Not too long ago my Toyota MR2 was having a weird problem. The car would be able to start, but after running for a while it would stall, and have very bad air fuel mixture, black smoke and all. After consulting with several owners, one suggested that I check for leaked capacitors on the ECU as that might be one of the causes.
ECU located at the rear boot
The ECU on second generation Toyota MR2, in this case a 1991 3S-GTE version, is located at the rear boot wall that is seperating the boot from the engine, just behind the carpet. You will need a Philip screw driver and if I remember correctly, a 9 or 10 mm wrench to undo the mounting.
look at the 3 connectors at the bottom of the ECU
There are 3 connecting sockets located at the bottom part of the ECU. You should be able to disconnect them without much effort. The 3 connectors are of different sizes, you can’t go wrong when putting the thing back together. Do note that you should always disconnect the car battery and properly ground yourself before diving into such endeavour.
the 3 connectors
The ECU consists of two PCB boards, take off the 2 screws connecting both and you will be able to expose the innerds. In my case, I did not find any leaking capacitors. The problem was later found out to be caused by a faulty airflow sensor.
the capacitors on the ECU
More technical info on MR2 can be found on this page, including the Engine error code and how to diagnose them.