I recently got my Miata up and running with an alright tune and ran autotune the last two nights to clean up my Fuel VE Table a bit. My AFR's look alright under normal load and even under boost. My startup idle is really lean, though. It richens up as the car warms up but it is pretty lean (20 ish) on cold startups. I also run a steady 22.4 any time I am in gear and let off the gas entirely to decelerate. What can explain these really lean readings and am I putting my motor at risk with these? I have attached my .msq file, AFR Target Table, and Fuel VE Table. Any insight would be appreciated!


Also, on a mechanical issue related topic... Anytime I am under full boost at like 4.5k-5k RPMs, my tachometer will just fly right up and my car acts like it's in neutral. This is my clutch slipping, yeah? I was hoping my stocker would hold up for at least a month or two...
Is the Flyin' Miata Stage 1 clutch kit with 10 pound flywheel a solid option for a replacement?

 

Lean on deceleration is decel fuel cut. When you start the engine the o2 sensor has to warm up.

 

Have you used the Warm Up Enrichment Wizard? It autotunes your WUE.

Yes, your clutch should have been replaced prior to turbo. Everybody knows that.

The decel fuel cut referred to above is called overrun. It is turned on to save fuel and aid in engine braking. OEMs use it too, but people don't commonly notice.

You are not actually running lean while downshifting. You are running lean while decelerating in gear.

 

No fuel = lean.

your tires are spinning your motor so you don't need ignited fuel to do it.

 

I have not used the Warm Up Enrichment Wizard yet, just autotune... I will look into it this evening after work.
How do I tackle running lean while decelerating in gear, then? Or are you saying that it is normal to read 22.4 since I don't have any fuel going through the system? My vacuum pressure while decelerating in gear is 24 in. HG. That's normal, yeah?

 

It's not merely normal, it's something that the ECU does intentionally. You don't need any fuel at all while decelerating in gear, so the ECU just stops injecting altogether after a short delay, and then starts injecting again either when the RPM drops below a threshold or you get on the throttle again.

All of this is adjustable in the overrun cutoff screen, but most people never touch it and just leave the default values in.

The fact that your O2 sensor is indicating any value at all is kind of weird (I'd expect it to just go off-scale), but not indicative of any problem by itself.

 

Ah, alright. I understand now! My AFR's will bounce up to 10 and 12 when my car backfires while decelerating in gear. That makes sense because it's reading the un-burned fuel going through the exhaust, enriching the AFR.

The leanest that my gauge will read is 22.4 so maybe that's some sort of null value that it's preset to display when it's not reading any fuel. Is there any way to make it zero out while decelerating in gear? I have an Innovate MTX-L.

 

O2 sensors don't read fuel, they read O2, oxygen. Unburned air/fuel mix will results in a LEAN reading as there is more oxygen in the exhaust.

 

Er, right. Anyway, I want my gauge to zero out instead of reading 22.4 while decelerating in gear. Any advice?

 

Damn I'm dumb... Nevermind. It makes total sense for it to read 22.4. If it read anything else, it'd be entirely wrong. The engine is like an air pump while decelerating in gear because the injectors are shut off so of course it'll read as lean as possible. Got it finally

 

The AEM UEGO displays --- when it goes to max lean.
Sounds like the MTX shows 22.4 at max lean?

 

Yep, that's right! At least the way I have it wired and programmed, 22.4 is the preset value for max lean. Max rich is 7.4.

 

.