AFR spikes lean - mild misfire?
 

I've been wrestling with what I thought was noise in the AFR signal from my LC1, but I'm starting to think this might be a misfire problem instead. Wondering if anyone else has seen similar behavior.

Here's what it looks like at cruise:

https://www.miataturbo.net/attachmen...1&d=1336450348

And here's a particularly ugly idle:
https://www.miataturbo.net/attachmen...1&d=1336450488

I have rerouted the grounds several times, and at this point I have the heater ground soldered to a lug connected to the engine block ground point, and the sensor ground run directly to the ECU ground at the Megasquirt. The character of the "noise" hasn't changed noticably so I'm starting to think grounding wasn't the problem in the first place. Also my power signal isn't perfectly clean but doesn't seem to be unusually variable, so I don't think that's the problem either.

At this point I suspect the sensor is actually working, and what I'm seeing is misfires resulting in extra O2 showing up at the sensor. If they are misfires they are exceedingly mild as the car feels smooth and I don't even see anything on the RPM trace in the logs.

My question: do you think misfires might be the culprit?

I put in new plugs and wires only 3k mi ago ... but I'm going to swap them out again this weekend. Also just ordered a torque wrench so I can torque the plugs correctly ... any other suggestions?

Thanks all -

DJD

You did read this in the install manual right?



car looks to be running really smooth when the AFR is still erractic. I'd expect the see the pw, dc%, idle% and RPM on that log in those areas all go crazy as well if the AFRs were real.

Thanks for the reply, Brain.

I did read the manual and in fact I followed their "optimal" instructions in my original setup. I saw these spikes making me think there was noise in the signal.

So I tried rewiring the 12v source from the blue connector near the drivers' side headlight to another source on the fuse block and eventually to the lighter socket. That didn't help. Then I followed the advice of several threads on the topic and separated the heater ground and the sensor ground. Still no luck.

My main question is whether I should still be troubleshooting my LC1 or whether I should be looking for a misfire. I feel like I've done everything I can think of on the LC1 so I'm starting to look elsewhere. But if "true" lean spikes of this magnitude are accompanied by RPM, PW, etc spikes ... well then maybe the LC1 is the problem after all.

I'm going to do further investigation of both hypotheses this weekend.

DJD

is that real idle?
why is your PW so high at idle, it should be around 1.6-1.7 something is wrong possible indeed.
take a timing light to all spark plug cables and make sure the light is going off.
check for arching spark out of the cables.
post up your tune

possible you have one coil pack not working?
I once had it where one coilpack wasn't getting spark, so I was running on two cylinders smoothly but I had high pw at idle, the wideband was reading stoich~ ok but I smelt unburnt raw fuel and the engine was smooth enough not to notice the two cylinders not working.

where is your map sensor vacuum line is routed to? tee it off the fuel pressure line.

also, cleaning my injectors smoothed out my afr a bit.

What does a high PW imply?
 

Yes, the second graph is a real idle - about 900 RPM. I'm not sure I understand what PW represents - it's related to duty cycle and different for different injectors, right? I guess I'll go do some thread searches. Incidentally, I'm on stock injectors, the original ones on my 100kmi car, so it's entirely possible they're clogged or otherwise tired.

I won't have access to the car until this weekend, will try the spark tests then.

Coilpack seems unlikely; the performance is good, I can't imagine I'm getting this kind of performance on two cylinders. But I do smell unburnt fuel and the fuel consumption seems very high.

Tune attached.

Thanks guys!

Soviet, do you think the MAP signal in the logs is too noisy? I thought of that as a possibility but it looks pretty good to me.

sounds like you aren't sparking on two cylinders, or fouled plugs or arching wires or bad iginitor, etc
you can have raw fuel unburnt and still get ok readings on the wideband from the other two.
If one cylinder out it would be noticeable, but with two sharing the same coil pack won't be as notiecable.
PW is how long the ecu is keeping the injector open for.
map looks ok to me.

so idle pw should be around 1.7+-1 depending on injector\how much fuel you throwing at it during idle.

What injectors are you using?

Just stock 1.6 injectors.

Let me see if I understand - PW is determined by the Megasquirt, which is trying to hit an AFR value using the fueling in the VE table. This much I get. But if two cylinders aren't sparking, why would the PW increase?

Hmm..reading this thread makes me wonder....

I have a CALIFORNIA '93, which if I understand correctly was set up for sequential injection, unlike '93's sold in other states. Do I need to set some kind of sequential injection setting on the Megasquirt? Could that be related?

OK, to be more precise, I have a MSPNP. Refreshing my memory on this (and reading threads), it appears the MSPNP is already set up for batch injection on the extra wires. That is, it should be plug-and-play, just like they say: it just runs the four injector wires on the harness in batch mode instead of sequential. So I shouldn't have to wire anything different and I shouldn't have to set my MSQ for sequential.

Right? So that's not the answer. Damn.

depends how old the MSPNP is, the first release of it didnt have the wiring for the 93 california models...it was later fixed.

easy test, just pull a injector at idle and watch for difference in idle. If it changes then the injector\spark is probably working fine, if you get no change at all, suspect injector/spark

Thanks Techsalvager, will give that a shot this weekend. Will also check my MSPNP version to see if it might not be wired for my '93 CA per Brain's suggestion.

My pulsewidths are also about that high at idle (around 2.6ms) on a 1.6 with stock injectors. reqfuel ~13.4 with 1ms open time, stock plugs, igniter, and wires

what injector dead time are you using southermx5? and can upload the tune?

1 ms at 12v... same as the mspnp basemap