mercruiser backfiring, destroying distributor caps
 

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.

mercruiser backfiring, destroying distributor caps
 

can you post or e-mail a picture of the cap issue. Lets see the inside.

"dave" wrote in message
om...
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.

mercruiser backfiring, destroying distributor caps
 

"Woodchuck" wrote in message
...
can you post or e-mail a picture of the cap issue. Lets see the inside.

"dave" wrote in message
om...
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.

As a non mechanic I'd suggest getting new sparkplug wires (radio suppression
type). There is a resistance
in them to RF so that they don't radiate. This RF resistance will reduce
the peak current and maybe extend rotor and cap life. Beyond that I'd guess
maybe a bad connection or bad electronic module. One would be hard to find
and the other expensive.
I'd schedule an appointment with a mechanic that has a automotive scope.

mercruiser backfiring, destroying distributor caps
 

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.

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

mercruiser backfiring, destroying distributor caps
 

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.

mercruiser backfiring, destroying distributor caps
 

"Woodchuck" wrote in message ...
can you post or e-mail a picture of the cap issue. Lets see the inside.


I emailed some photos to you. Thanks.

mercruiser backfiring, destroying distributor caps
 

As a non mechanic I'd suggest getting new sparkplug wires (radio suppression
type). There is a resistance
in them to RF so that they don't radiate. This RF resistance will reduce
the peak current and maybe extend rotor and cap life. Beyond that I'd guess
maybe a bad connection or bad electronic module. One would be hard to find
and the other expensive.
I'd schedule an appointment with a mechanic that has a automotive scope.


Thanks. All of that has been done! That's why it is so frustrating.

mercruiser backfiring, destroying distributor caps
 

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.

mercruiser backfiring, destroying distributor caps
 

dave wrote:

As a non mechanic I'd suggest getting new sparkplug wires (radio
suppression
type). There is a resistance
in them to RF so that they don't radiate. This RF resistance will reduce
the peak current and maybe extend rotor and cap life. Beyond that I'd
guess
maybe a bad connection or bad electronic module. One would be hard to
find and the other expensive.
I'd schedule an appointment with a mechanic that has a automotive scope.


Thanks. All of that has been done! That's why it is so frustrating.

Greetings:

It certainly sounds like you have really looked into the problem and
I seriously doubt if I have any solution, but I thought I'd mention an
experience a friend of mine went through. He had serious pinging issues,
and by the time I got down to look the problem was easily located. When I
went to put the rotor back on after pulling for inspection purposes, I
noticed it rotated in the forward direction almost all the way to the next
contact. Upon turning it over and shining a light there was no key inside.
The plastic key had worn completely off from contact with the metal
distributor shaft. In this case, new rotor meant problem solved. A new
rotor will fit tightly on the distributor shaft. Check to see if the rotor
is loose on the shaft after seeing the problem. If it is something
(vibration most likely) is causing an accelerated/premature wear.

Another thing we used to do when high energy ignition systems first
came out was to put a tiny little dab of silicone grease on the part of the
rotor that rotates to distribute the spark in order to quiet ignition noise
in police/fire radios.

As far as high energy is concerned I'm also wondering if an
excessively high voltage is presenting a destruction that resembles
corrosion. A couple of wild guesses here on my part, but it does sound like
you've covered all the bases already. One thing I do in a situation like
this is employ reverse logic. If I can't figure out what the problem is I
then try to eliminate what it isn't. Sometimes you get lucky and back
yourself into a corner where only one "problem" is left. Good luck to you!

-Jason

mercruiser backfiring, destroying distributor caps
 

That cap looks like it has more than 100+ hours versus only 5 or so as you
say. Here is what I would do.

1-use a ohm meter and check the resistance of each wire
2-is the coil primary side wiring reversed. when this hsppens the spark
jumps the wrong direction (polarity)
3-it is possible the coil is arcing internally causing the tach to go wacky
4-your coil resistancce being to low may be an indication of an internal
issue.