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Spark plug wires replace-gctid373393

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    Spark plug wires replace-gctid373393

    While having the manifolds/risers replaced on my 5.7 mercruisers, the mechanic has recommended that I replace the plugs and wires back to the distributor.

    I am going to do it since the access is soooo much better with the manifolds removed. He says that the wires are all carbon based now to reduce radio interference and only last about 300 hours. He says that you experience a slow performance reduction unless changed.

    I did not know this, I assumed that plug wires either work or you start getting a miss but what have others found. I do know that the new electronic ignition systems really put out a high voltage to the plugs. His advise in other areas seems good. I have about 350 hours on a 10 year old boat.

    Curious what others have been doing.


    I had an older Chris Craft with 454 Commanders, basically GM engines. When I rebuilt them, I used a DMM to measure the resistance of the spark plug leads. With the meter probes on each end, I pulled and rotated the wires, easily finding any that had broken down. Have a look at these articles:, The most important feature one of the wire manufacturer told me, was to ensure was to use at least 8 mm diameter wires.


      Both good links. Here's another one with lots of good technical information.

      Your mechanic is right in that OEM ignition wire sets and most after market wire sets do have a finite life time, some shorter than others. At 10 years of age, if I were you, I'd replace them. They are relatively inexpensive and just one more thing that you can do to ensure your engine is running as well as it can.

      I use Magnecor on our boat for several reasons. First and foremost because I cannot tolerate any Electro Magnetic Interference or Radio Frequency Interference. Secondly, things are rather "cozy" in my engine room and changing plugs and wires is difficult. The plug boots can get more than a bit reluctant to let go of the plugs so I wanted the highest quality wires and boots I could find to minimize any chance that I could damage the wires when disconnecting. Another reason is fit. Off the shelf wire sets are somewhat generic, even for a specific engine. Magnecor will custom build you a set with boot styles of your choice at no additional cost so the fit up will be as good as the measurements you supply to them.

      If you can't justify the cost of a Magnecor set, then any name brand carbon core set will work just fine for several years, probably more given the low hours you put on your engie.


        I've been having poor engine performance the past couple of years. Misfires, fuel smell in the exhaust, backfiring, etc. Tried rebuilding the carbs... no luck. Did a complete gas treatment thinking I had water in the fuel... no luck. Checked the plugs, they looked very good. Changed them anyway. Still no luck. Changed distributor caps and coils... no luck. I finally changed the spark plug wire.... Viola!!! I wish I had done this in reverse order.
        Greg A.
        1994 4788
        2002 Capri 175
        "Little GFI"
        Boat on the Sac/San Joaquin Delta
        Willow Berm Marina


          Classic! Spark plug leads are disposable. Thanks for sharing that.


            After three years of ownership and four seasons, we had our 32 tuned up last year. New plugs, wires, rotor, cap, points and the timing checked/adjusted.

            What a difference it made. She starts easy, runs smoothly and now will hit max RPMs of 4200. Before the tune up I was only getting 3800 RPM out of the engines. That's what let me know it was time for the tune up.

            I don't have flow meters but I'm sure we're getting better GPH.

            During this years spring shake down cruise, I got the boat up to 28 MPH--GPS.........That was fun.... just wish I could afford to run her that fast more often.