I haven't been able to find another thread with a similar problem, but then again I'm not very good at searching threads, but I have these weird problem going one with my 3.0 that started yesterday morning. I have a Bluetooth reader for the OBDII port and the Torque app on my phone to keep and eye on what's going on.

All of this started pretty soon after I added an entire gallon to my cooling system because I wasn't getting any decent heat coming from the vents, these was done last week. On Tuesday I was going to work and saw that an alarm went off on my app saying that my coolant temp reached 210F and it kept going up, so in a panic I turned on my heat full blast and it went up to 240F then just dropped to 160F - 170F and stayed there for the rest of the ride. This has happened every time my truck has sat long enough to fully cool off and I start driving home, or elsewhere.

I will be doing a complete flush of my cooling system and replacing my rad cap, t-stat and checking all hoses but it's way too cold to do that right now. I don't know what kinda of damage having the temp go up to 240F a few times can do, but I don't want to see.

So my conclusion is that I have a T-stat that gets stuck and when it reaches the extreme temp of 235F+ then the T-stat opens and super cold coolant rushes in and cools it off. I'm also not entirely sure why my heat isn't that great, the upper rad hose is pretty warm and so are the 2 hoses that go to the heater core, I'm just hoping that my truck doesn't need a new heater core because i don't want to remove my entire dash to do so.

What do you guys think?

Truck info: 1999 Ford Ranger, 3.0FF, auto, 135,000+ miles, RWD and stock

Thanks!

 

240degF won't hurt engine.

Yes could be t-stat.
Could also be By-pass valve/heat control valve.

I don't have a '99 3.0l so you will have to look and see what you have.
Ford changed this system often.
Find the two heater hoses at the firewall
Follow them, do they BOTH run to a plastic connector with 4 hoses in total?
If so then you have a by-pass valve, it will also have a vacuum hose connected to an arm the moves the valve.

I will assume you have this setup.
The by-pass is needed so engine doesn't get hot, it is the flow from water pump thru engine when t-stat is closed and also when it is open, so blocked or limited flow by-pass valve will cause random overheating symptoms.

When the valve is working and you turn on the Heat, the heater core becomes the By-pass.
Yes, if core is partially blocked then you also get the random overheating.
Heater core can be back flushed.
First warm up engine and turn heat on
Feel both heater hoses at the fire wall, the warmer hose is the IN(to core), colder is OUT(of core)

Remove both hoses and put garden hose on OUT, turn on water and hold it in place this will push out any larger debris that has got stuck in the small tubes, you are Back Flushing.
You can put a hose on IN and point it down at the ground or into a bucket, this makes it less messy.

You can also get some warm water and mix in CLR, pour it into core and let it sit 20 min., then flush it out and see if flow gets better, repeat as you see fit.

Faulty by-pass valve could account for the running hot and no heat in cab, but so could blocked heater core if you have the Heat set to hot all the time.

 

Awesome, this is some great info.

I did some more testing after work and decided to leave the heater on to see what would happen. I turned on the truck, and started to drive it and had the heater on. Once it got up to around 150F there was cold air coming through the vents, so then I get on the highway and the load that's put on the engine while getting up to highway speed was enough to send the temp skyrocketing. It was going up in increments of 5 to 10 degrees and i started to panic a little when it got back to 240F, but all of the sudden the temp started to drop, dramatically, and boom before you know it, it was running at 165F and some good heat was coming out of the vents that i never shut off.

My conclusion to all this is my t-stat getting stuck closed until the heat is enough to open it, which would cause cold coolant to rush into the block cooling off the engine and then hot coolant to make it's way into the heater core. Does this sound right?

So my next step will be doing a coolant flush, replacing the t-stat, replacing the rad cap and I'll have to do this tomorrow because I don't want this thing to get worse and I might as well flush the coolant because it looks pretty nasty. But also thanks for the BACK FLUSH, I'm going to do this tomorrow when i flush out my system.

 

If the by-pass/heater valve is working then the heater should get warmer as engine warms up, not stay cold, this is direct circulation from the water pump when valve is open(heat set to HOT)
So that part is a bit odd.
It is common if there is air in the heater hoses to get an "air dam", this blocks flow of coolant thru the heater core.
But once heater gets hot that would mean "air dam" is gone and can not come back unless air is somehow getting into the system when it cools down, i.e. a coolant leak, which will suck air in as engine cools down.

High engine temp can also be air in the system, thermostat is at a high spot so air will collect behind it, and air doesn't transfer heat very well, so t-stat may not be slow to open, it may just have air behind it so isn't getting hot enough to open until coolant temp sensor is at 240degF.
But again, when t-stat does open that air would be gone for good, it would be stuck at the top of the radiator and could not flow down to lower rad hose to get back into engine.
So issue couldn't repeat on next warm up.

Newer Thermostats should also have a "jiggle valve", this is a small hole in the t-stat plate with a loose fitting metal pin, this hole should be at the 12:00 position when thermostat is installed, it is there to let out any air that may be trapped in engine when refilling the system or if there is a coolant leak.
So make sure new t-stat has a jiggle valve, and yes that is the technical term for it, lol, the metal pin jiggles to keep hole open and free from debris.
If for some reason you can't get a t-stat with jiggle valve then drill an 1/8" hole in t-stat plate and install it at 12:00 position.

And when refilling cooling system always remove heater hose to let air out while refilling.
3.0l on some years also had intake pre-heater hooked to heater hoses, these are high points so if you have this CAREFULLY remove one of these hose while refilling.
CAREFULLY because the pipes on the intake are know to break off, and that means replacing intake, or installing a by-pass tube between the two hoses.