The elusive misfire
 

So my car has had a misfire for the last like year. Pretty much since I bought it tbh. This misfire only happens when cruising (low load) around 2500-3500rpm. It doesn’t happen at idle or at WOT. I’ve changed coil, plugs, wires. Nothing. It still persists. Any ideas. It’s just like a random hiccup every so often while cruising

Have you logs, when it occurs?

Stock coils? I had a skip, usually while cruising in 5th gear, and always after it warmed up. Switched to COP setup and it's gone.

I have done a few cruise logs but didn’t look at them right away so don’t remember if/when it happened and didn’t see anything abnormal on the logs. I’ll take another one today and take a look at it and post it

I have a very similar (I think the same problem) with my track car for over a year now. I'm super interested in what you figure out / would be happy to help test something and work to help figure out the issue.

Not trying to thread jack here...but here is some info on my problem and what I've done to test / attempt to fix. Maybe that helps a little bit.

Problem: Car stumbles when at steady part throttle, at lower RPM (below 5K ish). It never stumbles at WOT / high RPM or at Idle. I'm fairly certain the problem shows up / gets worse with heat, but I would only bet the tractor not the farm on this. Normally on the track, I don't notice the problem except when I'm going through a sweeper a bit slower than normal due to traffic or on a cool down lap. I really noticed the problem, last August at Roebling Road, when I took my mother out for a couple slow parade laps & the car was almost un-drivable at slow speeds.

Info:
2000 Miata that's stock for our purposes
Changed / Tested

• No Codes when checked
• Plugs (pulled and they were clean & correct gap, 5k on them...)
• New Wires (no change)
• New Camshaft Angle Sensor (no change)
• Test Throttle Position Sensor (passed test)
• Coil (replaced with a used one that I was told was good...no change)
• Checked ground @ throttle body & back where the dipstick is & cleaned them (no change)
• "tested" O2 sensor (by pulling cable & seeing that a code registered)
• "tested" MAF sensor (by pulling cable & seeing that a code registered)

I don't think it's fuel related because it works fine at WOT at redline, although I read of a guy having the same issue & it was his fuel pump. I have read that it could be the CAS (which didn't help me after swapping) and the coil (mine was a "used" coil). rleete saying the coil fixed his problem makes me wonder about my "used" coil test. I swapped to the "used" coil for a track weekend (AMP) in Oct. I thought I had fixed the car, it ran strong in the morning, but I had the problem (lesser extent) in the afternoon on a cool down lap.

Again, not trying to thread jack & if I can help test / figure out the problem, please let me know. (I am a bit limited on testing, my car isn't road legal.) I think we have the same issue & would love to solve it without just throwing parts at it.

It's not the COPs per se, it's the fact that they are no longer sitting in the worst possible location. Back of the head is the absolute hottest part of the engine bay, and the coils get heat soaked. I also tried "new" coils (used, but tested), new wires, different plugs. Nothing helped. Every day, after the car warmed up, when cruising at highway speeds, it would just miss.

I had similar issues with my nb2: it would misfire from 3kish or any time more than partial throttle. The solution for me was new boots for my coil packs. The aftermarket coils that come with are the incorrect boots for a bp.

may not help, but I’ve been missfire free ever since

NGK (58914) CPB-Z003 Coil on Plug Boot

Heat soaked totally makes sense to me. The car ran super hot before coolant bypass & upgrade radiator (some of the very first changes I did when I bought the car). I’m not running a mega squirt. I would if I was keeping this track car, but building an LFX Miata as my new track toy. I’ll have to read up and see what I can run COP wise with stock computer. If I must run an aftermarket computer...maybe a brand new coil pack would last a season instead? Any idea how the spec Miata guys handle this? Just trying to fix, but not sink a bunch of time and money on something I’m going to sell in 3-6 months. I think COP's & aftermarket ecu is the ultimate answer if I was keeping the car

My COPs run on a stock ECU. I bought a premade harness from someone on this site, cops from someone else, and fabricated my own bracket.
https://cimg2.ibsrv.net/gimg/www.mia...f288175fd0.jpg
https://cimg6.ibsrv.net/gimg/www.mia...717c6fc0f2.jpg

I'm sold on trying it. Ordering up some parts. Thanks rleete for the help! I'll update this thread once I have them up and running so 95RedM & others have another data point.

So you are running COPs in wasted spark mode? I would almost figure the little coils would overheat or have issues firing every rotation, but I assume you don't have any control over the coil timing, I guess it would be ok on a stock ecu. If you ran them in wasted spark on a megasquirt and changed coil times you would be very likely to damage them, am I wrong? Ofc, there is absolutely no reason to do that on an ecu that can control each individually, but I am just curious.
The main benefit here is just moving the coils away from the hot spot on the back of the head?

Coils don't really heatsoak on their own. They can and do overheat, generally from being overdwelled - not from being overused in wasted spark, and the temperature delta of hot/cold engine bay is a drop in the bucket of internal operational temperatures. Stock na coils are dwelled at 5-ish ms, toyota at 3.5ish. LS coils will auto ignite on overdwell (about 3.5ms).

Y'all barking up the wrong tree IMHO. Fuel combusts easier at faster piston speed - compression heat make it more volatile. Basic differential diagnosis is misfires at higher rpm is fuel, lower rpm is spark.

OP needs more details/log/msq. Trubokitties basemap is 3.5ms which will underdwell stock coils.

I did a log where the car misfired a few times. I made it a short log so it was easy to sift through the data but I couldn’t find anything alarming or obvious. I’ll post it when I get some time I’m using my phone currently.

Both my 90 and 95 have had similar weird issues. After lots of expensive parts replacements, the issue on the 90 was corrosion on the "Engine" fuse. Put in a new one and now the main relay gets 12 volts instead of only 8 volts. The issue on the 95 was corroded/burnt fuel injector connectors.

These are old cars. Wiring issues are cropping up. They are frustratingly difficult to diagnose but cheap to cure. Before embarking on expensive parts replacement, check the basics like grounds, corroded/burnt terminals, corroded fuses, etc.

Just a heads up - while you're logging in tunerstudio, if you press spacebar, it'll "mark" the logfile. Makes hunting down specific interesting stuff much easier.

hey thanks man! Good info