Troubleshooting a misfiring cylinder

bogle · 4 July 2022

Hey all, it appears cylinder 4 is not firing in my car. Looking for some help debugging. What am I missing?

I started the car today and everything was all good. It felt normal. I have 2 widebands: one in the front bank and one in the rear bank. Generally when it's warming up, both widebands show ~12.5, then eventually chill out to around 14.5 at idle when warm. I let the car warm up in the driveway and drove through the neighborhood, 1st gear, 2k rpm. I constantly check the widebands, so at the very least when pulling out of the driveway the agreed: around the 14's.

I pulled out of the neighborhood onto the main road, 1st gear up to ~4k (driving it like an accord, ~10% throttle), shift. On the shift, there was a backfire / pop, I look down and the front bank is super lean, like 2 points leaner than the rear bank. I cruised at 3k rpm for a block: rear bank was mid 14's, front bank was ~16-17.

Basic gist: front bank is running really lean. It sounds like garbage and feels like one cylinder is not firing. Based on it being super lean, I assume one cylinder is not getting any fuel. Could there be another cause? If there were no spark, the front bank would be super rich, right? Am I crazy?

Notes on the car

* AEM series 1 ECU
* CTSC
* RC 550 low impedance injectors (2.5 ohm, PL9-550)
* Stock injector resistor box
* O2 feedback is turned OFF
* Seems like injector connectors are still stock? (so no bad crimps?)
* There is what looks like aftermarket electrical tape around the front bank injector harness, so maybe something crazy in there

What i've tried

Before taking the car out today, I messed with the air temp sensor. I unplugged it, removed it, and tinkered with it. This is important as I had to move around the front bank injector wiring to get to the air temp sensor. My first thought was that maybe I disrupted some solder or butt connection in the harness. But it seems like the injector wiring is stock? Anyone have a pic of the stock injector plugs? They are normal Jetronic EV1 injector plugs, right?

I narrowed it down to cylinder 4 (front, right side cylinder). I pulled injector plugs on cyl 5 and 6 and they both changed the front wideband reading + made the car run worse. Pulling the 4th injector plug had no change in either. So it seems like the #4 injector is not firing.

* Tested resistance on all front bank injectors: 2.5 ohms on all
* Tested signal path from pos side of injector plug to resistor plug: all good
* Tested signal path from neg side of injector plug to ECU pin A2: all good
* Tested injector resistor box: all pins are 6 ohms

My next check is to put a scope on the injectors to see if there is actually a signal. I do have a scope, but it wouldn't turn on today (of course) and wont take a charge. I have new battery coming.

My thoughts right now as far as possibilities:

* Wiring issue (but it seems fine?)
* Bad injector
* Burnt injector driver in the ECU
* Injector resistor box. Do these go bad??
* Something real bad inside the engine

Not sure what else it could be. What else should I check? What am I missing? Really hoping it's not some internal issue...

bogle · 4 July 2022

Here is me confused limping back home. (Red is O2 #1 , Blue is O2 #2 , grey: RPM, green MAP, pink: throttle)

And when it started, looks like right after shifting into 3rd at 3k rpm, 7-8% throttle. You can see here right before the shift that the o2 sensors basically agreed in the 12's, then after the shift they start to diverge (red way higher than blue)

Valhalla · 4 July 2022

I have some questions for you after reading your posts and looking at your logs;
* O2 #1 should be the rear bank of cylinders as those are cylinders 1-3
* Correspondingly that make O2 #2 the front bank with cylinders 4-6
* You are running the car in closed loop and not open loop, why? Do you not trust your tune, wide-bands, O2 sensors?
* If you are running a stock (OEM) fuel rail system with stock fuel pump, the front bank generally runs richer than the rear bank because the fuel runs through the front bank first. Your Bank 2 (Front bank) should be running pig rich on the wide-band if you have a cylinder 4 down, not lean.
* You state you are running AEM Series 1. Can I assume your wide bands are AEM LSU 4.2's? Have you checked the controllers and the condition of the O2 sensors themselves? FYI the 4.2 controllers are no longer made and you will have to upgrade to the 4.9 controllers, sensors and adjust for these in your sensor tables.
* You state that the injectors seem to be in good shape and that the harness(es) are intact. Did you pull the coils and check the plugs? Front bank injectors are less prone to failure than the rear from water intrusion but I would check to see if you have spark and verify by looking at the spark plugs.

I recently had some issues and have since corrected all of them. First thing I would do is correct the placement of Banks 1 & 2 to correspond with your O2 numbered 1 and 2. Second thing would be pull the coils and plugs to verify. Let us know what you find.

bogle · 4 July 2022

Valhalla said:
I have some questions for you after reading your posts and looking at your logs;
* O2 [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=1]#1 [/URL] should be the rear bank of cylinders as those are cylinders 1-3
* Correspondingly that make O2 [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=2]#2 [/URL] the front bank with cylinders 4-6
* You are running the car in closed loop and not open loop, why? Do you not trust your tune, wide-bands, O2 sensors?
* If you are running a stock (OEM) fuel rail system with stock fuel pump, the front bank generally runs richer than the rear bank because the fuel runs through the front bank first. Your Bank 2 (Front bank) should be running pig rich on the wide-band if you have a cylinder 4 down, not lean.
* You state you are running AEM Series 1. Can I assume your wide bands are AEM LSU 4.2's? Have you checked the controllers and the condition of the O2 sensors themselves? FYI the 4.2 controllers are no longer made and you will have to upgrade to the 4.9 controllers, sensors and adjust for these in your sensor tables.
* You state that the injectors seem to be in good shape and that the harness(es) are intact. Did you pull the coils and check the plugs? Front bank injectors are less prone to failure than the rear from water intrusion but I would check to see if you have spark and verify by looking at the spark plugs.

I recently had some issues and have since corrected all of them. First thing I would do is correct the placement of Banks 1 & 2 to correspond with your O2 numbered 1 and 2. Second thing would be pull the coils and plugs to verify. Let us know what you find.

Hey thanks for the response. Did you have a misfire? How did you solve it?

* My setup has the O2 #1 as the front bank, O2 #2 as the rear bank. The car didnt come with a proper stock o2 setup, it appeared in the manual wiring diagrams that the O2#1 was front bank and it made logical sense to me to have the front be #1 .
* Yeah it always is in open loop. I've been meaning to turn on O2 feedback now that both widebands are in there and the cruise tune is close to ideal. But I just havent, it's been running really well, and I didnt want to rock the boat. It's also just another thing to tune...
* The widebands are both AEM 30-0310 inline controllers with LSU 4.9 sensors. The front one is ~6 months old, and the rear one is ~1 year old. You can see both O2 inputs follow the same pattern, just the front bank is significantly leaner now. TBH I trust them. These controllers are pretty sensitive and will fault for the slightest of reasons. Also If it were just the O2 sensor, the car would still run normally cause open loop.
* The rail system (comptech?), pump (walbro 255?), FPR (aeromotive) is all aftermarket
* The rear bank (O2 #2 in my setup) does tend to run a little learner than the front during normal cruising, but only by .1-.5 AFR. You can see this in the 2nd pic above where the red line is often below the blue in the chart
* I dont know for sure if the injectors and injector wiring is in good shape. The injectors do all pass resistance measurements, and the wiring has continuity, but an injector still could be bad or clogged.

I did pull the front bank plugs yesterday and they look fine. All 3 plugs look pretty similar, plug 5 had a bit more soot on the ceramic, but generally all the same.

I also ran it for a min yesterday and touched each front bank's header pipe. Cyl 4 was touchable while cyl 5 and 6 were not...

My gut right now is either :

* Bad injector
* Bad coil
* Maybe bad igniter

I'm leaning toward injector because it's running lean and this misfire thread had the same symptoms solved by a replaced injector.

But I have some used coils and a used igniter coming now just to exhaust cheap options.

Still moderately worried something really wrong internally, but oil looks fine and coolant looks fine. I have a compression tester and leak down tester coming as well...

Valhalla · 4 July 2022

To make this a bit easier i would do the following in this order;
* - Move spark plug 4 to another cylinder and see if the problem follows, if not
* - Move the #4 coil pack to another cylinder and see if the problem follow that, if not
* - move the #4 injector and see if the problem follow.

I doubt it's an igniter, they rarely fail and when they do it's catastrophic. The fact that that bank goes lean is puzzling. Good luck and let us know what works.

As to my issue, I posted it all in "Build" thread.

eff10mm · 4 July 2022

I had a similar issue with non NSX. The wideband shows a nonfiring cylinder as a lean condition. I chased my tail a bit to find it because I had two issues that caused the same effect. I had to replace a spark plug in one cylinder and a plug wire on another to get it running correctly.

bogle · 4 July 2022

Thanks! Yeah I’ll move some things around and put in the new parts as them come in this week.

eff10mm said:
I had a similar issue with non NSX. The wideband shows a nonfiring cylinder as a lean condition. I chased my tail a bit to find it because I had two issues that caused the same effect. I had to replace a spark plug in one cylinder and a plug wire on another to get it running correctly.

Great to know even an ignition issue would show as a lean condition. A little counterintuitive, but really helpful

After all this nonsense is over, I’ll probably end up with all new EV14 injectors and k series coils as preventative maintenance…

docjohn · 4 July 2022

every so often my 96 threw a lean code after about 18 years on old injectors...RC took care of that...:wink:

Old Guy · 4 July 2022

The first observation is that the front bank may not be running lean. I underline may because O2 sensors do not measure air / fuel ratio, it measures free O2 in the exhaust. A truly lean fuel mix results in excess O2 in the exhaust because there is insufficient fuel to consume the O2. The O2 sensor measures that free O2 and interprets it as a legitimately lean fuel mixture. However, a true misfire (failure to ignite the air fuel mix) dumps both unburnt fuel and unburnt O2 into the exhaust. The O2 sensor knows zip about unburnt fuel; but, measures that free O2 and interprets it as a lean fuel mix; however, the actual 'numerical' fuel mixture may be correct, it just didn't get burned. So you can have a legitimate lean fuel mix caused by an injector / fuel delivery issue or you can have a cylinder failure to fire (caused by lots of stuff) which is tricking the O2 sensor into displaying a lean fuel mix. Note that a lean fuel mixture can also cause misfires so there is always some grey zone of uncertainty as to the actual cause.

Your improvised header temperature measurement is a good indication that cylinder #4 might be dead and the likely candidates are the #4 injector and its related wiring or the ignition system on #4 . Your inspection of the plugs could be consistent with that diagnosis. If the #4 injector is stuck permanently closed with no fuel delivery the plug will remain white and show no damage because there is no fuel being delivered to create heat problems. If the ignition on #4 is dead the plug can also be the normal white color; but, the plug will likely come out wet with unburnt fuel.

If your #4 plug came out looking 'high and dry' then I am inclined to think fuel delivery problem. The first thing I would do is use your scope or an injector test light to confirm that the electrical signal is making it to the injector. If the wiring appears OK then its a choice of install a new injector or try sending the injector out for testing / cleaning (or all the injectors out for cleaning). If you want to confirm that the #4 injector is the problem before buying new / cleaning you could switch if with an injector from the back cylinder bank to see if your lean condition moves from front to back. That is a function of how much extra stuff you want to disturb. I tend to minimize fiddling with the fuel system because of the inevitable spilled fuel / create a new leak problems.

If your #4 plug comes out looking wet then it is likely an ignition problem. I would try replacing the plug first (they do fail) and then switching a coil from the back side to the front side to see if the problem moves from the front to the back. Despite being labeled front and rear and having different part numbers, the front and rear coils are the same. The difference is that the base gasket which has a little notch in it is rotated 180 deg to allow for correct drainage (down). If the problem moves front to back it is ignition related and the coil is almost certainly the problem. If the problem does not move it could still be ignition related; but, the problem may be in the wiring / ignitor / ECU. The best way to confirm that you do not have some kind of wiring control issue is to scope the ignitor terminal of the coil and look for the voltage transient that occurs when the coil is switched. Your scope will need an input that can tolerate around 400 V because the switching transients can be pretty high. If your scope won't tolerate that voltage then you could use a resistor divider to drop the measured voltage to something that your scope will tolerate.

If your plugs come out looking "I don't know whether this is wet or dry" I would start with the ignition system diagnosis just because I personally hate fiddling with injectors (I think I mentioned that already).

As a second observation, the fact that you were running open loop was a good thing. I don't know what the AEM ECU does; but, the OEM ECU uses the front O2 sensor to adjust all three front cylinders together. If the front O2 sensor 'reads' lean the ECU cranks the injector pulse width up on all three front injectors to try to bring the mixed exhaust stream to the correct AFR. If one cylinder causes a problem then the ECU can crank the other two cylinders in the opposite direction to try and bring the blended exhaust to the correct AFR. Closed loop can hide problems affecting one or two cylinders. I sometimes think people like closed loop because it means that they can be lazy about setting up their base fuel pulse width map and just use the O2 sensor to bring fuel mix to a desired target AFR. If you are going to rely on closed loop to do your tune for you, you also need to monitor your fuel trims / EGO correction because you can have a problem that closed loop is hiding. Closed loop control is what the OEMs use to insure that as stuff in the engine wears out the exhaust gas stays at 14.7 which is what the catalytic converters need to do their clean up and insure the car will pass its emission tests for the emission warranty period. If you have your base fuel maps set up so that you are meeting your target AFRs closed loop control is a less important feature.

bogle · 4 July 2022

docjohn said:
every so often my 96 threw a lean code after about 18 years on old injectors...RC took care of that...:wink:

Sweet, I ordered a new RC 550. We shall see

Old Guy said:
The first observation is that the front bank may not be running lean. I underline may because O2 sensors do not measure air / fuel ratio, it measures free O2 in the exhaust. A truly lean fuel mix results in excess O2 in the exhaust because there is insufficient fuel to consume the O2. The O2 sensor measures that free O2 and interprets it as a legitimately lean fuel mixture. However, a true misfire (failure to ignite the air fuel mix) dumps both unburnt fuel and unburnt O2 into the exhaust. The O2 sensor knows zip about unburnt fuel; but, measures that free O2 and interprets it as a lean fuel mix; however, the actual 'numerical' fuel mixture may be correct, it just didn't get burned. So you can have a legitimate lean fuel mix caused by an injector / fuel delivery issue or you can have a cylinder failure to fire (caused by lots of stuff) which is tricking the O2 sensor into displaying a lean fuel mix. Note that a lean fuel mixture can also cause misfires so there is always some grey zone of uncertainty as to the actual cause.

Your improvised header temperature measurement is a good indication that cylinder [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] might be dead and the likely candidates are the [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] injector and its related wiring or the ignition system on [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] . Your inspection of the plugs could be consistent with that diagnosis. If the [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] injector is stuck permanently closed with no fuel delivery the plug will remain white and show no damage because there is no fuel being delivered to create heat problems. If the ignition on [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] is dead the plug can also be the normal white color; but, the plug will likely come out wet with unburnt fuel.

If your [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] plug came out looking 'high and dry' then I am inclined to think fuel delivery problem. The first thing I would do is use your scope or an injector test light to confirm that the electrical signal is making it to the injector. If the wiring appears OK then its a choice of install a new injector or try sending the injector out for testing / cleaning (or all the injectors out for cleaning). If you want to confirm that the [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] injector is the problem before buying new / cleaning you could switch if with an injector from the back cylinder bank to see if your lean condition moves from front to back. That is a function of how much extra stuff you want to disturb. I tend to minimize fiddling with the fuel system because of the inevitable spilled fuel / create a new leak problems.

If your [URL=http://www.nsxprime.com/forum/usertag.php?do=list&action=hash&hash=4]#4 [/URL] plug comes out looking wet then it is likely an ignition problem. I would try replacing the plug first (they do fail) and then switching a coil from the back side to the front side to see if the problem moves from the front to the back. Despite being labeled front and rear and having different part numbers, the front and rear coils are the same. The difference is that the base gasket which has a little notch in it is rotated 180 deg to allow for correct drainage (down). If the problem moves front to back it is ignition related and the coil is almost certainly the problem. If the problem does not move it could still be ignition related; but, the problem may be in the wiring / ignitor / ECU. The best way to confirm that you do not have some kind of wiring control issue is to scope the ignitor terminal of the coil and look for the voltage transient that occurs when the coil is switched. Your scope will need an input that can tolerate around 400 V because the switching transients can be pretty high. If your scope won't tolerate that voltage then you could use a resistor divider to drop the measured voltage to something that your scope will tolerate.

If your plugs come out looking "I don't know whether this is wet or dry" I would start with the ignition system diagnosis just because I personally hate fiddling with injectors (I think I mentioned that already).

As a second observation, the fact that you were running open loop was a good thing. I don't know what the AEM ECU does; but, the OEM ECU uses the front O2 sensor to adjust all three front cylinders together. If the front O2 sensor 'reads' lean the ECU cranks the injector pulse width up on all three front injectors to try to bring the mixed exhaust stream to the correct AFR. If one cylinder causes a problem then the ECU can crank the other two cylinders in the opposite direction to try and bring the blended exhaust to the correct AFR. Closed loop can hide problems affecting one or two cylinders. I sometimes think people like closed loop because it means that they can be lazy about setting up their base fuel pulse width map and just use the O2 sensor to bring fuel mix to a desired target AFR. If you are going to rely on closed loop to do your tune for you, you also need to monitor your fuel trims / EGO correction because you can have a problem that closed loop is hiding. Closed loop control is what the OEMs use to insure that as stuff in the engine wears out the exhaust gas stays at 14.7 which is what the catalytic converters need to do their clean up and insure the car will pass its emission tests for the emission warranty period. If you have your base fuel maps set up so that you are meeting your target AFRs closed loop control is a less important feature.

Noted on the reason for a lean reading even though it may have fuel and no spark. Good to know!

The #4 plug was dry, and it looked slightly white, but not so different to the other front bank plugs. Though I was only idling before I pulled the plugs, and I let it cool down a bit before pulling them, not sure how wet it'd be at idle with fuel and no spark.

Good to know on the coils being the same with only a gasket difference. I bought 3 used ones on ebay, and they didnt specify which end; great I can use them anywhere.

On the o2 front, definitely a blessing in disguise to be in open loop. The AEM ECU would do the same as the factory ECU: adjust all 3 front bank cylinders with the front sensor. I guess it's configurable and I could install 6 widebands, but not going to do that! There is a strategy to help the AEM ECU not totally mask stuff like: you tune to get the maps really close to where you want, then set limits on correction. Also prevents it from going way lean when a wideband dies as they may read full rich.

I have several parts coming now

* New plugs
* A new injector
* 3 used coils cause who knows
* A used ignitor, though hard to test with the CTSC. I may make a wire extension if necessary instead of pulling the blower
* Dielectric grease
* Batteries for my dumb scope
* A temp gun so I can actually measure the header temps better than with my meat sticks
* Compression tester
* Leakdown tester

I agree on the fuel front! I dont want to disrupt the fuel system unless I have to. My plan next weekend:

* Try new coils + plugs
* Scope the injector(s)
* Fix any wiring if necessary
* Try the new injector
* Try the new ignitor
* Attempt to make sure all other cylinders are getting fuel / spark
* Still broken? Check compression and do a leak down test

Valhalla · 4 July 2022

I have a box of RC 550's that i had cleaned by RC laying around somewhere. Hit me up if you need 6 of them.

bogle · 4 July 2022

Oh man, awesome, I’ll PM you

bogle · 9 July 2022

IT was a coil!! Or a plug… but so far things seem back to normal with a new to me used coil and a new plug in cyl 4. The coil that came out had some yellow paint on it maybe indicating it came from a junkyard had been replaced before?

Used no 4 legend coil cause it was available plus scoping the no 5 cylinder just to see the pattern.

So happy it was easy. The real test will be going for a drive today.

I may still upgrade to k coils just out of paranoia and to remove the igniter from the system.

Back to rc the new injector goes!

Valhalla · 10 July 2022

Glad you got it figured out and it was easy. Enjoy your drive.

Old Guy · 10 July 2022

If you are considering the K coils, just because they can bolt in may not be the only consideration. The logic level output of the ECU may be able to drive the integrated ignitor in the K coil directly; but, chances are the dwell requirements of the NSX coil and the K coil are different and their required dwell versus voltage characteristic may be different. If you have an aftermarket ECU those parameters are usually programable. If you have chipped your ECU and are using Tuner Pro you should be able to adjust those parameters. Of course, you have to know what the characteristics of the K coils are. There was someone selling the Bosch integrated igniter coils from the Audi R10 for use on the NSX; but, in the original version they were just for use with aftermarket ECUs.

If you already know that the dwell requirements for the K coils are a good match for the original NSX coils then the conversion is easier.

bogle · 10 July 2022

Valhalla said:
Glad you got it figured out and it was easy. Enjoy your drive.

Thanks! The drive went well and I was able to experiment with the air temp stuff I originally set out to tinker with last weekend. It’s all back to normal, values look good [emoji28]

Old Guy said:
If you are considering the K coils, just because they can bolt in may not be the only consideration. The logic level output of the ECU may be able to drive the integrated ignitor in the K coil directly; but, chances are the dwell requirements of the NSX coil and the K coil are different and their required dwell versus voltage characteristic may be different. If you have an aftermarket ECU those parameters are usually programable. If you have chipped your ECU and are using Tuner Pro you should be able to adjust those parameters. Of course, you have to know what the characteristics of the K coils are. There was someone selling the Bosch integrated igniter coils from the Audi R10 for use on the NSX; but, in the original version they were just for use with aftermarket ECUs.

If you already know that the dwell requirements for the K coils are a good match for the original NSX coils then the conversion is easier.

I do have an aftermarket ECU (AEM series 1), so I will definitely need to update the dwell tables. There is a plug n play k coil kit here I ended up buying yesterday: https://www.foundry3studios.com/product/7nsx-k-coil-swap-kit He supplies the dwell values and all that. He has a kit for the R8 coils as well, but I went with the k coil kit to stay in the honda family.

The k coils each have an integrated igniter similar to the Bosch coils, so should be ok I think. We’ll see soon!

Thanks again for the explanation on why there would be a lean condition even with no ignition

docjohn · 10 July 2022

foundry3 is a solid guy..

Troubleshooting a misfiring cylinder

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