I'd check to make sure the thermostat isn't broken before doing anything
else.  I'd also look in to make sure the whole radiator isn't clogged
with scale.  Then I'd check to make sure the water pump was happy.

I would not be surprised if all three of these were issues.  

Note that an acid flush to clean all the scale out may cause the radiator
to start leaking if it's already in bad shape and marginal.  But it's
not a bad idea if you're intending on keeping the thing running for a while,
and if the water pump is good.  

Maybe the fluid isn't circulating at all. (Water pump?)

Maybe the flow regulation isn't working, and so the coolant isn't
getting to the engine block. (Thermostat?)

Also, maybe there is inadequate pressure in the system due to a leak
somewhere. (Radiator cap?)

A car engine's coolant, which is largely water, has to heat far above
100 degrees Celsius without boiling. This is posssible because the
cooling system is closed, and pressurized, kind of like a pressure
cooker.

Without the pressure, your coolant would boil off quickly.

But too much pressure is, of course, undesireable. That's why, just
like a pressure cooker, there is a way to relieve pressure. This is
typically in the radiator cap, which is spring loaded so that its seal
can lift under pressure.

That popping sound you hear, could it be your radiator cap bleeding off
pressure prematurely?

That would explain why you are losing coolant, and why the engine is
overheating.

If the liquid boils inside the engine block due to inadequate pressure,
the resulting vapor can't do a useful job of keeping the engine cool,
you see. Water vapor has very little heat capacity compared to liquid
water! Once the coolant passages in the engine block are full of
nothing but vapor, you are going to overheat.

So, first thing, try replacing the radiator cap.

throwaway340...@hotmail.com wrote:

Maybe the fluid isn't circulating at all. (Water pump?)

Maybe the flow regulation isn't working. (Thermostat?)

Also, maybe there is inadequate pressure in the system due to a leak
somewhere. (Radiator cap?)

A car engine's coolant, which is largely water, has to heat far above
100 degrees Celsius without boiling. This is posssible because the
cooling system is closed, and pressurized, kind of like a pressure
cooker.

Without the pressure, your coolant would boil off quickly.

But too much pressure is, of course, undesireable. That's why, just
like a pressure cooker, there is a way to relieve pressure. This is
typically in the radiator cap, which is spring loaded so that its seal
can lift under pressure.

That popping sound you hear, could it be your radiator cap bleeding off
pressure prematurely?

That would explain why you are losing coolant, and why the engine is
overheating.

If the liquid boils inside the engine block due to inadequate pressure,
the resulting vapor can't do a useful job of keeping the engine cool,
you see. Water vapor has very little heat capacity compared to liquid
water! Once the coolant passages in the engine block are full of
nothing but vapor, you are going to overheat.

So, first thing, try replacing the radiator cap.

Where does the coolant go when bled off by the cap? Either just into
the atmosphere, or into a reservoir from which it can be sucked back
when the pressure drops, creating a vacuum (which the cap is also
designed to relieve). It depends on whether you have an open or closed
type pressure relief system.

   Among everything people have mentioned here, check the oil as well....
you say the fluid keeps getting low, but none on the ground.... is it
leaking into the crankcase??  If so, it may not such an easy fix...

Good luck..

Fwed

Say, would that be the 3.0 V6 in your truck?  (not a bad idea to add
important info like that, ya know)

If so, your headgaskets are blown at cylinders 1 and 6.  Call the
Toyota dealer and see if your truck still qualifies for the headgasket
recall.

The 22RE also can blow it's headgasket and cause similar results, but
it isn't nearly as common a problem nor is it covered by a recall
campaign.

Toyota MDT in MO

I am not a mechanic but the local junkyards in my area scrap out the
radiators from toyota trucks as they are almost always clogged.

Just my 2 cents,
Good Luck,
Chris