a couple months ago i had to change a headgasket in my 99 OHV, replaced all the gaskets i could and such, everything was running great, for 5-6 weeks then the gauge on my dash was saying my engine temp was going wacky. it would stay rock solid for a while them it would go to over heating, then drop to half way between the half way point and overheating. it doesnt do this all the time only sometimes. i also replaced the engine temp sensor, thermostat and water pump, all brand new. could this be air trying to get out of the system?

before i worked on the head gasket my fan would work like normal, after the gasket its stuck on windshield. so i baffled at that.

and finally it seams my truck doesnt like to shift into 5th gear, its an automatic. it will shift into it but it seams to try and stay in a lower gear unless i push the O/D off/on button or mess with the peddle, is this a shift sensor or something else?

 

did some reading, it seams i most likely have air in my cooling system, plus what my gauge does seams to conform it, the fan thing, still no clue, ill be stopping by my mechanic to see what he says.

 

I missed this first time around.

When you replaced the head gaskets did you have the heads tested for cracks, common issue with 4.0l OHV?

Yes, your issue reads like air in the system but.............where is the air coming from?
The cooling system is self purging for air, it might do what you describe one time right after refilling, but as soon as thermostat opened all the air would go to top of rad and be purged out to overflow tank and bubble up and be gone, so when you shut off engine and it cooled down only coolant would be sucked back into rad from overflow.
Air would be gone

A leak from a cylinder keeps pumping air into the cooling system.

Just to take it off the table I would do a Glove Test and see if a cylinder is pumping air into the cooling system.

Cold engine
Remove rad cap
Remove overflow hose and plug outlet, putty, vacuum cap ??

Place latex glove over rad cap opening and seal it with rubber band, you can also use a balloon or even a condom for this test.

Unplug coil pack, you want a no start

Crank engine, watch glove
If it starts to bounce up and down you have a leak from a cylinder
If it just lays there you don't.

If it does bounce then start removing 1 spark plug at a time and repeat test, when glove stops bouncing the last spark plug removed was from the leaking cylinder, reinstall it to confirm.


When refilling cooling system, remove 1 heater hose from by-pass valve(heat control valve) or from heater core at firewall, then fill system until coolant is coming out of that hose, attach hose, air is out.
The air gets trapped in engine because thermostat is close and it is a high point, heater hoses are higher so will let the air out.

 

the head was replaced because i dropped the old one, the air most likely comes from when i drained the system to replace the gasket. then after replacing the gasket, i had the engine running for 30 seconds before i remembered i didnt top off the coolant. i am thinking my issue was i didnt run the engine without the radiator cap with the air on hot to bleed the system. after i replaced the gasket i did do a glove test, hence the 30 seconds i ran the engine with little coolant.

 

You can run the engine for a few minutes without any coolant so that wasn't an issue.

Air in the system after a refill isn't a major issue with Rangers, as said they are self purging.
If you want to make sure the air is out find the heater hose that goes to lower intake manifold just above thermostat housing.
Follow that hose back towards firewall, at the first connection(heat control or firewall) unclamp it and pull it off.
Refill until coolant comes out, all the air is out of waterpump and engine now, reconnect hose.

Start engine and let it warm up, leave rad cap off for a few minutes, 3 or 4, then top up and put cap on.