I've just about had it with this truck. I had a blown head gasket a few years ago because the water pump rusted out on it. I spent $2,000 to have it fixed. Last year the same thing happened! Instead of spending $2,000 again, the shop told me they had a guy with a 3.0 v6 that had 66k on it and they'd put it in the truck for the same price. Ok done. Now the stupid thing has air in the heater core and when I park and shut it down, air bubbles come up in the overflow reservoir! Is this another blown head gasket? I'm just about to scrap this stupid truck!

Things I've done/replaced that haven't helped:
thermostat
radiator cap
replaced all of the coolant and parked it on a hill to remove the air in the system

 

3.0l is not known for blowing head gaskets even when overheated a bit.

Obviously any engine with a head gasket will crush it if overheated long enough, metal expands when heated, physics 101, head and block are metal.............head gasket is softer material and in between head and block so..............crushed head gasket.

Doesn't matter how new or old the engine or gasket is, crushed is crushed.

Yes, from what you describe, air from a cylinder is being pumped into the cooling system, so head gasket or cracked head is most likely the reason.

You can do the free and easy Glove Test, it will tell you instantly if that is the problem.
Remove rad cap
remove overflow hose from rad and block that opening in rad, gum, vacuum cap putty.....??
Place a latex Glove over rad cap opening and seal it with rubber band, a balloon or even a condom also works.

Disconnect coil packs 4 wire connector, you want a no start

Crank engine and watch Glove.
If glove bounces then you have a leak

If it just lays there then you don't

If glove is bouncing you can remove 1 spark plug at a time and crank engine again
When glove stops bouncing the last spark plug removed was the cylinder with the leak, reinstall it to confirm.


If the used 3.0l was from a car and the head gaskets were not changed then that could have been the problem that caused this.
Car engines are mounted sideways so coolant flow is from front to back and out, trucks flow is front to back to front, so different head gasket design for best cooling.
That a long shot though.

head gaskets just don't go that often any more, materials are better and parts are better.
So very odd for this to happen 3 times, well one that you know was from over heating really doesn't count because that's like crashing a car into a wall at 50mph and blaming the bumper and hood for bending, lol.

A used engine is always a gamble since usually there is no history with it, it may have been overheat prior to you getting it, can't really test for that.

 

I'll give that simple test a try. Is there any other reason why as soon as I park it (and according to the gauge, it's not overheating) and turn off the truck that air bubbles start coming up in the coolant reservoir? It happens every single time. The truck never overheats but like you said, who knows what happened to the engine before I got it?

 

Air bubbles after shut down only.

Rad cap has a pressure rating, lets say it is 14psi.
Cooling system itself has no pressure cold.
The pressure comes from coolant warming up, when you warm up a fluid it expands, that's how "pressure cookers" work.

So as engine/coolant warms up the coolant volume expands and pressure builds up inside the engine/radiator.

When it gets above 14psi(cap rating) the rad caps larger valve opens and allows coolant to flow out into overflow tank, so coolant level in overflow tank gets higher, the WARM mark on the tank.

When engine is shut off the coolant cools down and shrinks in volume, when pressure drops below 0psi, the smaller valve in the rad cap opens and sucks coolant from overflow tank back into the radiator, when done coolant level in overflow should be at COLD mark.

This system needs a good seal on overflow hose, or it would suck in air when it cooled down, air is easier to to move than coolant.
Also needs sealed engine/rad system, any leaks let coolant out of course but will allow air to be sucked back in.

Air bubbling into overflow tank after shut down but NOT while engine is running, means air is getting into the system from somewhere, if not a bad head gasket then a leak.
The air will end up at the top of the radiator that's the high point as coolant circulates.

When you shut off any engine coolant stops circulating so the coolant in the head will get hotter, and expand more, this is totally normal.
So pressure in the system will go up and rad cap will open to let it out.
If there is coolant at the top of the radiator it will flow in to overflow tank, if there is air...........bubbles in the overflow tank.

So it is normal for an engine it get warmer after shut down, and it is normal for it to purge some coolant at that time, the air means system is not sealed

 

Could it be an exhaust leak? Since the put the new engine in, it has always done this. Wonder if they left something loose.

I also had a new rear main seal put in afterwards (not sure why they didn't just do that at the same time!) but it did it before then. I'd love some ideas on what to check.

 

Do Glove Test


After engine cools down over night remove rad cap, if coolant is not all the way to the top then your coolant recovery system is not working right.
Explained above.

Check overflow hose for cracks or breaks, replace rad cap

Also find a picture or drawing of heater hose routing and connections for your year and engine, "they" may have reversed a hose and that is causing a flow issue, so engine is getting hotter than it should on shutdown.

 

Turned out to be ANOTHER blown head gasket! The new shop put in a new gasket, new head bolts, new radiator (they said the old one felt like it was 50 lbs heavier than it should have been!), new thermostat, and did all of it for $1500 plus tax. Next week they are going to fix my suspension issue. :)

 

I have used car gaskets on all my 3.0s they seem to work

 

Yes you can, but it does create hot spots in a few cylinders, "it works" is about the best you can say about it.

3.0l in a car is transverse mounted, so coolant flow thru the engine is front to back and out.
In a Ranger coolant flow is front to back then to front again and out, so head gasket coolant passage holes are different, only a few of them but they are different.
So you "can" use car head gaskets in Ranger 3.0l, but you won't get the best coolant flow thru all parts of the head/block.