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Presure in the radiator/overflow jug

How long or how long should it take for the presure in the radiator/overflow jug to be normal? After my call sat for about 8 hours at work i went to remove the cap from my overflow jug to add more fluid and there was still considerable pressure and the cap kinda popped off as i was unscrewing it. After 8 hours should the pressuer be pretty low to nothing?

Pressure will remain for some time, but after an hour or so you should be able to SLOWLY and carefully remove the cap.

Unless there is a leak in the system I would think it could hold some amount of pressure a pretty long time.

Used to be that the overflow jug wasn’t pressurized, and the cap was on the radiator. With very little air in the system the pressure would drop pretty quick. Now that a pressurized overflow jug is more common, the air in that jug can maintain pressure quite a bit longer.

How hot was it outside?

low 90’s…

I thought the overflow jug was vented, I’ve never seen one that wasn’t but I haven’t owned a Ford. Part of the jug has an overflow pipe that allows coolant to come out if too much goes in. This can be caused by over filling the jug or by the engine overheating and boiling over the radiator. Never saw one build up pressure. Radiators, however, can hold pressure for a long time in hot weather.

It’s not an overflow jug. Ford doesn’t use overflow jugs anymore. It’s a pressurized reservoir / degas bottle. The “radiator cap” is on top of this reservoir. The side tanks on the radiator are smaller now, so the pressurized reservoir makes up for that. There is an expansion air space in the reservoir, no there’s no overflow to worry about. However, many parts places still call them overflow jugs. I guess it’s just like how many people call R-134a Freon, even though it isn’t.

Another make that uses pressurized overflow tanks is VW (and, I assume, Audi). VW has used them at least since my '83 GTI.

Well with my engine having a headgasket issue also will create more presure inside this reservior, but i was just thinking that after sitting for 7-8 hours the presure would decline to where you could remove the cap without the hissing sound of escaping gas. If you overfill this bottle where does the fluid go?