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.

 

Your sure it wasnt the oppossite?

Top sequential trailing plugs fire half as often as the lower leading waste spark plugs.

 

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.

 

Was it more fouled than the others? Sounds like weak spark.

 

I'd take a hard look at your leading coil---The FSM has a test for it as well.

 

If the leading coil is faulty, wouldn't it send an equally weak spark to both leading plugs? Should I check for a bad wire on the front plug?

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,

 

If still a stock FD ignition system, then yes, both leading plugs are fired by the same coil in a wasted spark setup so if the coil were failing you would have problems firing both leading plugs. So yeah, it would be wise to check for a bad spark plug wire or just replace them.

 

There are two terminals on your leading coil...... I've seen a situation more than once over the years where one of the two terminals is bad. You'll typically see a white powdered finish inside the terminal. Also as mentioned makes sense to inspect your plug wires. What brand and how old/how many miles on them?

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/

 

Thanks for the link. The car has 61,000 miles. Coils and wires are original.

 

Are the undersides if the earth lobes more worn on that plug? Electron flow direction can effect which side errodes too.

 

Yes, the outer electrodes (earth lobes?) of the rear leading plug, like the center electrode, are more worn than those of the front leading plug. But aren't the outer electrodes/body of all of the plugs negative polarity?

 

......... you're running almost 30 year old spark plug wires?

 

Hah! That's nothing -- the wires in my house are 3 times that old.
Seriously though, they can't be too bad -- I can still hear AM radio stations.

 

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.

 

I understand -- I was joking about the AM radio. I've had faulty plug wires other cars and know the signs. My 1986 VW GTI 16 Valve was notorious for it, and my 1998 Corvette needed new wires regularly.

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.

 

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.

 

Your comment prompted me to measure the resistance of the old plugs to see if that was a factor. Results were inconclusive. (I'd measure the wires too, but I can't take off the throttle body at the moment to get at them.)

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.