I have a 91 z/24 that bucks at low rpm. It reacts just as if you are pushing the accelerator to the floor and then off again repeating the process continuously. The Check engine light is on but shorting the ALDL or plugging in a scanner doesn't show anything. In fact the scanner doesn't read a thing. It is as if the ALDL is not connected. Also, the fan is now on all the time. Does anyone have any ideas?

If you're not getting anything from the ALDL check all the fuses... or it may just be a loose wire to the harness.

Easiest and most common issue with bucking are plugs and plug wires... you don't have the wires like the 2.2s, so I'd check the plugs.

Thank you for your response. The bucking problem turned out to be the digital egr but the check engine light is still on and the aldl is still dead. And, although the engine performs well it is far from optimum. There is obviously a problem somewhere as the check engine light indicates. (I had disconnected the battery to clear the codes) I am now wondering if I am having a problem with the computer itself?