I am in a troubling situation and not sure where to go next. I bought a 1998 3.0 Ranger with 168k miles on it. The cooling system is very odd, I have pressure-tested the cooling system and it only dropped 1 psi in over 35 minutes of testing. I have done a leak-down test and it passes along with a compression test where all cylinders test 120 psi or very close to it. I have replaced the thermostat, lower radiator hose, and flushed the cooling system. When I burp the system I can drive forever and the temp gauge never does anything odd and the heat constantly works. I leave the truck sit overnight and go to drive it the next day and the temp gauge will drop and then come up and then drop and come up again, etc. The heat will work and stop working as the temp gauge goes up and down and it pushes the coolant into the reservoir tank, could I still have an air bubble in the system?

 

Welcome to the forum

You have an Overflow cooling system
This means the Rad cap has TWO valves, the larger one with the big spring and a smaller valve in the center with small spring

When any liquid is heated up it expands in volume, so 1 gallon of coolant at 70degF becomes 1.2 gallons at 180degF, just physics
This is where the Pressure in a cooling system comes from, the expansion of the coolant as it heats up

Rad cap's big valve is rated at 14 PSI, if pressure in the system goes to 15psi then the valve is pushed up and hot coolant will flow thru the overflow hose to THE BOTTOM of the overflow tank, using the bottom of the tank is important

When you shut off the engine and it cools down if ANY coolant went over to the overflow tank, as the coolant shrinks in volume it will reach -1psi in the cooling system because coolant is missing, at that time the small valve in rad cap is sucked open and coolant is sucked back in from THE BOTTOM of the overflow tank, topping up the coolant level in the radiator to the very top, there should be 0 air in radiator when you remove that cap after full cool down

The overflow tank is Vented, NOT sealed
So if there was air at the top of the radiator it will stay there and as coolant warms up and pressure goes above 14psi the AIR will be pushed out and bubble up from THE BOTTOM of the overflow tank
When engine cools down again COOLANT will be sucked back in so air is gone, a self purging system

So make sure the overflow hose is air tight, no cracks/leaks, and clamped at both ends, and clean out the overflow tank as any debris in the tank sits at the bottom so can prevent coolant from being sucked back in

Top up radiator when its cold, drive like normal
Next morning, or when its cold again, open rad cap, should be no NO AIR, 0, none, nada, el zippo, if there is air then something is wrong with the system
If there is air then check overflow side first, no leaks or cracked hose
Change rad cap



BUT.................your description of the temp gauge going up and down randomly after warm up reminds me of a clogged heater core
The heater core is the "water pump bypass" on 1998 Ranger 3.0l, so it always needs to have flow out of the lower intake manifold and into the water pump
If there is limited flow thru the 2 heater hoses from intake to water pump the temp gauge will go up and down randomly

You can remove the 2 heater hoses at the firewall and use a garden hose to try and flush out the core, in BOTH directions, you can usually tell if flow is limited
And reverse the hoses at firewall when you put them back on, reversing the flow thru the heater core, you should do this every 2 years on any vehicle
See if you can get some flow back
In a 1995 and up Ranger you need to pull back/out most of the dash to swap out a heater core

 

Thank you for the help, I tried this before however it does not seem like the heater core holds any fluid and I was able to blow through the heater core, could it still be clogged?

 

"...it does not seem like the heater core holds any fluid" ??

You said previously that the heater in the cab would blow hot, only way it can do that is if hot coolant is flowing thru it at a good rate
Blowing air thru the core is is not the same as a fluid, so need to send some water thru it to see if it has good flow

Another test for that is after engine is warmed up and idling, turn heater to hot and fan on HIGH, set as Defrost
Feel the air coming out of windshield vents, should stay HOT, if it starts to cool down heater core is partially blocked

 

The heater core holds fluid as when I burp the system the heater core lines get very hot. When I let the truck idle I hear no noise in the cab however when I get up to about 2500 rpm it sounds like little bubbles coming through it. The heater blows heat when the temp gauge is up however when the gauge drops for an unknown reason even when up to temp the heat stops working and then the gauge comes back up and the heater works again. I thought maybe the heater core was clogged so I unhooked it and then I was able to blow through it with my mouth without any restriction at all. I dumped fluid into the core and it held very little before washing out the return hose. The heater core hoses go down in temp as soon as the temp gauge starts to do weird things.