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Hesitations when accelerating in 3rd/4th gear, only when hot

My 1992 Honda Civic DX (engine D15B7) is experiencing stalls/hesitations, but only under certain circumstances (fully warmed up/hot, heavy load/acceleration, low rpm, and 3rd/4th gear). It feels as if one of the cylinders stops firing occasionally, and I started noticing it about 2 months ago. When cold, it runs just fine. background: I replaced fuel filter about 10k ago. I drive very little, maybe about 15 miles per week in 4 short, 3.5 mile trips. I changed timing belt, rebuilt the distributor (burned up coil), and replaced spark plugs and wires only about 7k ago. Plugs are between .04 and .045 gap. Timing is right at the red tick (16 degrees TDC), as Haynes Repair Manual recommends. Mpg has been suffering, down to 30 mpg instead of standard 38+ in town. Having had other vehicles which required periodical O2 sensor replacement, and since I’ve never replaced in the 50k since I’ve had the car, I suspected a bad O2 sensor. The manual talks about how at cold, the sensor gives off between .1 and .2 volts and the ECM runs open loop, but at about 600F, the ECM starts to operate closed loop from the O2 input. I unplugged the O2 sensor, and put a multimeter on the sense line to watch it as it warmed up. As predicted, it started at .15v, then warmed up steadily (idle speed), right up to the .9 volt (“oxygen low, fuel rich”) signal and maintained. Even when I periodically re-plugged in the connector, because I didn’t know if running without any sense input would cause the ECM to run open loop and ruin the experiment, it stayed at .9 volts. Also, there were no codes in the ECM memory, until I unplugged the o2 sensor, after which it read off a code 41 (O2 heater sensor). So, my question(S): What’s causing the hesitations? Is it related to the o2 sensor? What would cause an engine to run rich all the time? Should the O2 sensor always read .9 volts, or did I do the experiment incorrectly?

I would try some sea foam and a good workout on the highway. Fouled injectors is my guess.

Sea Foam, had never heard of it. Sounds like good stuff. Why would fouled injectors only act up when the engine is hot? When cold, the engine works great.