97 Cherokee overheating issue? also heater isnt working

I have an issue. I commute in a 97 cherokee. I have recently noticed that the heater is blowing cold air from time to time. Then tonight while driving my 45 mile commute, the temp spiked to hot, 260 or what ever it is, then would suddenly drop down a bit and then go back up again and back and forth. During this time my friend I commute with and I could hear a sound from under the dash. He could put his hand under the dash on the heater box and feel it rumbling when it made this sound. The heater would not be working correctly and then the heater would make this sound and some heat would blow through the vent?

My question is, is there a heater core under the dash of these rigs and could a bad heater core, cause the engine temp to spike like that??? I am guessing the heater issue is the core?

And if it is the core, how hard to change them in this type of rig?

thanks for any assistance.

Also is the thermostat in these things under that housing in the front of the engine and do these often go bad? How hard to change these?


Dave

The system is low on coolant.

Top off the coolant. If the same symptoms continue, it's most likely a bad head gasket.

I will have to say the head gasket also.. I know the 4l have an issue with that,,,