Recent engine rebuild - yesterday I noticed that there is only luke-warm air from the heater. Also noticed temp guage bouncing around. It was reading cold, then warm, then cold, etc.

Took out the thermostat and dropped it in boiling water. It did open, but did not _stay_ open for long. My question is how long should it stay open? The water bath that I dropped it in remains very hot (too hot to stick a finger in for very long), yet thermostat closes after just a few minutes (maybe five mins).

Parts stores closed today - I don't think I have a spare.

 

You probably have an air bubble in your system. Get the front of the truck up high as you can and top it off.

You are best off using a toyota thermo. Its one of the "toyota made it best" parts.

 

Raise the front end as high as you can and let it idle for a while to get the air pocket out with the radiator cap off.

The temp guage sensor is in between cylinders #2 and #3, I have seen it get build up on it and it affect the guage. I clean mine with a brass brush or a brilo pad.

Is the valve on the firewall moving to it full postions?

 

Valve on the firewall is moving, temp sensor is ridiculously clean (just finished a rebuild), and the system had been "burped" on re-assembly about 1100 miles ago. Pretty sure it is the thermostat. First test it did not open. Second test it opened and closed quickly, third test no-open again. Had a spare - it no worky either.

Will put a thermostat in it today.