Paul, back to the cooling system basics. Is this engine overheating, or is the gauge just indicating high. Known concern, sending unit and a simple fix. This is a Thermistor and does not indicate correctly if not submerged in fluid. Next this engine appears to have an inherent overheating concern. Google it, 800,000 hits in half a second. ................. As for the basics of the cooling system, this like all current systems is a closed system. That means that the radiator is filled to the top, and an overflow hose MUST be submerged in coolant at all times. This is why there is a cold/hot full mark on the overflow tank. As the system heats up and the coolant expands it pushes the excess into the overflow tank, when it cools and contracts, it pulls coolant back into the radiator, thus the radiator is always full and no air will be in the system. Increasing pressure by using a pressure cap, you increase the boiling point of the coolant, similar to a pressure cooker. Also the system uses ethylene glycol, 50-50 mixture, antifreeze also increases the boiling point. Thus if the unit is not throwing coolant, it is not overheating. Check the condition of the coolant, is there rust, dirt in the system. all thermostats have a bleed hole so there can be air passage across the closed thermostat. If this becomes plugged, it can cause the coolant to peculate in the block, Stant makes a stat that has a wiggle pin, to prevent plugging, in it and Chrysler products which are notorious for rusty coolant, I would always use a Stant thermostat in a Chrysler product or you could increase slightly the size of the hole so debris does not plug the vent hole. As for changing the coolant, depending on the color and the condition of the coolant, contaminated flush the system by dropping the lower hose, and removing the thermostat. Do not back flush as this will force debris back into the heater core which has much smaller tubes than a radiator and it will plug the heater core. The gold coolant is 100,000 mile, but costs 4-5 times that of ethylene glycol green coolant. Your owners manual should give you the proper service intervals. //////////////////////////////As for your 1984 Nissan 4x4, spray the wires and boots with salt water, the ionized water will conduct electrically leaking wires much quicker than plane water. If this does not indicate a leak, be sure to rise with fresh water. I would suspect with this old a vehicle, you are due for new wires and a good cleaning of the coil. Be sure you rinse any salt water off when you replace the wires.