Chasing a Misfire
 

The rundown:

1996 Miata. ID1000 injectors, MS3X upgraded from MS2 following Braineack's instructions at trubokitty, deleted cat (it was dead and I'll be fabbing my own downpipe and exhaust in a few weeks if I can get this problem worked out), normally-aspirated/stock everything else.

The car starts great but has a bit of a stumble at idle with AFRs that vary up to as much as 1.1 points and is causing the closed-loop idle to chase itself along with wanting a very rich mixture in the 13.5 range to quiet the misfire. It smooths out at higher RPMs and the car runs better but seems a little sluggish. I have found a misfire that seems to be present on all cylinders. I can see it with a timing light, and in the high-speed logger, it looks like an event is missing at a regular interval. The car has new NGK plugs gapped at 0.044 (checked with a wire gauge), NGK blue wires, and I've swapped a known working set of stock coils, along with tracing all connections, grounds, and my MS3 wiring. Compression was tested earlier this summer when I got the car back on the road and was within 5 PSI (with #3 being the lowest; I don't recall the exact numbers now but they were nearly at the top of the spec range). Idle kPA is steady around 27 and 30 under load (headlights and heater running; no stereo, AC, or power steering) and I've excluded vacuum leaks along with replacing every exhaust gasket from the manifold to the cat. I've gotten to the place where I'm checking things for the 10th or more time and am out of ideas. NOTE: Tune file and log with be attached in the next post; the uploader isn't loading on this computer.

Tune and log here. Also forgot to mention that I properly replaced the water pump and timing belt.

Was everything running fine till you replaced the brain-box or is this a whole new/recent build? Are the injectors new, cleaned and/or tested? My dyno guy showed me some bad plugs that hit the market in the last few months - the Made In Japan ones are fine but the Made In America with parts from Japan ones are problematic.

Everything was fine on the stock ECU, although I never did any kind of testing except to swap back and forth while I was straightening out my wiring problems and learning MS. The only code I ever got was from a bad rear O2 sensor. All the original O2 bungs have been capped and welded shut since, as I assumed my problem was learn2tune before I did any troubleshooting.

I swapped in another new set of plugs and wires with no change.

Ok. Agree that it points to the tune file. I can’t help you on that because I’m running a MAXXECU and cannot even open your files. Someone on here will jump in and help like they always do.

Appreciate the time either way.

Friendly neighborhood idiot checking in. I pulled the valve cover and the exhaust cam is one tooth off. Because I properly installed the timing belt. And I've checked it twice in the last two months and somehow never noticed it was off. I haven't corrected it yet, but I'm 99% sure this will solve the problem. I'll go crawl back into my hole of shame for a few more months.

good luck finding the issue.

Update: Exhaust cam was not, in fact, off. I pulled the front of the motor and double-checked anyway. Back to square one. Any ideas out there?

I've had the same problem too. Never had it with the stock ecu like you. I've been unlucky finding the cause but gotta be a spark related setting in Tunerstudio?

Quick update: I did another compression test and cylinder #3 was off. Around 2003 (I've had the car for almost 20 years) I had spark plugs and wires changed (this was long before I got into doing my own maintenance) and the shop cross-threaded the #3 spark plug and it blew out a few days later. It was fixed with an insert, and when I went to do the latest compression test this weekend, the insert came out with the plug. I had totally forgotten about it. While this doesn't completely explain the problem not showing up with the stock ECU, I got a reading of ~173 psi on that cylinder (compared to 190, 189, 188 on 1, 2, and 4) when I did the comp test. I'm not too thrilled about the idea of that sparkplug blowing out under boost when I get my turbo installed.

I have a spare motor I tore down yesterday and am going to swap the head this coming weekend. I'll report back with the results.

Updating this in the hope that when I find the problem it helps someone else.

Fuel pressure test: Perfect from idle to WOT
Replaced #3 spark plug insert and rechecked leakdown and compression: 2% leakdown/188 compression on #3. Same readings for the other three cylinders.
Pulled fuel rail and checked injectors (again): good spray pattern and all function properly
Spark looks good on all four plugs.
Rechecked mechanical timing. No change there.
I also worked on my VE table and the car runs excellent everywhere except for at idle. Pulls hard (for being stock) and runs far better than it ever has since I've owned it.

I also found this thread with nearly identical symptoms. I don't have an oscope and one isn't in the budget, but I'm going to see if I can track one down to use. Edit: Found an oscope identical to the one in the thread I linked. Will be here tomorrow.

Log screencaps at 13 and 14.7 AFR for reference.

Idle at 13 AFR
https://cimg6.ibsrv.net/gimg/www.mia...03ac40af91.jpg

Idle at 14.7 AFR
https://cimg7.ibsrv.net/gimg/www.mia...c3b2f1086c.jpg

I get the same symptoms at 13.0 and 14.7, just far more muted when running rich.

Scoped the signal at the coil connectors. 3.74v on each. This has to be a mechanical timing issue. Checking it again.:crx:

Update: Definitely not mechanical timing. Gapped plugs from .044 to .040. No change. New plugs. No change. Greased boots/cleaned connectors. No change.

I can't say that this is the full extent of your problem, but it sure does seem odd to me...looking over your idle settings, I get to your closed-loop settings and see that you have Idle Adaptive timing set, which is cool. But when I look at your Adaptive Advance timing curve i thought, "Whoa Nellie!"
https://cimg5.ibsrv.net/gimg/www.mia...58d80f57eb.png

Your "degree" values are set to static values (or "set values"), but your Idle Advance Settings specify that they are meant to be "additive". That means that even when your idle is dead-on to what you've set, the adaptive advance is adding 17.5 degrees to whatever's in your spark table (at least that's what my primitive MS2 2 would be doing, given these settings). If the MS3 does differently, then school me, oh wise ones. Otherwise, change the adaptive timing advance table to be the degrees you want to ADD to (or subtract from) the spark at idle.

Just for reference, this is what I use (not saying it's perfect, just what I'm using);
https://cimg9.ibsrv.net/gimg/www.mia...bbc19d1a2d.png

That might be an error between MS2 and MS3? I'm running MS3x with 1.5.2beta7 firmware, and my idle advance correction curve is nearly identical to yours (using Brain's basemap), although I've since turned idle advance correction off. Appreciate the reply though.