Possible cavitation?

[Mactheplumber]

94 4.0 manual w/ac.

Okay so I can't figure out why my truck is doing this. But here's what it's doing. At idle is where I notice it. So after it warms up, and it runs for a bit the temp gauge starts going from the L in normal, then drops down to a little before the N on normal. And it appears when it's reading on the cooler side of things, the engine will start kinda thumping/bumping.

But then when the gauge starts to climb the noise stops. But, if I kill it when it's bumping, it keeps bumping. Maybe for a minute or so.

So here's what I've done. New radiator cap with the red safety release. New thermostat. I took when I replaced the thermostat and ran a water hose through the upper radiator hose hoping to flush out any air bubbles. But, the bottom radiator hose was disconnected so might not have did a good job.

Do you guys think I need to replace the heater core, radiator, or possibly water pump? Only had the truck a few months now. And just so I know, the water pump pushes water through the lower radiator hose correct?

 

[2trux]

It pushes water out at the top and sucks it in from the bottom.

 

[Mactheplumber]

So the spring on the thermostat needs to point away from the block?

 

[bf750]

nope

 

[RonD]

Thermostat spring goes toward the heat source so the block.

And yes to 2trux, on all radiators, hot coolant goes in at the top and cooler coolant comes out at the bottom.

Temp rise at idle usually means Fan Clutch is going bad.
When driving the air flow thru the rad is from vehicles motion
Under about 10mph the fan is needed to keep the air moving thru rad or temp gauge will start to climb up


On the 1994 4.0l the Heater core always has coolant flowing thru it, if heater core gets partially blocked you lose Max heat in the cab but temp gauge will also go up and down randomly because water pump by-pass(heater core) is clogged.

Heater core is not too expensive and very easy to replace on 1994, 4 screws, 2 hose clamps and 20min the first time, 10min if you've done it before