I wonder if the gas pocket is in the radiator, not the engine?

Maybe the heater core is supposed to always have air, too. It is located at the highest position in the system after all.

This explains the purpose of the little hose that connects the top of the radiator with the side of the reservoir.

 

Air is a gas in case your unaware.

No amount should remain in the system. The system is force purged from flow/circulation BUT. The main problem areas that can trap a pocket within these systems is the core hoses because of their height relation compared to the degas bottle. Some use gizmo's attached these hoses to purge, - personally, I just raise the front end positioning the degas bottle higher than the hoses. It's just quicker and less messy for me since I the tools.

The worst spot that locks a pocket that can take forever to purge is when the engine has a ECT sensor. They eventually did away with them, but some gen 10 V8's do have them. The 5fours had them deleted in the 99 models. The 4sises had them deleted in 01. At times, the only way to purge that bubble or pocket is the crack the senor to bleed. Same method used bleeding diesel injectors. You crack the sensor loose until coolant flows from it. It's the only way since the models that have these sensors use a sensor block that is too tall. Ford redesigned a replacement block for the vehicles that have one. In order to get the much improved block, you have to order a new one. From there you'll get it, - yea the old style has been discontinued for quite some time. Anyway, if your system uses an ECT, sometimes you have to crack loose to purge...only way.

I guess the worst thing about all that, it can cause a fail safe mode default. For those who aren't aware of the issue, it may initiate a WTF moment and cause an unspecified amount of hair loss.

 

I dont really understand the problem with some trucks
On my 03 my procedure for filling coolant is like any other vehicle on road

add coolant
drive a few miles to open tstat
let cool down overnight, add more coolant to cold fill line

I never had a single instance of issues with my truck

I understood one of the 4.x engines was a bear for some reason

perhaps inadequate water pump flow, tstat, plugged heater core, etc reduces flow to point it cant flow enough to purge the air?

When I had bad HG, I could see foam circulating for several minutes after Tstat opened...Until the engine warmed and sealed the leak, and all the entrained air separated out in the radiator. It only took a few minutes to go from foam circulating...to no foam.

 

I know. Through the years I've heard many (not just a few) that do not have a problem with these systems. I think it's around 50/50 if I had to guess. Some just fly through normal procedure not a problem but man, it seems some of these trucks are tuff to purge.

When you flush, do you always fill through through the T-stat housing first? The Ford documented procedure?

 

These two statements seem to conflict and I don't know which is correct:

It seemed like I got all the air out after I finished working on it last week, but then two days later I had another temp spike and it blew a bunch of coolant out the pressure cap again. If it had truly been full of coolant (no air) this should never have happened, correct?

Maybe this system works properly ONLY when the water pump is new, but fails when the pump loses its ability to create enough pressure to "move the air pockets" out of the heater core/hoses and into the top of the radiator or the reservoir.

 

You must have posted this while I was writing my last post. I had the same thought. Maybe this is why some trucks have this problem while others do not. Differing amounts of restriction combined with the relative efficiency of the pump?

But no matter how good the pump is, it's not going to purge any air from the engine/heater core until AFTER the tstat opens.

I did NOT do this when I refilled my system after servicing it (I didn't know this was the recommended procedure). But this makes sense because it should allow air out of the engine/heater core before the tstat opens. I should try this tomorrow ...

 

Nope
Fill thru reservoir

I squeeze the hose to the radiator a bit which helps to displace air somehow

I can see the idea that system cant fill thru closed tstat, but wiith the little bypass hole it does, by squeezing hose at least

 

Thats why tstat has the bypass hole.
It has to always have some flow or hot. Fluid will never reach the tstat, and it wont ever open. Its located in spot where gas shouldnt be able to hang up and vapor lock pump either. Corrosion or debris or sealant could plug hole though or make jiggle pin stick

 

Its not common...but you can have a bad brand new tstat. New oem motorcraft on my reman wouldnt get above 168F. And it wasnt stuck open, it was closed at rt. Open when hot. Just too open. Replaced with stant superstat.....195 like it should be.

 

I used a Stant 195 because I thought it was the best. I did not test it before install, but if I fill from the tstat housing tomorrow I'll test it.