Failed secondary system? Where to start?

[Athens91ZR1]

Hello all,

I am new to this forum and am helping a family friend (original owner) get their 91 ZR-1 ready to sell. The car only has 24k original miles and is 100% stock.

Current Situation:

The car drives beautifully in the Normal engine mode but when turning the key to the Full engine mode, it falls flat on its face and it feels as though the ECU is more or less preventing the car from accelerating at all (too lean or too rich of an air/fuel mixture I am guessing). I am aware of the injector issues that the early years had due to ethanol content and the owners are saying that they had the injectors replaced about 10 years ago. But I am still skeptical about how good of a job the previous mechanic did installing them.

Where should I start to diagnose this issue? (I am mechanically inclined and have an E85 converted 3000GT VR-4 that people say is a hard car to work on so am not afraid to take off the plenum if need be and/or take on a new challenge to fix this ZR-1 for its next rightful owner).

Thank you for all the help and tips, in advance! Oh and if anyone in Colorado is looking for a great condition ZR-1, then message me to come check out the car.

Jordan