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Merc 5.7 Mag missing/intermittent wont start

K

KVsteve

New member
1A602189
SN 1A435211

Replaced cap, rotor, plugs midway through last season and we ran well.

This season it ran fine for the first ride after lunch about 15 minutes. Next trip out it would start missing at cruising RPM starting at about 2000 to 3200. Above 3200 it seem to smooth out although it would occasionally backfire at WOT.

Put it away, couple days later went out and it would not start. Engine cranked, seemed to occasionally fire but then not start.

Left it again for a couple days went out and it started right out. Got in and drove it and it would not miss after about 15 minutes.

Couple days later started it up went out and after a few minutes with the engine up the temperature it started missing in mid range again progressively getting worse. This time it would not even smooth out if you put the throttle down. Eventually it was running rough below 2000 and eventually quit on the water and would not start again.

Next morning went out and it started but driving in a bit it would consistently miss when throttling up.
Hold the rotor everything looks clean no signs of arcing.
Pulled the fuel filter dump bowl and let it settle and see no trace of water. Fuel filter is a little discoloured black but not bad.
Removed the aluminum hose barb coming from the tank and the check valve was perfectly clean. My boat mechanic advised me to knock the check valve out of that fitting For better fuel flow so I did that.
Disconnected the barrel connecter on top of the engine hit it with a little WD-40 to make sure there is no moisture issues.

None of that worked still doing the same thing.

Wondering if it could be coil or ignition module?
Any other thoughts?
 
what brand of cap and rotor did you use? Those HVS caps are known to have finite lifetimes, especially the cheaper ones.

Surely it would be beneficial to scan the ECU to see what data it may provide...
 
KVsteve said:
1A602189
SN 1A435211

Replaced cap, rotor, plugs midway through last season and we ran well.

This season it ran fine for the first ride after lunch about 15 minutes. Next trip out it would start missing at cruising RPM starting at about 2000 to 3200. Above 3200 it seem to smooth out although it would occasionally backfire at WOT.

Put it away, couple days later went out and it would not start. Engine cranked, seemed to occasionally fire but then not start.

Left it again for a couple days went out and it started right out. Got in and drove it and it would not miss after about 15 minutes.

Couple days later started it up went out and after a few minutes with the engine up the temperature it started missing in mid range again progressively getting worse. This time it would not even smooth out if you put the throttle down. Eventually it was running rough below 2000 and eventually quit on the water and would not start again.

Next morning went out and it started but driving in a bit it would consistently miss when throttling up.
Hold the rotor everything looks clean no signs of arcing.
Pulled the fuel filter dump bowl and let it settle and see no trace of water. Fuel filter is a little discoloured black but not bad.
Removed the aluminum hose barb coming from the tank and the check valve was perfectly clean. My boat mechanic advised me to knock the check valve out of that fitting For better fuel flow so I did that.
Disconnected the barrel connecter on top of the engine hit it with a little WD-40 to make sure there is no moisture issues.

None of that worked still doing the same thing.

Wondering if it could be coil or ignition module?
Any other thoughts?
Click to expand...
What make & model of boat and year of production?
 
makomark said:
what brand of cap and rotor did you use? Those HVS caps are known to have finite lifetimes, especially the cheaper ones.

Surely it would be beneficial to scan the ECU to see what data it may provide...
Click to expand...
It was an OEM set from mercury not aftermarket. Probably has 3 hours on it.
One suggestion was possibly a fuel pump issue so may try and put a gauge on it tonight and see if I can get it to fail.
Will have to see about the scan.
 
KVsteve said:
It was an OEM set from mercury not aftermarket. Probably has 3 hours on it.
One suggestion was possibly a fuel pump issue so may try and put a gauge on it tonight and see if I can get it to fail.
Will have to see about the scan.
Click to expand...
You need to look at the system to find out what causes the fuel pump to activate, whether temprorarily or constantly. If the key is off, the pump is required to be unactive, to prevent pumping fuel and feeding a fire. Find out if the engine has crank sensors or if it uses the distributor/IC module to trigger spark and fuel.
 
No codes present.
Quit while testing and we disconnected the wired from the ignition module and reconnected.
Fired right up and has run for a good hour without issue so seems it was an electrical connection.
Thanks for the responses.
 
KVsteve said:
No codes present.
Quit while testing and we disconnected the wired from the ignition module and reconnected.
Fired right up and has run for a good hour without issue so seems it was an electrical connection.
Thanks for the responses.
Click to expand...
Did you do a visual inspection of wires, connectors, etc?

At training, we were told"

Verify the complaint
Do a visual Inspection
Do a wiggle test- wires, connectors, etc and check/measure/find data
Identify problems and repair them
Verify results of repairs
 
FWIW, there's a product called NYOGEL that is great for protecting the contacts on electrical connectors and it works much better than silicone dielectric grease...its not cheap.
 
no, nyogel 760, to be more specific - may have a letter after the -760...its similar in color to silicone, but thinner in consistency...
 
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