Your thermostat is stuck open. The gauge reading below normal is a symptom of this. Here are a few pics I took to help you replace it.
- Gasket sealer and new t-stat
- a pair of adjustable pliars
- 8mm socket
- 6-inch or longer socket extension
- and a socket wrench.
As you can see in the pictures below, the thermostat is housed in a black plastic water neck that bolts to the face of the cylinder head.
You can see the round opening of this neck in this image. The upper radiator hose attaches to this with a spring-lock clamp. Release tension with the pliars and move the clamp "up" the hose so you can pull the hose off the water neck.
- Coolant will spill out of this portion of the hose. Be prepared with a proper catch pan or other device to capture at least a cup of coolant.
- Make sure the engine is COOL and the pressure on the radiator has been released by removing the cap after the engine has cooled.
Here you can see only ONE of the 2 (TWO) bolts that attach the t-stat housing/water neck to the head. You may have to remove the bracket that supports the A/C line that runs in front of the engine (visible in this image) to access the other bolt on the opposite side.
The thermostat has a clip which locks it into the plastic housing. You probably won't need any gasket sealer since you're thermostat housing is sealed with a rubber o-ring. However, a thin layer of gasket sealer is good measure while you have it apart to ensure no leakage occurs.
Reconnect the radiator hose after replacing the two bolts that hold the housing on.
Take the truck for a test drive, then check for leaks!
Your gauge should return to a middle reading and stay constant.