Sorry in advance for my stupidity. I will try to keep this brief. 2000 turbo Miata, MS3. After engine rebuild (second time). Has been tuned professionally. Roughly 350 relatively trouble-free miles.

Suddenly, when cruising tonight, my AFRs started to go lean at idle. It got worse and worse, to the point where it would no longer stay running. Under load, it went right back to a good ratio and ran great. But as soon as I let off the throttle in neutral, it spikes to 18-20, stumbles, and eventually dies.

I'm finding it's doing this even under part throttle but with no load. I have checked everything that could be leaking. I believe it truly is lean and not a faulty o2 sensor because it consistently runs like garbage and feels rough when it gets to 17+ AFR. I've had leaks at the manifold/turbo connection, and other places, but nothing has ever caused it to go this lean.

DW300 fuel pump, FlowForce 725cc injectors (checked for leaks at rail). What's going on?

I will also add something. First of all, I am a noob with TunerStudio and tuning in general. I sort-of know what I'm looking at but I cannot properly tune myself. That said: I block off the vac line from intake manifold to the ECU itself, and the car goes max rich instead of max lean. Does this help rule out fuel pressure?

 

As someone using flow force 725 as well, i have found that they are very difficult, non-linear, and heat soak sensitive in the 1.6ms-1.9ms pulsewidth range. Which unfortunately is the range for idle and low load on pump gas. I had the most success by adding a flex fuel sensor and enabling fuel temperature correction to model injector heat soak. In addition, some success messing with small pulsewidth data.

If truly no vacuum leak and only 350 miles since your professional tune… it could it be the weather warming up and your underhood temps are higher now. Try consulting your tuner?

 

This is good info, thank you. I should clarify, and this is kinda dumb of me, I know -- The car was tuned prior to the engine spinning a bearing. It ran great; blown engine was user error. I rebuilt it with a used block and exact same specs/internals (84mm bore, forged rods/pistons of exact same compression etc). Stock rebuilt head.

I know I should get it re-tuned, despite it being theoretically the same. It's just very weird to me that it actually did run great for a while on this new rebuild.

As far as temps, it was marginally warmer yesterday when this issue arose. But I have significantly better cooling this time with Singular Motorsports hood vents, and an oil temp gauge. Nothing was abnormal and it never had this problem last summer with very high ambient temps.

 

You aren't going to get any meaningful help unless you post the tune file and a log of the issue...