Coolant Problem
 

Hello all,
Yesterday my 01 Ranger Edge with the 4l v6 sohc started to overheat a bit. I did not let the truck go to the red line on the coolant temp gauge. I bought new coolant, opened the cap to the radiator and it was empty. As was the coolant reservoir. Ive had the truck for a week and this is the first major issue ive had. So I filled the radiator, started the truck and allowed engine to heat up and thermostat to open. It opened as coolant "burped" out of the radiator fill then coolant leveled off. No bubbles or overflowing. I put cap back on and filled up reservoir half way. Im just wondering how the truck ran out of coolant in the first place? I see absolutely no leaks. I know the head gaskets are solid as well. Any ideas?

Common issue with the 4.0l SOHC is a leaking thermostat housing, coolant evaporates so no leak to the ground is seen

Obviously if coolant got low then there is a leak somewhere.

Have a look here: How To Replace the Upper and Lower Thermostat Housing on a Ford 4.0L V6 SOHC Engine

And see if you can see coolant residue around your housing

Another place to check is the Overflow hose, pull it off the rad cap opening and blow into it, bubbles will come up in the tank of course, but you need to make sure it isn't clogged.
And make sure that hose isn't cracked or leaking anywhere.
It only gets HOT coolant so evaporates fairly fast so no ground leak.

Pin hole leaks, these are the toughest to track down because they only leak when system has pressure, so after coolant is HOT, and it can spray on warm engine part so disappears quickly.

You can rent a Cooling system pressure tester, usually $20-$25 for the day, some auto parts stores do loaners, worth a call
Basically it is a rad cap fitting with a hand pump and a gauge
You can then pump up a COLD system to 20psi and then look for a leak, dropping pressure will confirm there is a leak, and on a COLD engine it is much much easier to track down

Thanks man. Ill look into the thermostat housing as well as getting a psi test kit. Overflow isnt clogged.

EDIT: I lifted up the passenger carpet. Found it was flooded with coolant. So I assumed heater core and after 6 hours I had it replaced and everything bolted back together. Filled coolant and purged, blah blah blah. Took it for a 10 mile drive to see if coolant leak is gone. It is still leaking on the passenger side front wheel well, very slowly. If i open the coolant reservoir while engine is running, every few seconds an air bubble will arise, indicating that coolant is being consumed. There is no coolant around water pump or in weep hole nor is there any signs of leakage at the thermostat housing. Im not seeing any leaks comming from the radiator either. Does coolant flow through anything else that could cause my leak on the passenger side?

Thanks for the posts on V6~4.0L~SOHC engines & TStat housings; good reference info.
Replaced the old leaking FRPlastic TStat Housing with a new Aluminum TStat Housing
in 2019Aug on a 2001 Ford ExSportTrac; no leaks as of 2020Mar after ~5K more miles.
My full write up with pics on that repair:
http://2001fordexplorersporttrac.fandom.com/Coolant_ThermoStat_Housing