After checking that it was actually plugged in and the connectors were clean, I pulled the 15A EFI fuse from the fusebox in the engine bay for about 30-seconds to clear the codes stored in the ECU. The codes disappeared, so I assume that the car was started prior to the oxygen sensor being plugged in after the engine swap a couple of weeks ago and the codes were never cleared.
I shut the bonnet and took the car for a spin around the block. As soon as I got to about 3,000rpm in 1st gear the car bogged down like crazy and was coughing and spluttering, and changing to 2nd didn't make things any better. I didn't know what the hell was going on so I quickly turned around and went home. Had no codes so decided to reset the ECU again by pulling the EFI fuse.
Went for another drive and had the exact same problem
. However, this time I decided to hold my foot to the floor and it suddenly roared into life. It got over it's tantrum and starting accelerating like it should do. Accelerating in 2nd gear was still a little sketchy but that soon corrected itself with a heavy plant of my right foot. From there on out it was fine. I went home, parked it up for about 10-minutes, checked once more that the error codes hadn't come back (didn't touch the EFI fuse this time), and then went for one final drive. To my shock, the car was hesitating and stumbling all over again, but fixed itself by me once again giving it full throttle. I went for a good long 20-minute drive, thrashing it's ass off whenever it was safe to do so and didn't have any other problems apart from some slight hesitation in 3rd gear at low rpm when entering the motorway.
It was like the ECU was 'relearning' itself, so curious to know what the hell had just happened I jumped on Google and found a really interesting post on another forum. Figured I'd post it here as other's may find it interesting too. The original post can be viewed here. I take no credit for what is written below.
The 4AGE ECU is a pretty incredible unit and not something most people give due credit to. I've been trying to learn as much as I can about it regarding it's "intelligence" and I thought I'd share what my good friend Nitin has been telling me. Nitin has been working towards getting his mechanics certificate for a while now, and he's one of the few guys that seems to really take to the electronics side of the business as well as the usual pulling wrenches bit.
Here's what I've learned, put into easy to understand terms...
The 4AGE ECU is an adaptive or "learning" unit. The ECU will take the information from the many sensors in the car, match that to the way the car is being driven, and work out the best way for everything to work/fire/run to keep everything as efficient and smooth as possible.
Whenever any engine electrical work is done (moving sensors, making adjustments, etc.) it's always a good idea to reset the ECU. You can do this by either disconnecting the negative terminal from the battery post for a couple minutes (or more depending on how cold the car is), or removing and reinstalling a fuse in the kick panel near the driver's feet. Doing this gives the ECU a chance to forget what it's learned in the past so to speak, and start with a clean slate. If you do this it will seek out fresh readings from all those sensors and use those to make the engine run smoothly, instead of piling the new readings on top of the old bad ones and trying to get a good working operation from the combination.
So that's the engine/sensor side of teaching the ECU. You adjust all the sensors and stuff within spec, then the ECU learns what it needs to automatically. Pretty easy.
But the other part of teaching the ECU comes into play when the car is actually being driven. The ECU will need to know the answers to questions like: How do the sensors react when the car is accelerating? Decelerating? Coasting? What's the best way to manage spark in those situations? What about timing? etc...
When the ECU learns the answers to these questions, it's hoping that YOU the driver will know how to properly train it, which is something I've understood for quite sometime, but never knew exactly HOW I should drive the car so it learns what it needs to in a proper way.
This is what I've been instructed to do:
Find a nice, long, and hopefully deserted road. After getting the car up to normal operating temperature, and after resetting the ECU, drive the car up to 60km/h and then let the car slow down to 30km/h WITHOUT touching the brakes. I assume you should leave the car in gear. Once you hit 30km/h, take the car up to 70km/h and then once again let it slow to 30km/h. Repeat this procedure for 80, 90, and 100km/h. Doing this should give the proper and consistent readings it needs in order to decide how to run the engine in the most efficient way, and apparently this works for most EFI systems used by pre-96 vehicles.
Yes, I'm definitely sure that this isn't a case of me wanting to believe something than finding out the actual truth. I was just happy that my car was running properly again, but the issue had me wondering why it happened in the first place and why it miraculously fixed itself by driving it. I know that when we installed a Mine's chipped ECU in my brother's Caldina GT-T that it took about 2-weeks to fully adjust the tune by itself to suit our slightly lower 98 octane gas and his driving style. I've never seen any Toyota literature about the ECU being capable of training itself based on what the sensors are doing, but it's an open subject that everyone can have their own opinion on 
