any help on this serial number?

I just bought this caterpillar 311. Serial num 5pk00323. Anyone can help on the year this was made. I've been looking everywhere... I also ran it for the first time today and it came close to being over heated, one dash from being in the red. It was very hot in south Louisiana but seems a little odd. I have rented equipment before and no problems. Equipment was very clean, including around the radiator, also radiator was full with coolant. Belts, including water pump belt, looked fine. Also seemed like it wouldnt have any strength in the low idling (1-3). Thanks for any help!!!!