2000 Toyota Tacoma • 148,000 miles

Cooling issue, 2.7L, AT, AC, 4WD
Truck is consuming coolant, about 1/2 gallon every 300 miles.
No obvious leaks, have pressure tested numerous times at 13 to 15psi, holds a steady pressure and again, no visual leaks or drips.
No codes, runs fine.
I have replaced the thermostat and water pump with new, not rebuilts.
Doesnt seem to cycle coolant through the pump at operating temps, bottom radiator hose is considerably cooler than the top hose, which is hot but not overly hot to touch.
I really dont think this is cycling at all?
Head gasket leak at a cylinder and dumping into cylinder?
one would think it would smoke like a train, miss and foul plug and a CEL maybe?
How to proceed?
When coolant is low, allows no flow into heater core and will blow cool air.
October 27, 2012.

Check the radiator cap condition.

Oct 27, 2012.
Cap appears fine and tests out good with the pressure tester.
While running the truck for 1 1/2 hrs today, with the pressure tester on it as well as running with the cap off, I never had flow into the water pump. The lower hose remained much cooler than the top.
You should see cycle flow when the thermo opens up. I did not.

Oct 28, 2012.
If the engine is not overheating, the coolant must be flowing and the warmer upper hose as compared to the lower hose is an indication of that. Your issue here is coolant depletion and the cooling efficiency seems good.

When idling the coolant flow is not fast and if the coolant level is up to the neck of the radiator cap, you might not see any actual movement. Increase the idling speed to at least 1500 and see what happens.

Pressure testings sometimes has to be done on a cold engnine and at times on a warm engine. Is the recovery tank being depleted or incresing in coolant level?

Oct 28, 2012.
Recovery tank gets sucked dry within the 300 mile of use as well.
Physically, the engine does not act hot, appear hot and the guage does not read hot, although it has once when I first got onto this issue.
Will check recover tank for leak or crack.

Oct 28, 2012.
If the recovery tank is being sucked dry, that is a better sign than it increasing in level. There is a leak somewhere in the system and sometimes pressurre test are inconclusive as the leak is minimal and a rather long waiting time is required to note a drop in the pressure gauge reading.

Most often leaks would be at hose joints. Look out for signs of whitish powdery substances(color could be nearer to the coolant) at joints and the lower portion of water pump. If you still cannot get the source of the leak, maybe it is time to try a dye where any leaks would show up with a UV light.

Oct 28, 2012.