'91 Voyager stalls and runs rich

[yga367]

I have a ’91 Plymouth Voyager 3.0L with 63k miles on it. Several weeks ago the check engine light came on and the van would stall. My mechanic put a computer reader on it and it indicated the thermostat and temperature sensor should be replaced. The thermostat was replaced about 6 months ago because I hoped it would help the van warm up faster. I replaced the sensor, but had the same problem. The reader again indicated the two items were defective so the thermostat was replaced…same problem and reading. The sensor was replace again, but the problem remains. I’ve noticed that the check engine light goes out after these items are replaced and I’m able to drive until I leave it parked for more than a day. The light then comes on again. Even when I’m able to drive it, it runs very rich; black exhaust and about 14 mpg rather than the 20 I usually get. Please help!