Q: Lack of power 1991 Nissan 300ZX

Hello and thanks for writing in. There are a number of things that can cause the motor to over fuel or run rich. When this happens, it is usually related to things like faulty fuel injectors, a failing mass air flow sensor or a faulty fuel pressure regulator allowing too much fuel to be supplied to the combustion chamber to be burnt. I would recommend having an expert from Your Mechanic come to your home to diagnose your fuel system.

Hi there - first task is to check for problem codes, even though this is an OBD I vehicle. Assuming your engine electronics are still relatively stock, there should be some information available on problems. You mention that all the plugs were black with soot - this suggests that fuel injection is too rich for all cylinders, because of something like a faulty coolant or air temperature sensor that is reporting a "cold" condition to engine management. A faulty ignition can appear as a misfire as well obviously, and also results in unburned fuel on the plugs. I recommend a loss of power inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, and give you an accurate assessment of damage and cost estimate for repairs.