Spark plugs and cap replacement interval

I've owned my 2004 Merc Mag 350 since 2018. I've never replaced plugs, wires, or cap. It runs fine. I have no idea if the previous owner ever replaced anything either.
I was reading on several boating forums that a lot of people recommend replacing the distributor cap every year and the plugs every year or two. This seems incredible often to me.
Apparently the flat top "crab" cap I have is very problematic but I don't know that mines ever caused a problem.
The boat has about 700 hours on it. Should I be replacing these parts or just let it ride? What's the recommend interval?

Perhaps just buy a set and a cap, and leave your old ones as a spare. I think in a marine environment, there’s such a wide range of service variables, which could lead to a wide range of service intervals that you’re either throwing money away or not replacing it before it fails. Only you can determine how hard you are on these items.
73!

The manual for my 2002 5.0 MPI specifies plug changes every 300 hours. I don't have it in front of me but I don't think it specifies a time frame on plug wire changes. I've changed mine once in 1,680 hours. I change the cap/rotor about every 5 years but I trailer in fresh water and have enclosed storage so I probably don't experience the condensate problems so many report.

Change everything. Every 3 years. Dealers need the money for their 85' yachts with a pretty crew.

The crab caps seem to be more prone to issues and cross firing Causing a miss. I change mine every 5 years or so in fresh water as the terminals get crusty. Still runs fine but just try to stay ahead of it.
I keep a spare in a freezer bag on the Boat

I think it really depends on the conditions your run you boat: fresh vs salt, mooring conditions, amount of use, ect. I put 15-25 hours a year on mine (7.4 merc), in fresh water. Bilge is spotless, rarely has water on the bottom. Stored indoors. Recently did a full ignition tune up after some starting issues (plugs, wires, cap, rotor, sensor, coil), and based on my use, I think ill be fine for about 10-15 years at least. BTW everything i swapped out was factory equipped, circa '95.

While “it depends” as KD and Tango point out and there are structured service internals, etc., IMO the key to knowing when to replace is annual inspection. Think of it this way: you have not replaced the items and unsure when last replaced, so the only time you will know needs replacement is when it fails. And say you are following a schedule like every 5 years, again without an annual inspection, you may find does not last 5 years.

Also plugs can and do rust, so not pulling them out occasionally can lead to issues.

My suggestion: Annually do this:

• Pull the cap and rotor for inspection of them while also looking at the dizzy (under the cap) for any signs of contamination. Reinstall if all looks good. Carry a spare cap and rotor.
• Pull the plugs to read them (a good indicator if something amiss). Replace every 3 to 5 years.
• Run the motor and move your hand all around the ign wires, if getting mild shock, replace, Else replace every 3 to 5 years. Can also use a multimeter to check internal resistance.

More: most do not pay attention to all the rubber hoses and fuel lines. These get brittle over time and may look fine until its not. One method to check them is to squeeze the hose and if it feels crunchy, time to replace. My intended replacement internal is 10 years for rubber but like all good intentions, that usually gets pushed out. From my experience getting these hoses for our older engines (I have 2005 VP 5.0 GXi) is getting harder and certainly more expensive. I spent a couple of years poking around to collect all the needed rubber > it spread out the expense and since did not need now, allowed me to look for good deals.

Where the boat is is most important about parts life.
Salt in Florida. Or freshwater in New York. ?

Thanks for all the replies. I did indeed leave out the use conditions. Freshwater only. Mostly it stays on a trailer. I have snap on covers but it lives outside. I probably run about 50 hours a year.

Thanks for the update. And given your situation, and that corrosion happens even when sitting on a trailer, the annual inspection outlined in #7 is still my suggestion.

I have always done a. As a problem STARTS to occur. Do the inspection. Replace as needed. P M of parts and labor ? Just another way of giving money to a company.
Companies still want oil changes and filter every 3,000 miles. Is modern oil so terrible ? Are the air filters so terrible ?

ESGWheel said:

• Run the motor and move your hand all around the ign wires, if getting mild shock, replace, Else replace every 3 to 5 years. Can also use a multimeter to check internal resistance.

Click to expand...

Ouch! Don't do this! Where did this advice come from, ESG? No offense intended (seriously), but this sounds like some AI generated hooey. That's a great way to get hurt - possibly seriously. Not from the shock, but from the reaction to the shock, or possible cardiac issues if someone has heart problems to begin with - especially on a high-energy ignition system.
Plus, a multimeter might be able to tell you the end-to-end resistance, but it won't tell you the dielectric breakdown voltage of the insulation, which is the most important factor. And that end-to-end resistance will vary greatly depending upon the length, and without knowing how much resistance per foot that wire is supposed to have, plus the tolerance allowed to be outside of that resistance, the multimeter won't really help.
The simple way to check if your wires need to be replaced is to look at them while the engine is running in a dark place - after dark on a moonless night. If you see the coronal discharge anywhere along the wire, it's time to replace them. Modern high-quality silicone wires can last a decade or two easily, depending on how hot they get - and our engine compartments on our boats is a relatively cool place for them to live. They are usually broken by mistreatment when removing and installing them before they age out.

Not AI, old school. Customer would come in complaining about misfiring or hard to start on rainy days, we would run our hands around the wires looking for that tingle. With the high voltage igns that exist these days, you are right that I best remove that from my bag of ticks. Thanks!

LOL - no worries - I just remember getting bit more times than I'd care to admit, and the shock wasn't typically what hurt, it was the flailing and hitting everything sharp and pointy that could possibly be hit while jerking my whole body back in wincing surprise!

kd4pbs said:

LOL - no worries - I just remember getting bit more times than I'd care to admit, and the shock wasn't typically what hurt, it was the flailing and hitting everything sharp and pointy that could possibly be hit while jerking my whole body back in wincing surprise!

Click to expand...

Good point and I have been there too.
Somewhat related, I can tell a story where I finally learned not to wear any metal during my knuckle busting days > my metal band watch (which I was very proud of and a gift from my GF) shorted across the batt pos post and the fender well. Needless to say, it took a while to get over that one in more ways than one

That ran a chill up my spine and made me cringe!
When I used to turn wrenches, that was one of the things the shop manager at the first job told me. No jewelry.
Later on in life when I got a real job working around transmitters with 35,000+ volt power supplies capable of delivering it at several amps, or as time went on and things turned to solid state, having less-than-lethal voltage but enough current capability to weld railroad rails together in one flash, it was a no brainer. Even the wedding ring comes off, lest it become a welding ring!