Spark plug mistery?
#1
Spark plug mistery?
I just replaced the stock plugs on my mostly-stock '94. The old plugs (platinum) had about 10,600 miles on them. The center electrodes on both of the old trailing plugs had rounded down a little at the end of the tip, which seemed normal enough to me for that mileage.
However, the leading plugs were different -- the front rotor leading plug's center electrode was still full-length and looked almost new, while the rear rotor leading plug's electrode was eroded down to a point.
Anybody have an explanation for the different wear on the front vs. rear leading plugs? Is it a normal pattern? I don't remember noticing this difference the last time I replaced plugs on this car.
However, the leading plugs were different -- the front rotor leading plug's center electrode was still full-length and looked almost new, while the rear rotor leading plug's electrode was eroded down to a point.
Anybody have an explanation for the different wear on the front vs. rear leading plugs? Is it a normal pattern? I don't remember noticing this difference the last time I replaced plugs on this car.
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Retserof (07-31-22)
#3
As I said, what I'm wondering about was the difference in wear between the 2 lower Leading plugs. The rear rotor's lower Leading plug was the most worn of the 4, while front rotor's lower Leading plug was almost like new.
Is there something about the engine's design that would account for this difference, or is it just my engine?
I'm thinking that it's just my engine, and that the front lower Leading plug has been getting a weaker spark than it should, or the rear lower Leading plug had an unusually soft center electrode.
Is there something about the engine's design that would account for this difference, or is it just my engine?
I'm thinking that it's just my engine, and that the front lower Leading plug has been getting a weaker spark than it should, or the rear lower Leading plug had an unusually soft center electrode.
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Retserof (07-31-22)
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Retserof (07-31-22)
#6
None of the 4 plugs were fuel fouled.The ash deposits on all 4 plugs were fairly thick but mostly light brown/tan in color. The front leading plug's deposits were actually lighter in color than on the rear one,
#7
Rotorhead for life
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Retserof (07-31-22)
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#8
Original Gangster/Rotary!
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Here's a helpful old thread that quantifies how to test all of your coils for the proper resistance:
https://www.rx7club.com/3rd-generati...estion-890570/
Last edited by GoodfellaFD3S; 07-31-22 at 08:41 AM.
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Retserof (07-31-22)
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Retserof (08-01-22)
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Retserof (08-01-22)
#14
Racecar - Formula 2000
I had a bad plug wire when my car was only 2 years old, so it's certainly a possibility. And hearing AM radio OK is not a sign of being good - too high resistance will not cause radio interference, but it will cause misfires.
Last edited by DaveW; 08-01-22 at 02:02 PM.
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Retserof (08-01-22)
#15
My RX7 runs very well and has no misfires at idle or under heavy load/hard acceleration. (Not counting the occasional 3K hesitation.) I started this topic not because it was misfiring or idling roughly, but because I noticed that the L2 plug was a lot more worn than the L1 when I changed them after 10,000 miles of service. Since then, I've searched for pictures of used FD plugs on the internet and think I saw a similar pattern in some of them. While it is possible that deteriorating plug wires are responsible for the difference in electrode wear, I have to wonder if it is a quirk of the model, perhaps due to the difference in the length of the wires. Either way, I plan to replace these plug wires.
#16
Racing Rotary Since 1983
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just hypothesizing with little knowledge but running a wasted spark setup if there was somewhat of a problem w the energy going to L1, might the energy be transferred to L2? too much E going to L2 would eventually round the ground. since your motor was running O K on both rotors maybe it wasn't a complete blockage.
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Retserof (08-01-22)
#17
just hypothesizing with little knowledge but running a wasted spark setup if there was somewhat of a problem w the energy going to L1, might the energy be transferred to L2? too much E going to L2 would eventually round the ground. since your motor was running O K on both rotors maybe it wasn't a complete blockage.
Literally, the path of least resistance gets the E, right? Well, not so far as my L plugs are concerned. L2, which had noticeably more electrode wear, actually had about 2.6% more resistance than L1. I think this is a trivial difference in this application, but I haven't been able to find NGK's spec on this. The 29-year old plug wires probably offset this by far in one direction or the other.
L1: 3.81 K Ohms
L2: 3.91 K Ohms
Meanwhile, my old T plugs had pretty even wear despite a much bigger difference in measured resistance -- the T1 plug had 64% more resistance. Maybe it got dinged during removal and handling. Certainly new plugs should be closer in resistance.
T1: 6.51 K Ohms
T2: 3.96 K Ohms
I wish I'd measured the replacement plugs before putting them in yesterday. I just happened to have another set of new plugs (non-platinums) in my garage. The leading plugs had the bigger discrepancy this time, but they were within 12%. The trailing plugs were within 1.5% of each other. Ls: 4.74 vs. 5.31 K Ohms; Ts: 4.87 vs. 4.80 K Ohms.
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