I've got a bit of a mystery on my hands, I'm looking for suggestions or guesses to get my engine back in tune: Symptom: Intermittent surging at low RPMs under partial to full load, with oscillating lack of power every 1-2 seconds. Normal power once I get to ~3+ krpm at full throttle, but returns at partial throttle. Worst when warm. Slight surge at idle. Getting worse every day, now it's difficult to accelerate uphill from a stop light. Vehicle: 2001 ZX2 auto, all stock, 102k miles. Plugs changed 1000 miles ago, which didn't seem to affect this problem for the better or worse. Timing belt is super old (possibly original?) but looks OK and the tensioner is still showing at the correct position. Diagnostics: There are no codes in the computer and no MIL even after 4 weeks of this nonsense. Code reader app (Torque w/ bluetooth adapter) reports sensible readings for mass air flow, O2, vacuum, temperature, and throttle position sensors, which combined with a lack of error codes, suggests that all these sensors are working. However, the ECU status seems to go back and forth between "closed loop using O2" and "open loop due to component failure" after warm-up, but with no other errors or clues visible to me. The engine idles fine (albeit with a slight surge), but if I prop the throttle open slightly in park (e.g., to 2 krpm), the RPMs will slowly drop over the next 10-20 seconds all the way to bogging and stalling the engine. The whole while with reasonable readings from the sensor son the computer. But, if I let my foot off the gas before stalling, it will stumble but recover to a stable idle. So I think IAC and TPS are working fine. Next steps, unless someone else has a better idea: I think I'll start replacing sensors like O2 and MAF? Thanks!