Pressure in cooling system.
Thread Starter
Cruising
Joined: May 2023
Posts: 11
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Pressure in cooling system.
Hello. I have a 68 C3 327 automatic. The cooling system gets a lot of pressure when the engine is cold. Heats up very quickly but does not boil. It's fine to drive a trip with a lot of pressure. If I release the pressure when the engine is warm and put the lid on, the pressure does not return. Before cold start. Shouldn't be like this. Desperately need advice here.
Thread Starter
Cruising
Joined: May 2023
Posts: 11
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Hello and thanks for the reply. I have original radiator. Haven't checked if it's clogged, but looks good, and the lower and upper hose gets hot. But that's not a problem when I've released the pressure once.
Race Director
Joined: May 2006
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The bottom of the radiator cap should have a small valve that permits air to enter as the engine cools. Check the radiator cap. Look at picture of bottom of cap
Radiator Cap
Radiator Cap
Last edited by MelWff; May 15, 2023 at 12:47 PM.
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whats the cap pressure relief valve setting it should be on the top of the cap. Ive run 16 psi caps with no issue
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Cooling systems are always under pressure. Even when cooled off. It's the pressure that raises the boiling point of water. Without a proper Rad Cap, things would begin to boil at 212*.
Reading your opening paragraph Kjande, I don't see anything wrong. Rad Caps pressure specs run from 14 - 18 lbs of pressure. Beyond their maximum, the relief valve built into the cap releases and the coolant discharges into an overflow tank. Then in most cases, the coolant returns to the Rad when cold via the small hose on top of the Rad.
You never stated and no one asked, but what does your temp gauge show?
Reading your opening paragraph Kjande, I don't see anything wrong. Rad Caps pressure specs run from 14 - 18 lbs of pressure. Beyond their maximum, the relief valve built into the cap releases and the coolant discharges into an overflow tank. Then in most cases, the coolant returns to the Rad when cold via the small hose on top of the Rad.
You never stated and no one asked, but what does your temp gauge show?
Race Director
Joined: May 2006
Posts: 18,703
Likes: 2,572
Cooling systems are always under pressure. Even when cooled off. It's the pressure that raises the boiling point of water. Without a proper Rad Cap, things would begin to boil at 212*.
Reading your opening paragraph Kjande, I don't see anything wrong. Rad Caps pressure specs run from 14 - 18 lbs of pressure. Beyond their maximum, the relief valve built into the cap releases and the coolant discharges into an overflow tank. Then in most cases, the coolant returns to the Rad when cold via the small hose on top of the Rad.
You never stated and no one asked, but what does your temp gauge show?
Reading your opening paragraph Kjande, I don't see anything wrong. Rad Caps pressure specs run from 14 - 18 lbs of pressure. Beyond their maximum, the relief valve built into the cap releases and the coolant discharges into an overflow tank. Then in most cases, the coolant returns to the Rad when cold via the small hose on top of the Rad.
You never stated and no one asked, but what does your temp gauge show?
Melting Slicks
Joined: Dec 2014
Posts: 2,778
Likes: 2,305
From: Central UT
Hi kjande, as others have stated, pressure in a cooling system is your friend - up to the point of the pressure rating of your radiator cap, which in your case should be 15 lbs I believe. If your radiator does not have an overflow tank (our '68 does not), any 'excess' antifreeze will exit through the 'overflow tube' that runs from under the cap down the inside of the inner fender, to the ground. Once your radiator settles in on the 'right level' you won't have any excess antifreeze to be concerned about. Without an overflow tank, your radiator level will settle around 1-2" below the cap.
I was getting our '68 ready to crank up for the first time in 6 months last weekend. I opened the radiator cap to check the antifreeze level and was happy to hear pressure release when I cracked the cap - the system held pressure during the winter.
If you're running decent temps during the summer and don't have to continually top off your radiator, then you probably have nothing to worry about at all. If not, we can help with overheating, leaking, and other cooling issues - you just need to let us know how it's operating. Best, Paul
I was getting our '68 ready to crank up for the first time in 6 months last weekend. I opened the radiator cap to check the antifreeze level and was happy to hear pressure release when I cracked the cap - the system held pressure during the winter.
If you're running decent temps during the summer and don't have to continually top off your radiator, then you probably have nothing to worry about at all. If not, we can help with overheating, leaking, and other cooling issues - you just need to let us know how it's operating. Best, Paul
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Thread Starter
Cruising
Joined: May 2023
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I know the cooling system should have pressure. But here there is a lot of pressure after a few minutes before the engine is warm. Correctly perceive that this is not a problem, but something is not right. Have today measured out whether there is exhaust gas in the cooling system, it is not. Took out the thermostat and it got a lot better. Tested this one and it's ok. When pressure builds up, the hose becomes incredibly hard. But if I release the pressure, it won't come again until I start with a cold engine. Has worked professionally with cars for many years. Something is wrong. I bought the car in November of 2022, didn't use it that much... Can't remember it being that way then. If the hoses don't crack or be damaged by this and crack eventually, I can just have it that way. How is your C3 1968 327 engine after a few minutes after cold start?
Race Director
Joined: May 2006
Posts: 18,703
Likes: 2,572
I know the cooling system should have pressure. But here there is a lot of pressure after a few minutes before the engine is warm. Correctly perceive that this is not a problem, but something is not right. Have today measured out whether there is exhaust gas in the cooling system, it is not. Took out the thermostat and it got a lot better. Tested this one and it's ok. When pressure builds up, the hose becomes incredibly hard. But if I release the pressure, it won't come again until I start with a cold engine. Has worked professionally with cars for many years. Something is wrong. I bought the car in November of 2022, didn't use it that much... Can't remember it being that way then. If the hoses don't crack or be damaged by this and crack eventually, I can just have it that way. How is your C3 1968 327 engine after a few minutes after cold start?
Thread Starter
Cruising
Joined: May 2023
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Sorry about that... but don't see this to be the problem as long as it's just cold. Once the pressure is released, it never stays that way again no matter how long I drive. but going to check... must eliminate everything....
Race Director
Joined: May 2006
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The valve that I'm referring to only works as the engine cools down to draw air back in.
Racer
Joined: Sep 2011
Posts: 381
Likes: 90
From: Portland, the original!
These statements seem at odds with both the way it should work, and with each other.
1) you mention when you release the pressure it doesn't come back. Very odd. Do we have a definitional problem? The way it reads to me, you come to start the car first thing in the morning and if you were to remove the radiator cap it would sound like opening a can of coke. A whoosh of pressure released. If that were the case I'd definitely check the pressure valve on the rad cap as previously mentioned.
2) you say there is "here is a lot of pressure after a few minutes before the engine is warm". Well if the engine has run "a few minutes" it might not register much on the temp gauge, mine doesn't, but there will be more pressure than when cold because it has warmed a bunch from ambient.
3) "Once the pressure is released, it never stays that way again no matter how long I drive" I
"Took out the thermostat and it got a lot better. Tested this one and it's ok. When pressure builds up, the hose becomes incredibly hard. But if I release the pressure, it won't come again until I start with a cold engine"
No pressure in a balloon, it is soft. Add pressure and it becomes hard. Pressures go up, keeping boiling point/temps down. Indeed that is how it is supposed to work.
You say you have no combustion blow by so everything seems in order here save the fact you keep opening pressurized cooling systems.
Team Owner
Joined: Jan 2007
Posts: 53,648
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From: Southern Cal Ca
St. Jude Donor '22
try this
fill rad 1/2 way, level should be pump high
cap off
start engine take choke to low idle
turn heater temp to full hot
add coolant to pump level
the first thing that happens is the burp
coolant may come out filler
after burp add some coolant
wait for the thermo drop
fill add cap
fill rad 1/2 way, level should be pump high
cap off
start engine take choke to low idle
turn heater temp to full hot
add coolant to pump level
the first thing that happens is the burp
coolant may come out filler
after burp add some coolant
wait for the thermo drop
fill add cap
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Sounds like you are describing a restriction somewhere in the cooling system that is not allowing a complete circulation.
Quote:
Took out the thermostat and it got a lot better.
That might be the restriction. A sticking thermo.
Quote:
Tested this one and its ok.
I assume the thermostat was placed in a pot of hot water and you monitored at what temp the thermostat began to open using liquid thermometer.
If you buy a new thermo, drill a couple 1/8" holes in the flange. This will allow air & coolant to bypass with little effort.
Quote:
Took out the thermostat and it got a lot better.
That might be the restriction. A sticking thermo.
Quote:
Tested this one and its ok.
I assume the thermostat was placed in a pot of hot water and you monitored at what temp the thermostat began to open using liquid thermometer.
If you buy a new thermo, drill a couple 1/8" holes in the flange. This will allow air & coolant to bypass with little effort.
Racer
Joined: Sep 2011
Posts: 381
Likes: 90
From: Portland, the original!