Cooling System Vacuum
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Fleet Mechanic
Joined: Sep 2006
Posts: 1,491
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From: Fresno, CA
Cooling System Vacuum
Yesterday was checking some things on the truck and noticed the coolant hose between the thermostat and radiator was collapsed. Pulled the cap and the cooling system was actually under vacuum. Engine had been sitting all night, fully cooled off. Anyone ever seen this before? I think the cap is just a pressure release, not vacuum release.
I did just replace the radiator last week. I had opened the cap with the engine hot when checking for air and I'm guessing when it cooled the pressure was allowed to go into a vacuum as the coolant will contract when cooled off.
All radiator hoses in system are 6-month old Motorcraft, look in fine shape. Same for the degas bottle & cap. Radiator just got replaced.
I can't think of anything that would pull the system into a vacuum besides the thermal affect so it's probably due to me opening it when hot. Guess I've proved that the system is tight!
I did just replace the radiator last week. I had opened the cap with the engine hot when checking for air and I'm guessing when it cooled the pressure was allowed to go into a vacuum as the coolant will contract when cooled off.
All radiator hoses in system are 6-month old Motorcraft, look in fine shape. Same for the degas bottle & cap. Radiator just got replaced.
I can't think of anything that would pull the system into a vacuum besides the thermal affect so it's probably due to me opening it when hot. Guess I've proved that the system is tight!
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Fleet Mechanic
Joined: Sep 2006
Posts: 1,491
Likes: 295
From: Fresno, CA
So the system is still pulling the upper hose into a vacuum when it cools down. Also it feels like there isn't any coolant in the hose during this case.
I'm wondering if I've got an air pocket stuck somewhere in the system. Looking at the coolant flow diagrams it appears the only real spot that could hold air is the heads. The outlet to the intake is on the side of the head and not at the very top so I could have a pocket above that.
I've tried running a higher rpm for an extended period (2750rpm for 10 minutes) when I was going up a hill figuring that might push the air out but still happens. And the cold coolant level didn't change so I don't think that did anything. I seem to recall some procedure that uses vacuum to suck the air out of the system?
Need to get my coolant pressure sensor installed & hooked up. I'm wondering if the cap is blowing off at too low of a pressure. It's a 2yr old Motorcraft unit. That would definitely explain it.
Truck has no cooling problems so far. I'm doing my first camper tow of the season this weekend so we'll see how she does.
I'm wondering if I've got an air pocket stuck somewhere in the system. Looking at the coolant flow diagrams it appears the only real spot that could hold air is the heads. The outlet to the intake is on the side of the head and not at the very top so I could have a pocket above that.
I've tried running a higher rpm for an extended period (2750rpm for 10 minutes) when I was going up a hill figuring that might push the air out but still happens. And the cold coolant level didn't change so I don't think that did anything. I seem to recall some procedure that uses vacuum to suck the air out of the system?
Need to get my coolant pressure sensor installed & hooked up. I'm wondering if the cap is blowing off at too low of a pressure. It's a 2yr old Motorcraft unit. That would definitely explain it.
Truck has no cooling problems so far. I'm doing my first camper tow of the season this weekend so we'll see how she does.
Fleet Mechanic
Joined: Nov 2016
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I recall having a spring like winding inserted into coolant hoses. It would prevent collapsing.
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Fleet Owner
Joined: Apr 2012
Posts: 27,307
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From: Spanaway
The spring would be in the outlet side of the radiator.
The cold side. That should be the lower hose. I don't
recall if there is a spring in it or not.
The cold side. That should be the lower hose. I don't
recall if there is a spring in it or not.
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From: Foothills, CA
On some vehicles the spring on the intake side of the water pump (outlet of radiator) was to prevent the pump from collapsing that hose with suction - that's the reason for them that I recall...
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