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Recently I replaced spark plugs which fixed an idle issue, and ones which came out which only had ~30k look pretty rough (each in slightly different way) so I'd appreciate it if the experts here can provide a reading on what they might say:
Front Rotor plugs on left, Rears on right, arranged LTLT Same F LT, then R LT Minor leak on Front Leading, from the ring?
The rear trailing is the only one without any white stuff on it, and the leading ones seem worse for deposits. The leading front has the most gunk, including what seems to be fresh oil? The "oil" has come out a tiny bit onto the housing for what looks for a while now. The car is currently running well.
Looking into the plug holes I saw some deposits on presumably the rotor, so I'm thinking of using some Techron in the next couple tanks. Any advice in general would be awesome, thanks.
The deposits look normal-ish except for the one that's all black. I don't think that coil is firing.
They were due. I know this is car is new to you, but the oily soot is indication the car spent time idling or being lightly driven. You need to wind it out regularly to keep things clean. Harder driven cars have the more familiar brown ashy deposits instead of black soot.
you don't read plugs on a rotary like on a reciprocating piston engine, pretty much run as cold as possible without fouling
which is 11.0 or 11.5 heat range on a race rotary that sees a lot of continuous WOT, 9 or 10 heat range on a street car in warmer climates
vs 7 Lead, 9 Trail from the factory on an RX8, depends what you can tolerate on maintenance cost/work and possibly climate too. Cold winter climate should stick with OE, or maybe 6L/8T in some severe winter climates.
maybe test that one T coil that seems more fouled than the rest, or do a compression check.
The deposits look normal-ish except for the one that's all black. I don't think that coil is firing.
Wouldn't that result in misfire symptoms (like obvious ones at idle), or was it the one not firing around idle which is what led to the rough idle I was diagnosing?
Wouldn't that result in misfire symptoms (like obvious ones at idle), or was it the one not firing around idle which is what led to the rough idle I was diagnosing?
If it was a leading it would be more noticeable, but trailings are not primary ignition. They're there for complete combustion/emissions management and idle stability. So... rough idle.. fouled plug... sounds like probable cause.
But the idle problem appears solved (smooth idle now), so does that mean it was the plug and not the coil? So to clarify, the rough idle (and slight misfire at full throttle at low rpms) developed gradually over a couple months, then disappeared entirely after changing plugs. I just want to be sure I don't need to do more work for now, except maybe check compression or such.
As far as the ignition goes..... the combustion doesn't care if its a bad plug or a bad coil. It either fires properly or it doesn't.
likely this time it was the plug....
The coils are a well known issue so i would test them with an HEI tester to see if they put out like they should. It takes less output to fire a new clean plug than an old crappy one. You might be good.... or might find it failing again when the plug is more elderly.
If you still have stock coils and they are old you might want to think about retiring them....
Does the rx8 "computer" sense or somehow detect plug/coil issues? It's kind of curious the plug was problematic but it's not throwing codes or anything else in the OBD.
The computer doesn't know anything about that. Often the first actual symptom of coil problems is loss of engine compression. It's not the kind of thing you leave until you have symptoms.
As far as the ignition goes..... the combustion doesn't care if its a bad plug or a bad coil. It either fires properly or it doesn't.
likely this time it was the plug....
The coils are a well known issue so i would test them with an HEI tester to see if they put out like they should. It takes less output to fire a new clean plug than an old crappy one. You might be good.... or might find it failing again when the plug is more elderly.
If you still have stock coils and they are old you might want to think about retiring them....
Been on break with the car, driving my winter beater ATM. I'm think of trying HEI tester first since it's easier to reach than replacing coils (esp since I've already gone in that way). Which tester would be best?
I bought the nonadjustable HEI tester, and found the black trailing coil to be largely not working. The clip on the tester is a bit janky, and would lose the spring inside easily and then broke off complete even with relatively light handling.
I then got 4 "c" revision coils and now it's working well, I was getting a tiny bit of what sounded like "misfire" at idle and full throttle at around 2-4k which is now gone. To do the replacement I only took off the side piece on the intake, and didn't move the airbox or anything else. Was quite tight.
Of the 4 takeoffs of "b" revision, 3 had a small bit of the white haze at the bottom, and the 4th (which I assume is the broken one, though I mixed them up a bit when removing due to some difficulties with tools during the process) has around a 1/2 inch white spot. Didn't replace the plug wires since it would require taking the wheel off again, and I thought it should be fine with 35k miles, unless the OEM wires are particularly bad?
12-12-2020, 08:48 PM
