Heya gents,

My 1999 4.6L is running well, but I'm using coolant / water to some degree, AND it's as if I have an over pressurized system.

I say over-pressurized because I blew an original heater hose once, but it happened AFTER I shut the truck off and walked away from it. At that time I knew that my filler-cap would occasionally whistle (letting out pressure), so I bought a new, and proper-pressure, filler cap for the truck shortly after replacing that hose.

But even with the new one I hear it off-gassing when I shut the truck off some days, and today I both heard it while driving AND the temperature gauge started rising at a stop light. (I thought that is was out of water, so I drove it some more to get to a parking lot, but the temp gauge went right back to normal and stayed there both at the next stop lights and in the parking lot, and when I finally could open the reservoir, it had plenty of water in it.)

So I'm left guessing at a few causes here.

Where is my water going to? Yeah, I don't want to think it is, but certainly a head gasket could be an issue - but I never see any steam out of the exhaust.

What is causing the over-pressurization? Well, that could be a head gasket, too, but could it be a stuck thermostat? I only ask based on what I saw the temp gauge do today.

I wish I could just find an external leak and know that that's where my water goes, but... ugh.

Still, if there is an over-pressurization, perhaps the water is being pushed out of an overflow tube since the cap is often letting pressure out (whistling)?

I really can't figure out why it would both whistle, and nearly overheat, if there was enough water in the system today.

 

I had the same thing happen on my non ford. It was a stuck thermostat. Or a semi-stuck thermostat .

 

Sounds like:
A) What Excursionator said about the thermostat
or
B) Something is plugged up in your cooling system