Could have air in the heater hoses, that will block the flow of warm coolant thru the hoses.
When filling the system with coolant remove 1 heater hose at the firewall, this will let all the air out, lift hose up above heater core outlet, when coolant starts to flow out the heater core, lower the hose down and coolant should be coming out of it now as well.
Reattach hose to core.
Now leave rad cap off and start the cold engine, top up coolant, and leave cap off
If coolant starts to overflow out of the cap opening then you have a problem....................
You have a blown head gasket or cracked head.
4.0l engines WILL crack a head if overheated even one time, they have a weak spot in the casing.
What happens with either blown head gasket or cracked head is that it starts off with a small leak, this small leak pumps "air" from a cylinder into the cooling system and that air goes to the high spots in the engine, which is the heads and heater hoses.
This air causes "air dams" blocking coolant flow so engine starts to overheat, reving the engine/water pump can break the air dam and engine will cool off again, but more air is still being pushed in from the leak.
Other sign of this is a Full Overflow tank, or a bubbling overflow tank as the system purges the air being pushed in from the leaking cylinder.
Coolant in the oil and White exhaust can happen at the end stages of a problem like this, once leak is larger
Search on this site for: Glove Test
Definitive FREE test for blown head gasket or cracked head
The heater core on the 1991 is the by-pass for the cooling system, so if it is clogging up, engine will start to overheat.
Remove both heater hoses, at the engine ends.
Point 1 hose down at the ground and hold garden hose against the other hose to Flush the core, and test its flow.
Reverse directions as well, Flush in opposite direction to be sure of good flow.