Author Topic: negative pressure in cooling system?  (Read 537 times)

Offline zieg85

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negative pressure in cooling system?
« on: January 08, 2022, 09:09:51 AM »
My current daily has this issue I have never seen before.  The truck runs fine, heats up normally, has excellent heat, builds up normal pressure and the temp gauge is in the normal range.  What I find questionable is after I shut it off and it cools down it sucks the upper radiator hose flat.  Would this be a faulty radiator cap.  It has the overflow container hooked to the radiator.  As I write this I am thinking DUH... just change the cap.  Anyone else run into this issue?  It is a 95 F-250 with a 7.5L not that it should matter.  Thanks in advance!
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline LILT77

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Re: negative pressure in cooling system?
« Reply #1 on: January 08, 2022, 10:17:48 AM »
Sounds like the cap to me. If you want to test that first, take the cap off when this happens (be careful obviously) and see if the hose returns to normal.
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Offline zieg85

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Re: negative pressure in cooling system?
« Reply #2 on: January 08, 2022, 10:53:48 AM »
Sounds like the cap to me. If you want to test that first, take the cap off when this happens (be careful obviously) and see if the hose returns to normal.

It does, it holds pressure fine.  It has a Motorcraft radiator cap.  I guess the vacuum created would normally suck the antifreeze back in the radiator but my cap is sealed so well it sucks the hose flat.  Maybe someone put the wrong cap on for a sealed system.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline JohnnyPopper

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Re: negative pressure in cooling system?
« Reply #3 on: January 08, 2022, 02:32:31 PM »
Probably a wrong or defective cap.

When you have it off, does it have a spring loaded disc in the center, that you can tug off the rubber seal?
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1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Online bd

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Re: negative pressure in cooling system?
« Reply #4 on: January 08, 2022, 03:10:40 PM »
Is the recovery bottle on that truck vented or is it a completely sealed system?  Otherwise, I agree.  Replace the cap.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline zieg85

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Re: negative pressure in cooling system?
« Reply #5 on: January 08, 2022, 07:29:22 PM »
Is the recovery bottle on that truck vented or is it a completely sealed system?  Otherwise, I agree.  Replace the cap.

Recovery bottle lid is not a sealed type, the lid just snaps down.  I'll change the cap and see what happens.  Thanks
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/