3Slow symptoms of air bubble in cooling system

[Bgunner]

OK before I get into what I have observed some times I swear that I have OCD when it comes to my vehicles so I need more than one symptom to be sure there is actually some thing wrong. Even my mechanic buddy had mentioned this about me before so this is why I'm here before tearing into the cooling system to alleviate my OCD issues and try to confirm what the issue is. Just throwing this out there could also be a thermostat stuck open, best way to stick but harder to diagnose.

Symptoms observed: not to hot heat during winter and the gauge stays around the "N" in normal on the gauge cluster.
The gauge works as the other day after climbing the mountain, back down and partially back up a different part I shut it off. When I fired it back up the gauge shot up to the "L" in normal and went quickly down as the fluid circulated but it was proof the thing actually works properly.

Since I have owned this truck I have had one leak or another off and on in the cooling system so parts have been replaced giving even more opportunity to trap a bubble in there.

MY question is other than the symptoms I have mentioned what else will an engine do when there is an air bubble trapped in the system? It does not overheat, hell some times I really think thermostat since I can still press the hoses in a little but can feel pressure.

I feel like an idiot asking this but it has been around 20 years since I have had to diagnose a cooling issue like this.

Edit: unlike other vehicles I have owned the gauge does not show the stat opening it just goes to that temp and stays there no dropping when the stat opens and then settles. it just goes up to "N" and stays there. Not having one of these before I don't know if these gauges are sensitive enough to read the stat opening.

 

[4.0blue98]

I've done a couple water pumps, hoses, tstat, fluid drain and refills and never had an issue with air in the system in my 98. I have to admit that I can definitely see the gauge respond when my tstat opens. If you haven't done the tstat recently, might be worth a swap. It won't hurt.

 

[RonD]

Trapped air can block flow in a head causing over heating
Easiest way to refill system is to remove either heater hose at the firewall and refill via the radiator, any air will come out the heater hose or the core side.
Reattach hose when coolant comes out both the hose and the core

The Rangers use a self purging system, the overflow tank is open to the air
Any air in the system will eventually make its way to thermostat and upper rad hose and then to the top of radiator
When engine heats up enough coolant expands and creates 14psi pressure in the system
At 15psi pressure the rad cap's larger valve opens and hot coolant flows over to the overflow tank, unless there is AIR at the top of the radiator, then the AIR flows out to overflow tank and bubbles up to the top, this is why the hose is at the bottom of the tank

When engine cools down the coolant shrinks in volume and pressure drops, it gets down to 0psi, and then if ANY coolant or air was sent over to overflow tank the pressure goes to -1psi, then the smaller valve in rad cap is sucked open and coolant flows back in to replace the air, or coolant that was sent over.

So when engine is cold open rad cap and coolant should be at the very top, NO AIR at all
If so all is working well
If coolant level in overflow tank is going down then you have a leak

If there is air at the top of rad then you have a leak


If heater is not as hot as it once was then core is partially blocked, try flushing it out with garden hose.

 

[Bgunner]

Thanks for the information RonD it has allowed me to diagnose it to a thermostat being stuck open. Since I changed the upper radiator hose and put new clamps on it I have not lost any coolant and it being driven long enough now to push any possible air bubbles into the overflow container I'm left with the thermostat being stuck open.

Is it normal for thermostats to have a, what I consider, odd open temps? I have normally seen them 175°, 180°, 185°, 190° ect., by 5° increments, but the part store gave me a 198° stat which made me feel a little off about it. Is this normal for this motor?

EDIT: Looking at the parts store computer monitor the Motorcraft said 197°, not in stock and that is why I snaged the one I did.

 

[RonD]

Yes, 192 to 198degF t-stats are the recommended temps for Fords

The temp reading on an OBD device is from the ECT sensor, so its location on the engine cooling system might indicate a lower temp than t-stat rating
And ECT sensors are not "calibrated" in any way, the computer just needs general temps for choke mode and warm running modes

Coolant in the head should be running close to 190-200degF for best MPG and best oil cleaning(burns off contaminants in the oil better), this was from a long study by SAE done back in the late 1970's and 80's
This is when Ford switched from the old 180degF t-stats to the 190degF t-stats

 

[Bgunner]

I have one more question before I install this stat, Looking over the How To Tech section for replacing the stat on the 3.0 it shows a flat housing gasket. The stat I have has a rubber seal going around the outer edge of the stat to seal it to the block and the housing. Is that flat 3 holed housing gasket necessary with the stat having the rubber gasket going around it or will high temp RTV work in its place? If it needs it I'll run out and get it.