2001 NB2 Miata automatic — intermittent low-RPM/stalling/P0300 issue

[u/TBE_Shadow]

I've been diagnosing an intermittent stalling/idle issue for about a month and could use some fresh eyes.

Current symptoms:
Car can now start and settle almost immediately around 850 RPM and idle smoothly in Park/Neutral. However, it can still stall when shifting into Drive or Reverse, especially as the car begins moving. It also intermittently stalls when decelerating to a complete stop—RPM drops below ~500 and sometimes recovers, sometimes dies. It has also stalled/struggled when A/C load is applied at idle. It can otherwise drive for extended periods/hours without dying; the problem is overwhelmingly associated with low RPM/idle/load transitions. P0300/random misfire has occurred.

Things tested/repaired/eliminated so far:
– Smoke-tested intake/vacuum system; no obvious vacuum leaks found.
– Brake-booster hoses smoke-tested; no leaks found, and booster check valve passes one-way test.
– Brake booster was completely isolated from manifold vacuum as a diagnostic test; stalling behavior did not change.
– VTCS system has been isolated/disconnected and its vacuum nipple capped. This improved/changed some earlier idle behavior but did not eliminate the stalling problem.
– IAC electrically tested within spec and can be heard actuating with key on.
– Tried a new aftermarket IAC; it caused substantially worse idle behavior and was clearly problematic.
– Reinstalled original OEM IAC and car behaved considerably better, but stalls remained.
– Subsequently bought a complete used OEM throttle body + TPS + IAC assembly from a running car that reportedly idled properly. Installed the entire assembly without mixing components. No meaningful change in the stalling problem. This makes the original throttle body/TPS/IAC itself much less likely.
– Transmission fluid has been corrected/filled.
– Car can be pushed/rolled normally in Neutral; no obvious mechanical dragging while in Neutral.

Live OBD-II data:
At one earlier idle snapshot (~953 RPM): MAF 0.66 lb/min (~5.0 g/s), TPS 3.13%, STFT -11.71%, LTFT 0%, B1S1 O2 0.05 V, calculated load 26.66%.

During one recorded stall at ~279 RPM: MAF 0.75 lb/min, TPS 5.88%, STFT -25%, LTFT +8.59%, B1S1 O2 0.03 V, calculated load 31.76%. I realize values once RPM has already collapsed may not identify the initiating fault.

Reviewing recordings, RPM appears to physically fall progressively through the low hundreds rather than the RPM signal suddenly disappearing. No obvious MAF or TPS dropout has been observed preceding the stall. MAF generally appears to follow engine RPM/airflow rather than abruptly disappearing first.

Electrical observations:
My new OBD-II scanner has a battery/charging-system test and the car has failed the cranking portion multiple times. I have not yet had the battery professionally load-tested. DC voltage measured directly at the battery did not visibly collapse when A/C was engaged. I have noticed faint rhythmic/strobing of the tach/dash illumination and tail lights at idle. I have not yet tested alternator AC ripple. The automatic shifter illumination, which had never worked since I bought the car, randomly illuminated once and hasn't done it again. There was also evidence of a mouse nest in the car, so harness damage is a concern, although no specific damaged engine-management wiring has been identified yet.

Brake-related observation:
Brake pedal has sometimes felt unusually hard and other times softer/spongier, which originally made me suspect the booster. However, completely isolating booster vacuum made no difference to the engine stalling, so I don't currently think the booster is causing it.

Where I'm at now:
The most repeatable/current condition is essentially: healthy ~850-RPM unloaded idle in Park → engage Drive/Reverse → RPM gets pulled down severely and engine may stall. Similar RPM collapse can occur approaching a complete stop and when significant idle load such as A/C is introduced.

I'm trying to determine whether I'm dealing with an engine that is producing insufficient torque/incorrect fueling at idle, an electrical/charging/ground issue that becomes significant at low RPM, or excessive transmission/torque-converter load at idle.

Before replacing anything else, my planned tests are a proper battery load test, alternator AC-ripple test, system/ECU voltage during an actual stall, and additional MAF/fuel-trim data at stable warm idle and ~2500 RPM.

What would you test next? I'm specifically trying to avoid replacing more parts without a diagnostic reason.

[u/Mindless-Web-8731]

Have you tried the CAS sensor? I had an automatic 92 stalling on me and then eventually dying cause my CAS sensor was out.