[Bob Lincoln]

So, if you shut it off for just a minute or two after full warmup, it starts and runs OK? It has to sit about 15 minutes before it runs rich? And if you were to wait an hour, it would be OK?

If it were the plenum leak, I would think that it would run lean, both on cold starts and until it expanded enough to maybe seal the leak.

Did you check for fault codes? Troubleshooting Chrysler Two-Digit Computer Fault Codes

I found with my Dakota V-6 (which is based on the 318), that both the coolant temperature sensor (2-wire) and the temperature sending unit (1-wire) had cracked and were falling apart. They are mounted on either side of the thermostat. They came apart as I tried to remove and replace them. The 2-wire sensor is the computer input for setting air/fuel mixture, and the 1-wire only delivers the signal to the temperature gauge on the dash. Check the fault codes and inspect these sensors, give them a gentle tug to see if they are cracked or coming apart; or if their wires are damaged.

After I replaced both sensors, the dash gauge read 1/3 of full scale when warm, whereas before it never went beyond the first warm mark. My brother had owned the truck new and said that it had always registered about 30-40% of full scale.