Let's start with a basic check. Turn the ignition on to stage one, do not start the car. Look at the standard boost gauge. If it is sitting in the middle it has passed the self check mode. This does not guarantee that the E.C.U. is perfect but it's a start. If the needle stays at the bottom or deflects to full, the E.C.U. definitely has a problem.
The main problem with the early units was that cheap capacitors were fitted at the factory and they tend to break down and leak, this then tracks across the board and incurrs damage to other components.
Yes you can buy a second hand unit off e-bay but unless it has been fitted with new capacitors and the board cleaned and re-lacquered it will most probably do the same thing. You will need a unit with an identical serial number. If your GTO is a Mk1 T.T. it will require a 9964.
We use the
ECU Doctor for our main repairs, i trust him and we constantly have around 20 GTO's at the workshop from all over the country.
I can't guarantee that this is your fault as i haven't seen or checked your car but it's where i would start, judging by your symptoms.
Check things yourself. Mitsubishi have about as much knowledge of these cars as i do of next weeks winning lottery numbers.
Remove the E.C.U. open it up and i'm fairly sure that you will see that the capacitors have leaked and damaged the board.
To do this, first disconnect the battery negative terminal, then remove both carpeted side panels on the centre tunnel. The unit is bolted upright at the front, just to the left of the drivers left foot. It will be a gold coloured unit with three yellow multi plugs going into it. Press the tabs in the centre of each multi plug and remove them from the unit, do not pull on the wires !
The lid is secured on the unit via four screws, remove them and you will see the capacitors, they will be black cylinders.
There is a chance that no visual damage is present and that the injector drivers have failed.