Coolant System Pressurized When Cold

Discussion in 'Street/strip 400/430/455' started by accelr8, Sep 15, 2022.

  1. accelr8

    accelr8 Well-Known Member

    Any idea why my coolant system is pressurized when the car hasn’t been started for several days?

    Here’s some background … I’m trying to solve a high coolant temp. issue that only happens at idle and in stop-and-go traffic, so I drained the coolant, removed the thermostat, and filled the system with water and a bottle of Thermocure. That was last week on Wednesday. I probably put 150 miles on the car between Wednesday and Monday. Monday night I parked the car. Wednesday of this week I decided to check the fluids before heading to the track. When I opened the radiator cap (car hadn’t been started for two days) water sprayed everywhere. The next part probably isn’t the smartest thing to do, but I scratched my head, cleaned up everything, topped off the radiator and headed to the track. I made four passes. The car ran great to, from, and at the track. I parked the car at about 8:00 pm last night. Today I drained the water/Thermocure mixed (which was pressurized again) and that’s where the car sits now.

    I did contact Evapo-Rust (makers of Thermocure) just to see if their product would somehow cause the system to pressurize ... and they said no.

    One friend theorizes that a head gasket isn’t sealing properly which is allowing a cylinder to pressurize the coolant system. He also thinks this could be causing my high temp. issue.

    Any thoughts?

    Thanks!

    Jim
     
  2. DaWildcat

    DaWildcat Platinum Level Contributor

    Seems like we had a recent thread about this, I'll have a look.

    I'd think no to a head gasket; initially I suppose cyl compression could pressurize the cooling sys for a bit, but I'd think that pressure should just equalize in a few moments past the rings in the other direction.

    Devon
     
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  3. DaWildcat

    DaWildcat Platinum Level Contributor

  4. DaWildcat

    DaWildcat Platinum Level Contributor

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  5. Schurkey

    Schurkey Silver Level contributor

    The pressurization cannot be coolant-heat related. When the heat is gone, the pressure would be, too.

    I suspect a cracked casting, possibly a head gasket. System pressurized via the leak between combustion chamber and cooling system when chamber has high pressure due to combustion.

    In other words, the thousand-psi of low-viscosity combustion gasses can get into the cooling system, with the rad cap limiting cooling system pressure. The 16 psi of higher-viscosity cooling system liquid pressure can't get into the combustion chamber.

    If you're LUCKY, this is a head-gasket problem. I think it's more likely to be a crack. Leakage past either could be affected by heat. Perhaps the leak "seals up" when hot...or when cold.


    The idea of "trapped air" in these older vehicles is largely myth. If the rad cap is the high-point of the cooling system, any "trapped air" will show up as a low coolant level.

    Similarly, if the rad cap is working properly you're unlikely to see retained pressure based on coolant temp. Excess pressure when hot blows-off via the rad cap, when cool the system either pulls in air or coolant depending on how the overflow bottle is plumbed, or if it even exists. But if the cooling system is pressurized when cold--by combustion leakage--then the cap would retain up to it's blow-off pressure.
     
    Last edited: Sep 15, 2022
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  6. accelr8

    accelr8 Well-Known Member

    Devon,

    Thanks for the response and the link! I read that post and that seems like a different problem than mine. The OP removed the thermo vacuum switch prior to lowering the coolant level in the system. So it appears it was just a matter of the coolant equalizing it's level once the thermo vacuum switch was removed. In my situation the "coolant" sprayed out of the radiator cap opening, which is the highest point in the system ... indicating that it is under pressure.

    One more detail that may be relevant is I never had this problem prior to flushing the system last week.
     
  7. Matt Knutson

    Matt Knutson Well-Known Member

    What sort of radiator cap are you using ( pressure rating)? What temp is it rising to at idle and stop/go traffic? Does the fluid circulate when you run the engine with the cap off?
     
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  8. Max Damage

    Max Damage I'm working on it!

    The Chassis manual has a simple test of running the overflow hose form the radiator into a clean jar of coolant and then starting the car. If the engine is pushing gasses through the cooling system, you will see bubbles coming out of the overflow hose.
     
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  9. DaWildcat

    DaWildcat Platinum Level Contributor

    Excellent point regarding a gasket leak/crack behaving like a one-way valve due to a drastic pressure difference.

    Devon
     
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  10. accelr8

    accelr8 Well-Known Member

    24 lb. cap (I didn't realize this until now ... seems very high)
    The temp. seems like it would keep climbing if I didn't start driving. Prior to this problem (3 months ago) it got to 220* so I pulled over to let the car cool down, because I couldn't get out of the stop and go traffic. After I put the flush stuff in last week it may have got a little bit above 200* a few times.
    I flushed it once awhile back by putting a hose in the radiator and disconnecting the upper hose and letting the water pump pump out the coolant.

    I'll have to get a lower lb. radiator cap and try this. I'm also considering doing a leakdown test.

    Unfortunately, I agree that this is an excellent point. Below is a picture of the water/Thermocure mixture I just drained. I'm not sure how well you can see the color, but to me it looks a lot more black than a rust color ... which to me would indicate Schurkey is correct.

    IMG_3245.JPG
     
  11. 12lives

    12lives Control the controllable, let the rest go

    Radiator shops have a device to check for combustion gases in the coolant. It can sometimes over-pressurize and "hammer" the radiator and cause it to blow out. Usually it is a head gasket. The flush may have loosened a deposit that was sealing the head gasket leak??? So it held 24 psi and you found out when you opened the cap! You may see more coming out with the correct cap.
     
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  12. accelr8

    accelr8 Well-Known Member

    Okay, I just read this article and now I'm thoroughly confused. I think my radiator cap rating is way too high and I think I overfilled my radiator ... so this article and Schurkey's comments make sense to me.

    So where to begin? I'm thinking a leakdown test first. If that checks out, then fill the coolant system properly, replace the cap, and see if the problem still exists. I guess I can also get the water/Thermocure mixture tested to see if there's any indication of combustion gases getting into the coolant system. Thoughts?
     
  13. DaWildcat

    DaWildcat Platinum Level Contributor

    Logical steps if you ask me, and taking a shot at a correct cap isn't a major investment.

    Devon
     
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  14. Max Damage

    Max Damage I'm working on it!

    Neither the high pressure cap, nor the overfilled radiator should cause the overheating and pressurization you described...

    The leak down might not show you anything, as it really doesn't develop the same heat/pressure as a running engine. It's really more to determine if your loss of combustion chamber pressure is due to poor rings or poor valve sealing. I would skip that.

    You could do a compression test, but that seems like it might not be informative either.

    Testing to see if combustion gases are getting into the cooling system should be the first move in my opinion. If they are, the engine needs to come apart to determine why.
     
    Last edited: Sep 16, 2022
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  15. rkammer

    rkammer Gold Level Contributor

    Start with the correct 15 or 16 lb. cap. Go from there. After driving enough to fully pressurize the system in my new 455 motor, it will hold just a little pressure after a few hours and sometimes after overnight there is a slight "poof" when I remove the cap. I consider this a non-issue.
     
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  16. DaWildcat

    DaWildcat Platinum Level Contributor

    Sorry, when I heard "leakdown" I was thinking of cooling sys pressure test.

    Devon
     
  17. accelr8

    accelr8 Well-Known Member

    Thanks for the responses!

    I think I'll get the coolant tested first. I've never had that done before, but I'm assuming it's relatively inexpensive and definitive ... either there's combustion gases in the fluid or there's not. Getting a new cap seems reasonable as well, whether or not it's related to the problem. It's inexpensive and the current 24 lb. cap seems unnecessary.
     
  18. Stevem

    Stevem Well-Known Member

    Man, some of you folks are really thick!

    What I am posting here assumes you have burped the air out of your filled up cooling system.

    it IS a issue and is NOT normal for a Cooling system to hold pressure for days on end!

    If you had a bone dry engine and cooling system and then let’s say you filled it all up on a 80 degree day and then ran the motor up to the temp that was needed to fully open the the stat and then stabilized the overall system temp then you would have also reached a normal system pressure.

    when you then allow sufficient time to get the whole system back down to the fill up temp of 80 degrees there should be no pressure in the cooling system.

    In fact if the temp when down enough to below the 80 degree temp and you then cracked the seal the rad cap you would hear vacuum pulling air into the system if the coolant was not right up to the inside bottom of the rad cap!
     
    Last edited: Sep 16, 2022
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  19. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

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  20. accelr8

    accelr8 Well-Known Member

    I just ordered the block tester kick. Thanks for the link Larry.

    Unfortunately, I do think there's a bigger issue going on, because I don't understand how a closed system can increase in pressure once it returns to its original temperature. (As others have mentioned.)
     

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