I have bean playing with a couple of things the engine temp sender for the ECU I have found that it ajust,s engine timeing so I tryed a few things. Find the resistance of sender cold and hot cold was 4170 ohm hot was 2600 ohm so I got a 3900 ohm resister and put it in the end of the senser plug once the car was running. Once hot as soon as i pluged it in it ajusted timeing it must have advanced so I went for a drive a big impruvment very good pick up compaired to before when hot. I will let yous know how i go i may be adle to make ajustable timeing for it ?
Ironically, I played with that sensors resistance as well, a couple of months ago. Below I'm attaching a chart with resistance specs.
I've tried various resistors, and with fooling the ECU to think engine coolant temp is lower than it actually is, I was getting a huge improvement in torque from stop, but above 2000rpms the car would ping like crazy.
Now I'm surprised you were able to use 3.9kohm resistor. Did you try it when the car was cold? Because mine wouldn't even start with anything more than 1kohm, but then my engine was already hot. Also, the higher the resistance, the more pinging I was getting, and it was hard pinging, detonation type, not slight knocking thats sometimes caused by lean mixure. I guess the injectors were dumping way too much fuel.
I guess the only fix is a standalone ECU, and proper tune. Maybe a piggyback would work, but I don't know. I've replaced a bunch of stuff trying to get to the bottom of it, but so far no luck. I had my ECU rebuilt, replaced O2s, MAF, full tune up, replaced TPS, coolant sensor, etc... Nothing worked.