I've been pulling my hair out for several weeks now trying to trouble shoot a random lean condition. I've been emailing with Bruce (olderguy) and he's been very helpful but I feel bad continuing to bombard him with my problems.

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 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.


Current ideas:

1) I can't log the non-boost NB signal. All I can see is the holding voltage. I suppose the NB signal could be erratic. Or perhaps it's getting messed up passing through the O2 clamp. I could take the O2 clamp out of the loop and see if it runs any better non-boost.

2) I don't have any means to view or log my fuel pressure. I guess I could buy a sensor and feed it into the Zeitronix unit and log it there.

3) Maybe it's still a grounding issue. I could take a look at the engine ground straps, etc.


Any ideas? I'm really at a lose here.

(I'm at work at the moment, but I can try to post some of my logs tonight if anyone wants to take a look).

Thanks in advance.

 

Have you had the injectors cleaned?

 

I have not. Would that be likely to cause a sporadic issue though?

 

What's interesting is that this issue appears to have been going on for a long time. I bought the car from forum member manstretch back in Oct 2013. Here's a post in 2008 where he had concerns about his off-boost and idle AFR being very lean.

https://www.miataturbo.net/e-manage-...nix-emb-15706/

 

Just a follow up. I wired the O2 signal directly into the ecu this evening, 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.

 

315cc injectors? so low-ohm?

and im guessing no actual WBo2?

 

I have both. Narrow band (heated 4 wire) and a wide band (Zeitronix that I data log). My reference to being rich or lean is based on the wideband.

As for the high or low ohm injectors, I'm not sure, they were installed by the previous owner. IIRC the PO referred to them as Supra injectors. They have a light green top. So I'm guessing they are perhaps the 7MGE injectors (305cc).

Fuel Injectors - Miata Turbo FAQ

This lists the 7MGE as low-ohm, and mentions the need to wire in a resistor. I'll have to look closer this evening to confirm what I actually have.

 

Yup--I wrote that.

you can quickly tell by the tabs on the injectors.

the 305s and 315s look very similar in person, both lt green, not as yellow as that picture shows.

 

Gotcha, as I paid a little more attention to the part #s I see there are two different 7MGE injectors. 23250-70080 is the high ohm and 23250-70040 is the low ohm.

I'll confirm what I have tonight, but I did do a visual inspection on the injectors last night. They appeared to be a direct plug in, I didn't notice any type of extra resistor in the wiring.

 

Confirmed, they are the high ohm injectors.

Also confirmed 12v at the fuel pump.

Installed a fuel pressure gauge inline. Ran out of time, but did see 36 psi at start up idle. Didn't get to run it long enough to get it hot or to drive it and see what it's doing when I experience the lean condition.

I also pulled the vacuum line on the FPR and verified that there's no fuel intrusion.

 

More non-progress.

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. The previous owner used an excessive amount of wiring, so I was suspicious that I 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.


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. Also I should note, the lean condition never appears until the car is warmed up. Never happens during cold start open loop.

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.

 

Longshot, but your exhaust is sealed and nice? If you're leaking before the wideband you could get lean readings, that shouldn't be intermittent though.

Oh, and egr? Sealed and nice? Thet shouldn't open in boost, but it would be one hell of a vacuum leak if it opened to air.

 

Thanks, I appreciate the input. I think the AFR reading is accurate, based on the way the engine runs. It feels like it's running out of gas (literally is).

Also, to my knowledge, the 1.6L engines did not have an EGR valve.


Shamefully, I've decided to try throwing a few parts at it. For starters, I grabbed a new TPS. However, I realized that to change that I had to break loose the throttle body. There I discovered heavy buildup of carbon in the TB. So I decided to remove it completely and clean it and ISCV.

I suspect this was the first time in 22 yrs and 161k miles that the TB has ever been removed because the TB gasket was carbon fossilized. I've never worked so hard in my life to remove a gasket. (repeated soakings with chemical gasket removed barely touched it).

Hopefully I'll have that back together tomorrow night.

I also ordered a transmission neutral switch. Random I know, but I read several threads where this had caused problems for some people (most surrounding idle droop). Exactly what the ECU does with this input is a bit fuzzy, but it likely tells the computer to go to a leaner idle fuel map when not in gear. I'd imagine the TPS and rpm signals would also play a factor here. If it fixes it, then I'll try to figure out the how and why.

Again, a bit of a long shot, but for $30 it's worth a shot. Should be here tomorrow.

 

If you log it, what does it show, same duty cycle etc? Or spikes?

 

Nothing crazy on the duty cycle logs. Typically inline with what you'd expect. During the lean condition the duty cycle does not spike upwards, like it's trying to send more fuel. Basically it's lean and the computer is ok with it, I guess. Though with a NB sensor feeding the ECU, the ECU wouldn't know the difference between 15:1 vs 19:1 would it?

 

Finished installing the throttle body and new TPS. No change. If anything, it's worse but that might just be my frustration talking.

I'm attaching my tune file and data log from the Zeitronix wideband. Though the tune file is rather irrelevant since I'm not under boost.


Duty cycle at idle is 1%. Light acceleration and cruise under 3k rpms, no more than 15%. That seem legit?

 

Yep, seems about right. Cant view logs due to ipad. Ok, so no spikes in duty basically rules out that it woul be an ecu problem, and the emb doesnt do much in cruise. Back to mechanical/injector wiring.

 

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.

 

Just a follow up after reviewing my fuel pressure data logs from tonights test drive.

Essentially zero change in fuel pressure during the cross over to the lean condition. 13 AFR to 16.5-17 AFR.

Fuel pressure ranges from 36 psi at idle to around 43 psi approaching zero vacuum at 3300 rpms, mild acceleration.

It's a little tricky to correlate the injector duty cycle and AFR because my Zeitronix unit logs the WB O2 while the EMB logs the duty cycle. But the best I can tell, the injector duty cycle drops from 12% to 10% when it kicks over to lean. Assuming all other factors held steady, how large of a change in AFR would you expect to see in a 2% drop in duty cycle?

 

Replaced the factory ecu. No change.