Ok, I’ve scoured the Internet. Is it
even possible to pressure test the coolant system on the 5.4l 3-valve? I can’t find a dang thing.

I’m having heater issues. Like warm now but that won’t work when it gets cold here. If I rev to 3k RPM then it gets hot. Temp gauge is exactly where it should be.

there seems to be little pressure under the cap so suspect a leak somewhere. Changed thermostat already with no new results good or bad.

I do not suspect a head gasket leak at this time.

 

Have you ever back flushed your system? The heater core may be partially blocked. Had the same problem yrs ago and the back flush worked for me.

 

After all it is 10 days later, but if the heater core is filled even partially with air , it will not produce sufficient heat.
With the engine idling just remove the upper heater hose slightly to allow the air to be expelled. If trapped air is the problem you should see an immediate difference.