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Misfiring and no power

2 February 2016
Irvine, CA
Yesterday I took my 93 out for a drive and I could feel intermittent misfires while driving around the neighborhood. Figured it was maybe bad gas or that the car just need to be exercised a bit. Drove down to the freeway, it felt mostly fine with intermittent misfires, getting up to freeway speeds was no problem but after reaching 70mph, the misfire became constant. I got off at the next exit, and from there forward, from idle on up, it ran like crap. I limped it home and let it cool overnight. Started it in the morning, and it felt like crap still. Took the coils and spark plugs out. They look fine, coils measure in spec, but the rears are a bit crusty. I ordered some aftermarket coils to test the failed coil theory. Will see if this fixes it next weekend.

Two questions, 1) is this what others who have experienced coil failure experienced? 2) any other things I should check?
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Fuel, spark, or ground are the most common issues for stuttering in any 90s Honda. Fuel as in the injectors are having an issue, spark is the obvious one, and grounds are just secure wiring. While you are waiting for new coils to come in you could check the grounds in the engine bay, which is a pretty easy check as it sounds like you have a multi-meter already.

23-15 has ground locations.
The resistance measurements for the ignition coils are not a particularly useful test. In order to be out of spec the coil pretty much has to be absolutely dead. Poor condition will not show up in the resistance tests. The most common failure is deteriorated insulation in the high voltage circuit and a normal ohm meter will not detect that problem. If the exposed laminations on the rear coils are a little rusty it may just be a bad electrical ground and you may be able to fix that by cleaning the rust off with some abrasive. If the rust is bad enough that rust has formed between the laminations which has caused the laminations to separate then the magnetic circuit is compromised and replacement is in order. Check the insulator nose of the coil looking for tracking which indicates insulation failure on the coil nose, also a cause for a trip to the garbage can. When replacing the coil covers make sure the front and backs are correct and that the sealing gaskets are in good condition to keep water out.

A lean fuel mix can also cause misfires. Unfortunately, the pre OBDII ECU is not as good at providing diagnostic information. Use your service check indicator to trigger the error code display to see if you have any stored error codes. If codes 42, 43, 44, or 45 display that could be an indication that you have a fuel supply issue of some sort. Its best to get a hold of the fuel system section of the service manual to help with diagnosis because there are a number of possible causes (fuel pump, injectors ...).

I agree with Superhatch that ignition is more likely your issue. In addition to the coils, I would also replace the plugs because tracking on the surface of the insulator can also cause misfires. Replace the coils and plugs and see if that resolves the problem. If not, check the grounds - particularly the ignitor module. If the problem persists then move on to checking the fuel system.
I have no CEL and no codes... checked the grounds from valve cover to bulkhead, it says 0.3 Ohms of resistance. I'll take a look at the igniter tonight.
I have no CEL and no codes... checked the grounds from valve cover to bulkhead, it says 0.3 Ohms of resistance. I'll take a look at the igniter tonight.

I was going to add checking the igniter unit too. They are known to fail on the early cars and can cause misfiring issues like you are seeing.
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I was going to add checking the igniter unit too. They are known to fail on the early cars and can cause misfiring issues like you are seeing.

Funny enough, I had read that before and bought a used igniter on ebay. I will search the service manual, but off the top of your head, is there any way to test the igniter?
Have you washed your car just before the incident?
Just feed on what others have suggested, but maybe moisture has gotten to the coils.
You might pull the coil packs and look for any rust, maybe rear first.
I have some coils that I bought as "just in case parts".
The top and bottom show some rust on a rear coil.
If I were to try to use these spares I would take a Dremel with a fine wire brush and try to doll them up.
Again, these are photos from a rear coil; maybe the best place to start
There is no way to test the ignitor module with something like an ohm meter. It is a module which consists of 6 power transistors. The ECU applies a low current current signal to the power transistor which then switches the high current associated with the ignition coil. The primary reason Honda used an external ignitor is that the ignition coil current is loaded with high frequency and high magnitude transient voltages and currents. By locating the power transistors outside of the ECU box they keep the transients out of ECU which can induce all kinds of nasties in the digital circuits. More modern cars now locate the switching power transistor right inside the ignition coil head eliminating the need for an external ignitor module.

If you are savvy with electronics, or actually have an active power transistor tester (pretty uncommon these days) you can set up to confirm that the 6 individual power transistors are still operational. However, the probability of failure of a power transistor is really low. It is more likely that a plug terminal is damaged or that you have a wiring harness problem. Grounding of the ignitor module can be a problem - particularly if you like to wash the engine a lot.

Unless you zeroed out the resistance of your meter test leads, 0.3 ohms is likely your test lead and contact resistance which indicates that the bulkhead ground is in good condition. If you just tested to the bolt on the bulkhead connection, you need to test the resistance between the bolt and another ground point on the chassis (not the engine). On the NSX the actual bolted ground connections can deteriorate if exposed to a lot of moisture. Assembly of the bolted connections with a conductive grease such as Ox-Gard can help eliminate grounding problems.
Thanks! Tonight I'll try the coils. If it's still running terribly, I'll replace the igniter when it arrives tomorrow, and if that doesn't do it, I'll pull the wiring harness, clean the connections, confirm continuity at each wire and reinstall... and if that doesnt do it, I'll move to the fuel system, though I think that's okay by the looks of the spark plugs (they're all the same color, no burnt ones and no gunked up ones, all 6 looked just fine). I also ordered a new main relay just in case. I just think it's super weird that I limped this thing home for over 5 miles and it didn't throw a single code. Are pre-OBDII cars just that forgiving?
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Update: Replaced all 6 coils tonight and that didn’t fix it. It’s still low on power, shakes hard at 3,000-4000 rpm and weirdly sounds like a Subaru boxer engine. Compression tests perfectly even across all 6 cylinders. I’ll have the eBay igniter tomorrow, and will swap that out to see if it’s that.
I also ordered a new main relay just in case. I just think it's super weird that I limped this thing home for over 5 miles and it didn't throw a single code. Are pre-OBDII cars just that forgiving?

If the engine starts and runs, albeit badly, its not the main relay. However, given the age of your car if the main relay is original replacement is not a bad idea.

It is not that the pre OBDII engines are forgiving, its just that the ECU lacks the firmware to detect a whole bunch of problems that the OBDII ECU can detect. There is absolutely no misfire detection system on the pre OBDII ECUs.

Because it is on the way, replace the ignitor (confirm that it has a good ground connection); but, I expect that is unlikely to be the problem. If you replace the ignitor unit and the problem persists, I suggest the next step is to test the fuel pressure and confirm it is in spec. This would be a particularly good place to start if you are still running the original fuel pump and don't know the number of miles on the fuel filter. If the problem has developed gradually then I would:
- check the air filter to make sure that it is not plugged
- check the manifold air temperature sensor for dirt build up
- consider sending the injectors out for cleaning and flow testing (good idea if the injectors are original and have never been cleaned).

If this problem happened all of a sudden, I would be inclined to think a failed O2 sensor. However, O2 sensors can also gradually drift with age.
Adding more color to the picture... this kind of just happened. The gas in the tank is only 2 months old. I fire it up at least once a week and let it idle until warmed up and will putt around to keep the catalytic converters happy. The car was idling fine on my driveway for 15 minutes. When the temp was up in range, I gave it a couple solid revs and it everything sounded normal. So I took it for a putt around the block, that's when the the car started acting up. It was a little jerky and missing under light load. I drove it back home, and the miss was intermittent but the car still had power. I figured it was maybe in need of a good stretch, so took it for a longer drive and it went like a bat out of hell getting onto the freeway, but then everything went to crap after that. The miss was now present across the entire RPM range. When the engine is cold, it starts and runs more smooth-ish, but is still low on power and smells super rich.

Plugs, coils and igniter weren't the issue... moving on to O2 sensors, fuel pump, FPR and injectors.
A little bit of solidarity here. I’m dealing with what seem like similar symptoms, and similar circumstances. Mine started happening suddenly as well last Saturday and I have a collection of parts to try out this weekend. Hope you figure it out quickly! I started a similar thread: http://www.nsxprime.com/forum/showthread.php/217959-Troubleshooting-a-misfiring-cylinder and this thread with a similar problem ended up being the injector: http://www.nsxprime.com/forum/showthread.php/193519-Misfire
'smelling rich' could be clue. The early ECUs do have error detection for the fuel system; but, the service manual is not explicit about what the conditions are that trigger those codes and how far out the condition has to be from spec to trigger a code.

Once the engine is up to operating temperature the AFR should be at 14.7 and stay there except when you press the throttle to the floor. The engine goes out of closed loop control some where around > 75% engine load. Innovate makes a little adapter which inserts into the tail pipe and allows you to measure AFR at the tail pipe with a wideband sensor. If you can borrow one of these adapters and a wideband controller from somebody you could measure your AFR which would give you a clue as to whether you have a fuel mixture problem.
Well… it wasn’t spark related or 02 sensor related. It was a stuck fuel injector dumping fuel down cylinder 5. Pulled all injectors and sent them off to RC to be cleaned up. Will go ahead and replace the fuel filter and do an oil change while I’m waiting on injectors. Let’s see if I can get it back to running right by the coming weekend.
Running rich on one cylinder usually does not cause misfires (failure to ignite the fuel) unless the spark plug becomes fuel fouled - which should have been apparent on the spark plug. It will cause the engine to run badly and in the case of the NSX (and most other engines) the O2 closed loop fuel control will reduce the fuel PW on the cylinders which will cause 4 and 6 to run lean making things worse. However, if it was really bad the ECU should have generated a fuel system error code. However, a drooling fuel injector is never a good thing so having them cleaned up by RC is a good plan.

A fuel pressure test can frequently spot a leaky injector. You pressurize the system and then watch how quickly the pressure decays. If it decays very quickly that can be an indication that one or more of the injectors is leaking; however, pressure decay can also be caused by a leaky check valve in the fuel pump or a problem with the fuel pressure regulator. If you have the gauge, do a fuel pressure test after you get your injectors back so that you have a base line for how fast the pressure decays. Also, remember the new crush washers for the fuel filter!