First off, forgive me for being a newb to your forum. Tried several searches and there was just to much to read through to find anything helpful.

94 Pickup 3.0 4x4 starts normal and the idle slowly climbs until it reaches 1800 RPM or so. It will stay there until the motor warms up.

The temp gauge isnt coming up gradually as it used to, instead it sits at nothing for several minutes then jumps up to about halfway, all at once. Once there the RPM will subside to a more normal 800 or so.

From here the temp will continue to climb until it the gauge gets right to the bottom edge of the red. Then I hear a noise from behind the dash, through the heater core I assume, (like water/air being forced past something maybe? If you've ever had a partially frozen garden hose, its similar to the sound of the water forcing its way past the ice.) and the gauge will show a drop in temp until it slowly works its way back up and repeats from there.

Sticking thermostat? Air in the system? Not sure where to start. Thanks in advance.

 

I'm new on here as well. I would try a new thermostat. Its a pretty easy job and fairly cheap. When you install the new one make sure that the air vent hole is on the top and that should eliminate any air in the system (if any is there). If that doesnt adjust it try flushing the heater core. Again, that is fairly easy to do as well. My guess is that the thermostat is stuck closed or partially closed, allowing only minimal flow through the core. I sure hope you're not driving it this way.

 

Check the connecting tubes on the fire wall for your heater and bypass, I'd suspect rust in them.

 

Well after 3 attempts to burp the system and continually adding antifreeze I think I got her fixed. Best I can tell there was air in there somewhere. Truck has run normal since last night after burp attempt #3.