the distributor, or excessive resistance between the
rotor and the spark plugs.
Check the voltage at the coil while running.
Have the coil checked on a scope, they can fail intermittently.
The moisture/contaminates may be coming up though the distributor shaft
housing because of excessive crankcase pressure. Check the engine
breathers and PCV system.
Al



Thanks. It sounds like you think like me. I have been through every
wire I can find, have disassembled and cleaned the ignition switch,
checked everything I can think of for correct continuity and/or
resistance. I put in a new MSD distributor and plug wires are new. I
have not checked the voltage at the coil while running. Coil is
original. I did just find on the coil that the resistance for the
secondary wiring tests at 7500 ohms, while the manual states
9500-11000. I'm not sure that means anything, though. I guess I
could throw on a new coil. Thanks again.



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dave
 
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Default mercruiser backfiring, destroying distributor caps

That is a problem that I am well aware of. I have checked this out
and as best I can tell the switch is working correctly. Thanks.



Jim Kelly wrote in message ...
Check the shift interrupter circuit. Since it grounds the coil, it will
upset the tach signal. If it interrupts at speed, it will cause a stumble
or backfire and could cause spark scatter in the distributor, damaging the
cap.

dave wrote:

Here's a stumper: 1998 (by serial number) Mercruiser 5.0 EFI that
runs like the proverbial swiss watch. About 150 hours total. Starts
easily, runs smoothly and strongly. Propped well. 4600 RPM/51 MPH in
2130 Chaparral. Used for family recreation, towing watersports.
Problem begins somewhere after 2-5 hours on a new cap and rotor.
Typical scenario is, while advancing the throttle to 2200-2400 RPM, I
begin to see the tach jump around like it's lost its ability to count
the firing cycles, or is maybe counting extra random firings. Tach and
engine come up smoothly to this range, then tach swings randomly up to
higher ranges, needle swinging back and forth. Once past this RPM
range, tach again comes into line and behaves. Engine is running
smoothly at this point and may continue to do so for a few more hours.
With continued running, however, engine begins to miss and backfire
when accelerating through this RPM range. Concurrently, tach is
having its problem and, as problem progresses, can't rev past problem
area. Inspecting cap and rotor shows excessive corrosion. Replacing
them right there on the water, engine still hot, solves the problem.
Again runs like a top...for a few hours. Only a new cap/rotor solves
problem.

Have inspected everything I can think of. New plugs, wires,
fuel/water filters, fuel lines, fuel pressure tested, all wiring
connectors, ignition timing (with correct timing tool). Even replaced
distributor complete. No water dripping onto distributor.
Disconnected tach to see if it is electronic feedback into the EFI/ECM
circuits. No luck. I even did 'sham' cap and rotor replacements,
where I opened the engine compartment, disassembled distributor, let
engine cool, then put old pieces back on...same problem. Can't figure
out why cap and rotor are getting destroyed.

Doubt serious mechanical engine trouble, as it runs too darned good
until this occurs, then runs great again immediately after cap and
rotor are replaced. Couldn't run this stronly with bad valves, cam,
compression, etc. Have not replaced the coil as it seems unlikely it
could run so well if that were malfuncioning...could a bad coil send
out multiple or random sparks? If so, why does a new cap and rotor
fix the problem? Are the new pieces just able to mask the multiple
sparks occurring?

Seems like corrosion is causing some sort of misfire, and/or
crossfiring (which perhaps could be what the tach is counting making
the needle swing, but the tach should get its signal from the coil or
at least the distributor shutter, not from what's going on in the
secondary wiring, right?). But I can't find the reason why the cap
and rotor get corroded so quickly, and their lifespan is decreasing.

One idea has to do with the advance mechanism on these electronic
motors. The distributor is fixed, unlike the old weight and spring
models that advanced the rotor and breaker cam simultaneously. On my
motor, the advance is electronic and the spark firing occurs earlier
and earlier in the rotation of engine AND distributor which means
that, at full advance, the spark could be trying to jump from rotor to
cap when it is still some distance from the cap terminal. Could this
be causing the terminal to 'burn' and/or the spark jump to another
terminal? If so, why doesn't this occur on all such engines. Why
does it occur on mine with both the stock disributor and the MSD I
replaced it with?

I bought this boat used with 40 hours on it in excellent condition.
I'm not suspicious that the previous owner had this problem and
decided to sell it (but that could be so) because when I first got the
boat a cap and rotor would last all season. Changing it the next year
didn't seem like a big deal. Well now I've had the boat for 4 years
and I'm changing the cap and rotor every time we go out. Any ideas?
I'm about ready to offer a reward to the person who can solve this
problem! Thanks in advance.

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dave
 
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Default mercruiser backfiring, destroying distributor caps

The suggestions I've received have been incredible. You guys are
excellent troubleshooters. Thank you so much.

Update: The more I thought about it the more I became suspicious of
that coil. I took the boat out last night and parked it on the
street. I fired it up in the dark and looked for any sign of
crossfiring or voltage leaking. It took a while, but once the engine
was hot, the coil began to ark to the block randomly. In fact, the
whole coil had a faint blue glow with occassional sparks jumping to
ground, coming from what looked like right through the resin body.
This is the familiar "late model" HEI system with external coil of
somewhat square shape that Mercruiser bolts right to the intake
manifold just in front of the distributor. The engine didn't miss,
but as we all know these ignition problems are worse under load which
I can't duplicate in my driveway! My theory is that this voltage leak
probably gets worse as the coil heats up, causes insufficient
secondary voltage and either 1) a compensatory increase in secondary
current (remember Ohm's Law?) which will burn the cap terminals and/or
2) random sparking which is not appropriately timed with the rotor/cap
alignment causing both the burning effect and the tach error. I put a
new coil on it today and did the same night time test and saw no
arcing and no glowing. I feel it is very likely that this is the
problem...a coil. I've experienced failing coils at one time or
another, but never with symptoms like this. I'll post a follow up
when I have a chance to get it out on the lake and test it, but it may
be a few weeks. Thanks again to all who chimed in.
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