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Solved: Misfire, hesitating steady speed driving, stumbling idle OBD 1303

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This seems almost embarrassingly simple to have a thread, but I could have used a similar thread a few weeks ago...

Symptoms:
1. Normal start and idle while engine cold
2. When warm, stumbling rough idle almost stalling every 10 seconds or so
3. Hesitation while driving at slow steady speed, but not at >50% throttle
4. CEL & TCS warning lights on. Service Manual :“flashes during misfiring and stays lit afterwards, storing the code(s).

Issue? Fuel issue like the fuel pump, bad gas, clogged injector, or an ignition problem like the igniter, coil, spark plug, ECU? OBD II gave code 1303, cylinder #3 misfire. I should have cleared it & repeated to rule out a random cylinder misfire just happening to be on #3 . BTW, cylinders are 1,2,3 from R to L in back; front cylinders are 4,5,6 from R to L.

I removed spark plug #3 , and compared it to plug #2 ; both perfectly gapped at 1.10 mm.
https://photos.app.goo.gl/3QUJkeThVpEeWjJ5A
If anything 3 (right) looked better than 2, but I haven’t looked at a lot of spark plugs. The coils looked pristine with supple seals and no sign of carbon arcing or moisture from a failed seal in the on the cover. Intact cover seals F & R. (I just installed ddf’s awesome coil guard, but it was absent for the last 27 years.) #1 , #2 , and #3 coil resistance on spec at 1.1 Ohms.

I switched the coils and plugs between cylinders #2 & #3 to see if the problem would switch to #2 with the coil and plug or stay with the fuel system on #3 . Test drive: CEL, TCS, stumbling, and rough idle. OBD 1302, cylinder #2 misfire. In hindsight, I should have moved spark plug #3 to cylinder #2 , and coil #3 to cylinder #1 , so I could also isolate the coil vs. the plug. When I moved original coil #3 to cylinder #1 , OBD still 1302, cylinder #2 . Diagnosis: spark plug.

To confirm, I switched the offending plug for a new NGK iridium plug. Result: normal idle, noticeably smoother acceleration, and no CEL or TCS light. (No double platinum plugs available locally and I was too impatient to wait. But that may be a good thing if the iridium will last any longer?) I replaced the other 5 as well.

It's now running great! Problem solved.

Because I can't stop myself, I reviewed the old service records; 6 new 3142’s January 2016 for a similar problem on cylinder #6 that eventually led to stalling while idling, and it’s gone ~5k miles since then. That seems early for double platinum plugs to go, but the service manual does say 6 years or 60k miles? Hmmm. Maybe I just had a bad plug. The only engine mods are the ’98 headers and ARC stainless exhaust. Thermostat was stuck open for a bit – would running cold accelerate spark plug demise? Perhaps the Iridium plugs will last longer?

Trouble code with Honda service check connector, MIL code 72, agreed with OBD 1302, but not as specific.

Why the TCS light came on with CEL each time? Also, why only misfire at idle and low throttle but work at higher throttle openings? I would have thought low RPM’s would mean longer time between firings, leaving more time to build up charge? But more fuel/air and higher pressure?

Related codes for searches: 1301, 1304, 1305, 1306, MIL 71, 73, 74, 75, 76
Sort-of related threads. I started a new thread because the title of these threads would not be helpful for search results:
http://www.nsxprime.com/forum/showthread.php/146531-Cel-73-Need-a-little-advice
http://www.nsxprime.com/forum/showthread.php/184307-Misfire-need-help!!! (1200 codes)
 
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Misfire at idle / low load; but, not under significant load is very odd for an ignition related misfire. As the density of the fuel mixture increases in the cylinder (higher engine load) it takes a greater voltage to fire across the plug gap. If there is an external weak spot in the high voltage insulation path (inside the coil 'nose' or a problem with the spark plug) the spark current will follow the weakest insulation path) and not spark across the spark plug gap - leading to an ignition misfire. If you don't have any signs of external arcing on the plug insulator or the coil nose I find your symptoms confusing.

I can't deny the empirical evidence that replacing the plug appears to have fixed the problem. Having said that, spark plugs do wear and as they wear the voltage required to fire across the gap increases. A fresh plug may have fixed the problem; but, may be disguising the true cause. Perhaps like @Honcho I am thinking that you might have a fuel injector problem. All injectors have a time delay in opening after voltage has been applied. Variability in the timing delay affects the fuel mixture at idle the most and has less effect under high engine loads. If you have issues with the fuel injectors which are skewing the fuel mix slightly lean at idle / low load this could be inducing a misfire caused by a weakness in the ignition system. Your symptoms
1. Normal start and idle while engine cold
2. When warm, stumbling rough idle almost stalling every 10 seconds or so
would be consistent with a fuel problem. While cold the ECU is altering the fuel mix to below 14.7 which makes the mixture burn more smoothly and easier to fire. Once you are up to temperature the ECU tries to run at 14.7 which is perhaps when your fuel / ignition problem materializes.

Since the engine appears to be running fine now, I might be inclined to let sleeping dogs take a short nap - for now. When it come time for the NSX's winter nap, I would be inclined to remove the injectors and send them off to RC or WitchHunter or whoever, to have them cleaned and flow tested. If they do the flow test correctly (high frequency PW as opposed to continuously open) that will highlight any issues with injector opening times.

If your misfire problem persisted up to high engine load I would say ' yep, ignition problem - plugs fixed that'. Since the misfire goes away at high load I am thinking underlying fuel problem at idle which creates a problem that shows up 'first' at one worn plug. Replacing the worn plug makes the symptom go away; but, does not address the root cause That is my current morning caffeine induced analysis.
 
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> I would be inclined to remove the injectors and send them off to RC or WitchHunter or whoever, to have them cleaned and flow tested. If they do the flow test correctly (high frequency PW as opposed to continuously open) that will highlight any issues with injector opening times.

I'll pile in on the bandwagon: the get injectors cleaned. It is likely you have the fuel dribbling out, the common symptom is less power and misfires.

If you like: number them with some sort of light scoring or heavy duty marker on the injector exterior if you wish to do some hardware to symptom tracking.
 
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I'll probably add the injectors to the list for this winter along with the EPS rebuild... I don't really follow the explanations, but I'm always looking for more work?!?
I'll see how the car goes this summer. Just got back from a 600 mile road trip to Michigan International speedway & the car was flawless, so I'm hesitant to mess with it.
With respect to the coils, I checked VERY carefully for any sign of arcing. The coils looked brand new and no sign of arcing on any plug insulator either: the misfire code followed the spark plug, not the coil.
 
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It's hard to say exactly what is going on without detailed photos and possibly some testing equipment. So heuristics.

A lean or poorly mixed fuel air can run hot and can overheat spark plugs.

When a small symptom occurs millions of times, the effect adds up and you get a misfire. The swapped plug may exhibit the same systems as it too starts operating in the same enviro as the failed one.

The NSX injectors are known to clog and/or dribble fuel out, it is pretty much inevitable.

So rule that out as cleaning needs to be done regardless.
 
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