Ok, I have been having this problem for a while, and can't figure it out.
While driving one time I got the smell of coolant. I looked under the hood to find that the coolant had came out of the overflow tank (at what must have been a pretty forceful rate, since the cap was blown open). Noticing that the coolant was dirty, I hoped a flush would fix it. I flushed it myself, and had it done professionally (with the machine they use) and this still happens. It does it every time I add coolant.

In what I assume to be a related sideaffect, my heater is very touch and go as well. It will blow hot, then cold, then hot again. With the weather the way it is now, this sucks too.

Sometimes after shutting the truck off, you can hear the overflow tank either gulping all the extra coolant, or putting it back into the overflow tank. It is all just very weird.

The temp gauge has never read above normal.

1999 ford ranger 4x4 3.0L

 

Also, I did change the thermostat. I didn't really expect it to help, but its cheap and easy so I did it anyway. No luck.

 

check for combustion gases in the coolant, might be a blown head gasket.

 

Where can I get this tested? Will most repair shops be able to do this?

 

Ditto on the head gasket.. when i had my old 76 grand marquis, it would blow off a bunch of pressure into the overflow.... Blew the left head gasket badly
3 months later.. i found alot of oily "scum" on the oil cap and floating in the coolant also.. but no coolant in the oil until the final blow..

 

you could try replacing the rad cap but the Head gasket is most likely your issue

 

Could be a number of things as has been suggested. To check for a head gasket problem, you could try & park with the front end up hill, remove the radiator cap with the engine cold, start & idle it & watch to see if you see any exhaust gas bubbles in the coolant.

A coolant system pressure gauge attached to the cap would also be able to measure for excessive coolant system pressure, if exhaust is entering the cooling system & over pressurizing it.

Do you have any signs of coolant in the crankcase oil, or condensation on the dipstick????

If so, you could do a cyl blow down test, to see if you get any air bubbles in the coolant at the removed radiator cap, this is kinda like doing the previous test with the engine running, except you pressurize each cyl with air, to find Which head gasket is leaking.

If no problems found, seeing as how the heater is also acting up, was it back flushed separately, so you know flow through it is good????

Seeing as how the coolant was neglected & we don't know for how long, nor how many miles are on this ride, or it's repair history, you might have other problems like rust, scale scum, sludge still mucking up the block, heater, radiator, ect. How much "stuff" did you get out on your flush & the machine flush???? How bad a shape was the coolant????

Has the water pump ever been replaced???? If not, badly neglected coolant can turn acid & eat it's vanes away & cause circulation problems.

Scum, scale, sludge, rust, can cause block & head hot spots that'll locally boil coolant & cause the system to throw up after a hard drive or shutdown.

Were/are you using a Ford thermostat, or one like it, with the built in air bypass/bleed valve & did you do a proper air bleed proceedure after you flushed & refilled the coolant system, so that your certain you don't have an air bubble trapped in the heater, or elswhere causing mischief?????
To do the purge, park the vehicle with the front end up hill, the steeper the better. With the overflow tank properly filled, hood up, heater temp control on Max heat, idle the fully warmed up engine for 10-15 minutes in this position, then shut the engine down & let it completely cool off (with the hood up, it'll cool off quicker). Doing this chases trapped air to the top of the radiator where the cap can burp it out & automatically top the system off with new coolant from the overflow tank as the system cools down.

More thoughts for consideration, let us know what you find.

 

tested the coolant, and we have a blown head gasket. Fun stuff. Selling it to my dad who has the time to fix it, because I definately don't.

 

Good feedabck, sorry to hear the news isn't good, but from the description of what was going on, it wasn't unexpected. Hopefully a headgasket replacement is all it'll need.

 

Yeah, it never actually overheated, and it still runs fine other than the coolant system tweaking out, so it should be good to go after the gasket is replaced.