If the coolant temperature sensor was accurate as far as when cold (was the same as outside temp) and then seemed to follow correctly as the engine warmed up, chances are it's okay.
If you were seeing black smoke, that indicates a rich fuel mixture. Have you checked fuel pressure to make sure the regulator is working?
Here is a link that shows in general how to test fuel pressure. I attached a picture of the regulator location. Lets do this to see if this is the issue.
Here are the directions for testing specific to your vehicle. The remaining pictures correlate with the directions. Also, the specifications are listed in the directions.
1997 Nissan-Datsun Truck D21 Hardbody XE 2WD L4-2389cc 2.4L SOHC MFI (KA24E)
Fuel Pressure Check
Vehicle Powertrain Management Service and Repair Procedures Fuel Pressure Check
FUEL PRESSURE CHECK
a. When reconnecting fuel line, always use new clamps.
b. Make sure that clamp screw does not contact adjacent parts.
c. Use a torque driver to tighten clamps.
d. Use Pressure Gauge to check fuel pressure.
1. Release fuel pressure to zero.
2. Disconnect fuel hose between fuel filter and fuel tube (engine side).
3. Install pressure gauge between fuel filter and fuel tube.
4. Start engine and check for fuel leakage.
5. Read the indication of fuel pressure gauge.
At idling: Approximately 235 kPa (2.4 kg/sq. cm, 34 psi)
A few seconds after ignition switch is turned OFF to ON:
Approximately 294 kPa (3.0 kg/sq. cm, 43 psi)
6. Stop engine and disconnect fuel pressure regulator vacuum hose from intake manifold.
7. Plug intake manifold with a rubber cap.
8. Connect variable vacuum source to fuel pressure regulator.
9. Start engine and read indication of fuel pressure gauge as vacuum is changed.
Fuel pressure should decrease as vacuum increases. If results are unsatisfactory, replace fuel pressure regulator.
Let me know what you find.
Images (Click to enlarge)
Friday, January 17th, 2020 AT 9:19 PM