Toyota Repair: Check Engine Light For 88 P/U 4x4, mass airflow meter, negative battery cable

Question
I'm at wits end! Been to Toyota dealers and other shops, with no resolution. I have a 88 Toyota Pick-up 4x4 with a 2.4L 4 cyl. EFI non turbo engine, with speculations for California emissions with less than 3000 on a rebuilt longblock. After replacing engine (within the last 8 months), have been experiencing a check engine light (code 25, too lean fuel/air mixture). This was not there before the engine swap. I have replaced the O2 sensor (Aftermarket & OEM) the mass airflow meter, & the throttle position sensor switch. Adjusted the valves, and replaced the ECM (computer). I have also replaced the thermostat, fuel filter, battery & exhaust manifold gasket(twice) and the donut on the exhaust during engine swap. The check engine light (code 25) appears about 20-30 miles after removing the negative battery cable, which to my understanding strips the stored codes from the computer ECM. The check engine light presents itself in the morning, during open road driving, but seems to be delayed, as it doesn't light immediately upon start or movement. It does present itself slower in early morning driving (At 6:30 A.M. it takes about 7 miles to show) than later on (8:00 A.M. when it shows within 2-3 miles). Yet I can drive around town without any check engine light showing up anytime day or night. I have tried to use a multimeter to test/check the resistance of the sensors, but am not sure of my readings. I also have had a fuel pressure test with a result of .36. Also to mention, it did once pull a code 14 during a shop scope, but hasn't shown back up. Any assistance you can provide or guidance where to go or look, would be greatly appreceiated.

Answer
You need to check the voltage signal of the o2 sensor with the engine fully warmed up at about 2000 rpm, open the diagnostic connector and connect a voltmeter between terminals ox and E1, there should be a varying voltage of about one volt, if the voltage stays the same the sensor is not producing the voltage that the ecu needs to see and it will interpret this as an open loop condition, this test is not real accurate but it will give you an idea if the sensor is doing anything at all, if you have difficulty with this the best thing is to hook up a scantool and watch the waveform of the sensor, it has a 5volt reference voltage and should go above and below this by about.5volts, if the sensor output is normal let me know, I'll try to do some more research.