I originally posted this in the e-manage section but since I don't think it's related to the EMB, so I thought I might pose this question to a broader audience.

And I apologize in advance for a very long initial post. I've been trouble shooting this issue for several weeks now, so this is a compilation of what I know and what I've tried.

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Here's what we're working with...

1992 Miata 1.6L. Greddy tubo, starion IC, 315cc injectors, stock fuel pressure regulator, Walbro 190 fuel pump, EMB with olderguy O2 clamp & Autotune. Zeitronix WB.

I'm data logging with the EMB and Zeitronix WB.

Problem:

Non-boost cruising, the car will go randomly lean. Steady rpm/load (or even at idle), AFR will go from ~14.5 to 17.5-19.5. Will typically stay that way and then may come back down randomly and stay. Sometimes when this happens, I can watch the AFR oscillate in a slow wave format, typically in the 15.5-17.5 realm.

The problem only happens once the car is warmed up, never in cold open loop. What's weird is that there's almost a "click" and all of a sudden the engine changes, runs smoother, and the AFR goes back to 14ish.

The problem isn't limited to just non-boost, it happens in boost too, though obviously that's a much bigger problem, so I've put in-boost tuning on hold until this is resolved.

Things I've tried:

Originally, it was idling very lean (17-19) with the injector compensation adjusting for 205 to 315. My current tune has the injector size at 250cc. Cold start AFR is very steady. Initially it's 12.5 and then slowly works up to 14.5 as the engine warms (open loop).

When the car isn't going lean, steady cruise AFR will be in the upper 13s typically.

I replaced the heated 4 wire NB O2 sensor which is in the downpipe, just below the turbo.

I also ran a new O2 signal wire and a new ground wire, thinking there could be a short somewhere. I've also tried a couple of different grounding locations, but no change.

I opened the AFM and cleaned the contacts. No real change.

I got another AFM and installed it. No change.

I installed a new fuel filter back in Nov 2013.

Confirmed that the injectors are high-ohm units.

I wired the O2 signal directly into the ecu, essentially bypassing the o2 clamp unit. No change. I also set the injector compensation in the EMB to zero and did a cruise. While significantly richer, it still had the random lean spike.

Confirmed 12v at the fuel pump.

Last week I discovered a short in my ground wire. The boost controller, autotune, NB O2, Zeitronix unit, boost sensor, and pressure switch (for the autotune), all share the same ground and they were all spliced into each other. The previous owner used an excessive amount of wiring, so I was suspicious that I had introduced the short by having to pull out all the excess wiring during my testing. I felt it was "A" problem, but not convinced it was "THE" problem.

Any ways, I basically rewired everything and changed the common ground location, just in case there was an issue. So the wiring is much cleaner, and no more electrical short, but the lean condition continues.

I also got a chance to drive it with the mechanical fuel pressure gauge hooked up. Obviously I can't see the gauge while driving but I was able to observe the fuel pressure at idle.

At least at idle, there's no change in fuel pressure, even when it's running lean. During warm up, open loop, the fuel pressure is 36 psi. After driving and getting it fully warmed up, the random lean condition kicks in. At idle, 17-19 AFR, the fuel pressure is still 36psi.

I've got a Zeitronix fuel pressure sensor that should arrive today, and I pre-ran the wiring this weekend. So it should be a fairly quick install and I'll be able to data log my fuel pressure. Though at this point, I don't think that's the issue.



I'm really baffled by this. I can't figure out if the ECU is commanding the lean condition because it's bad, or because a faulty sensor is telling it to go lean. Or if there's a vacuum leak somewhere.

The idle is pretty consistent though, aside from running a tad rough because of the extreme lean condition, so I don't think it's a vacuum leak. All the rubber hoses are in good shape.

The charcoal canister is disconnected from the intake, and plugged. The PCV lines are routed to a remote canister.

Does anyone have any insight on how various engine sensors (temp sensors, cam position sensors, etc) have an effect on the stock ECU's commanded AFR?

Thanks for any input.

 

My new thought of the day... what impact does the transmission neutral switch have? I read in another thread that it does communicate with the ECU, possibly telling it to go back to a leaner idle fuel map when not in gear. But obviously if I were driving, the air flow would be much higher, and thus an extreme lean? {off to do some research now}

 

I pulled the throttle body and cleaned it (probably the first time it's been off since 1992.). Replaced the tps. No change.

So I found that getting to the transmission neutral switch was going to be a pain. But after some research, I found that basically the ecu sends a voltage to both the neutral switch and the upper clutch switch. If either switch is engaged, it pulls the circuit to ground. So disconnecting both sensors would essentially be telling the ecu that the car is never not in gear.

So that's what I did this evening. Cranked up the car and let it warm up. Got to operating temp, and the AFR was still right at 14.7. Feeling good so I took it for a drive. All the way out of the neighborhood it was mid 13's afr. Finally get out on the road and by the time I get a mile down the road, it goes super lean again and stays there. Afr's are kinda all over the place but always very lean.

Come back home, car idling in the garage... Afr is mid 13's.

I'm wholly and truly baffled and I'm running out of things to guess at.

 

Well for posterity sake, I'll post the fix.

After I replaced the factory ECU with no improvement, I decided to take out the EMB completely. Still no improvement.

So I sat down with a pinout of the ECU harness and started checking the voltages pin by pin. The O2 sensor signal still wasn't where I thought it should be.

Two key points here. I previously mentioned 1) I suspected the PO had wired the 4 wire O2 sensor incorrectly. and 2) I ran a new signal wire and a new ground wire.

I did not however check the O2 heater wires. I made the assumption these were right. Classic mistake. I had also assumed that since a single wire O2 sensor is sufficiently heated by the exhaust, and that since my sensor is just below a 1000* turbo, that even if the sensor heater wasn't working correctly, the sensor would still be up to temp from the exhaust. Also a mistake. I'm not sure if the sensor wasn't getting up to temp, or if the sensor simply needs that circuit to be functioning to read accurately. But either way... it makes a big deal.

I found that the PO had wired both of the heater wires to 12V. That's incorrect. One is a 12V and the other is ground to complete the circuit. To be fair, the Bosch instruction don't actually state this, only that there's no polarity. I guess he was assuming the heater got it's ground from the ground wire. Also, both wires had pulled out of his electrical connector, so they weren't even getting the 12V he had intended. This wasn't visible though because he did an excellent job with the electrical tape, haha.

So I corrected the wiring and added my EMB back into the loop. A quick test drive revealed no more random lean condition. I've still got some tweaking to do but hopefully this is behind me and I can move on to what I had originally intended to work on 2 months ago... in-boost tuning.

I think I'm going to replace the O2 sensor again also, just in case it fouled any since it wasn't heating properly.

 

Hey dude, just reviving this thread. Hope you see this post! I'm having the EXACT same issues with my mx5. Mines pretty much stock with a PLX wideband O2 sensor simulating narrowing to a stock ECU. I've done this as I'm going ITB's and pretty lights are great and will be going megasquirt soon.

So I've got the same symptoms as you really. Really weird. Just decided to one day start playing up for no reason whatsoever. I've tried everything and still can't figure it out but hopefully your fix may actually work. If so, that will be amazing.

AFAIK, the wideband is wired up to his car the same as mine, and he's has no issues. All wires are connected to power and ground as they should. Either way, I'll trace them and see.

Grr.