I'm new to this site and having difficulties figuring out how and where to write what problems I'm having with my truck, my temp gauge has been fluctuating from cold to overheating, doesn't stay in either position long enough to overheat but I have replaced water pump, thermostat,temp sending unit, upper and lower radiator hoses and flushed system out to no avail, and now whenever I run the heat I get a thumping sound coming from under the hood, would like to know some possible reasons for this any and all info is greatly appreciated

 

Welcome to the forum

I too have a '94 4.0l and had the same problem, wait for it.............

heater core is clogged up
heater core is the water pump bypass so when it doesn't have full flow engine temp randomly goes up and down.
I replaced what you did AND the fan clutch, so well earned lesson

Good news is that it takes about 20min to change it, 4 screws and 2 hose clamps
And you don't have to drain the coolant
remove heater hoses at firewall and raise them up so coolant doesn't leak out
inside the cab put a towel down on passenger side
Look up under glove box, there is a bottom cover under heater core with 4 screws
3 are easy to find/see
There is one that is right behind glove box up higher than the others, if you empty glove box and flip it down you will see it

Remove/drop the cover
Now you will pull core back tilting it to get tubes out of firewall then tip it to get it out of heater box, and this is when some coolant will spill out on to the towel.
It will feel very heavy, because coolant can't drain out even if you flip it upside down.

You may need some weather seal foam tape to put around new core, you will see it on the old core but it will most likely be unusable, you don't HAVE to reseal it, heater will work fine without it

 

Thanks so much for the Intel, I was thinking that may be it but it's nice to have a little confirmation, and I was thinking that I would have to tear out the whole dash to get to it but that's great news that I don't have to again thanks soo much man