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Why is there a disappearing bubble in my upper radiator hose?


pentode

Well Known Cheapskate
Ham Radio Operator
Joined
Apr 30, 2024
Messages
862
City
Moncton NB
Vehicle Year
2005
Engine
3.0 V6
Transmission
Automatic
I did my timing cover a while back and I've been watching the system since then and it does a couple interesting things.
First off, I realize now that the truck was never building any pressure before the fix, and now it obviously does, which is good. One morning I went out and checked when the engine was cold and the upper rad hose was still hard, which I found strange, and when I opened the rad cap coolant sprayed out. Just that one time, hasn't done it since.
However, I've noticed a few times since then when I checked in the morning that there's obviously an air/gas bubble in the upper hose. You give it a shake and you can hear the coolant splashing around. Also, the coolant level in the reservoir is half an inch higher than normal.
Here's the fun part though. Obviously my first thought is I have a head gasket going. But I'll use the truck for a couple hours and check the next morning and the coolant level is back to bang on and the bubble is gone. This has happened a few times now.

So, why is this bubble forming? If it's not a head gasket (which there is no other evidence it is, coolant level is perfect long-term) then it's gotta be sucking air from somewhere right? The only thing I can think of is rad cap. But then where is the air going and how is it returning to normal? Anyone have any other theories? This is mostly a thought experiment at this point.

EDIT: Oh I guess I should add, yes, it was burped properly at the start, no bubbles.
 
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I would probably try a new radiator cap and check the overflow hose for cracks or holes.
 
You know what? I've been thinking cap so much I didn't really think to look carefully at the hose. Getting old. 😀
Just scratching my head trying to figure out how air is getting in and then back out with no drips.
 
If its not overheating id leave it be.
 
You know what? I've been thinking cap so much I didn't really think to look carefully at the hose. Getting old. 😀
Just scratching my head trying to figure out how air is getting in and then back out with no drips.

When the cooling system gets to temp, it pressurizes and pushes excess coolant by the cap, through the hose to the overflow tank. After stopping, the system cools back down and creates a negative pressure which should pull the overflow coolant back into the radiator. An old janky overflow hose can let coolant out just fine, and without leaking, but after cooling off it allows air in, instead of pulling the heavier excess coolant fluid back in.

It's easier to pass gas, than liquid. :)

For a couple of bucks, you can get a new piece of hose and two squeeze clamps. Best case, it stops your mystery bubble, and worse case you've done some preventive maintenance.

I have run into this on my trucks, and a new overflow hose has fixed it.
 
I understand the physics of it- or most of the physics anyway 😉 I just haven’t run across a situation where a hose that leaked air wouldn’t leak water too. The intermittent thing was throwing me too. Never too late to learn something new I guess! I’m going to tweak that hose and see what changes.
 

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