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Possible overheating problem

Discussion in 'Land Rover Discovery' started by Menash, Sep 3, 2020.

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  1. sierrafery

    sierrafery Well-Known Member

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    IMO that's impossible as long as there is coolant in the tank cos the coolant's pressure will always be above the ambient pressure even if it's residual after the engine cooled down.(provided the cap is not faulty)
     
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  2. Menash

    Menash Member

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    Believe me i trust your opinion and want to feel confidence that this is the situation. Sorry, but the last 6 months thoght me i can' t really trust my car, at least not yet. I am trying to get to a point witch i am fixing things before red lights turns on
     
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  3. NPG

    NPG Well-Known Member

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    Well not exactly because air making its way into an engine cooling system is not governed by barometric pressure alone. For instance, coolant pressure is constantly changing, not just with temperature but also with variables such as fluid velocity, engine speed and the cross sectional area of the conduits. Gravity will help too and it's not the first time that one sees puddles of leaked coolant under parked cars; coolant that has been displaced by air.
     
  4. sierrafery

    sierrafery Well-Known Member

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    I really doubt that air can get through a hairline crack into a pipe which has fluid in it under pressure , if air can get into a system which is full of coolant the leak should be very serious IMO not like in the OP's case
     
    Last edited: Sep 7, 2020
  5. Menash

    Menash Member

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    I believe that during the time that the engine works and the system is under pressure air will not get inside the system but when the system is cooling down and the pressure decline and going back to 0 the air will get inside and might be a problem the next ride
     
  6. sierrafery

    sierrafery Well-Known Member

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    The system is sealed at the top by the tank's cap and as long as that side is not open the fluid will not come out when the pressure drops cos the vacuum doesnt let it ...like in a syringe
     
  7. Stanleysteamer

    Stanleysteamer Well-Known Member

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    Ok, forgive my ignorance but doesn't the pressure cap do exactly that? If the pressure gets too high allow air above the coolant to escape, rather than pop the pipes and tubes off? And then let air back in once it cools? But only until the system reaches a certain pressure? Or is this too old skool?
    (This is what old ones used to do. Hence the spring.)


    OK, tin hat on time!
     
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  8. NPG

    NPG Well-Known Member

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    Hence why I said But this depends how small is small... in post #40.
     
  9. NPG

    NPG Well-Known Member

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    That's a good question that I could never figure out 100% - is the cap a one-way or a two-way valve ? I do notice sometimes when the cap is opened with the engine stone cold that you could hear a sound like air rushing in due to negative pressure in the system. The cap would also be quite hard to undo, like it's being sucked in, which could only be explained if the cap is a one-way valve
     
  10. Stanleysteamer

    Stanleysteamer Well-Known Member

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    But the caps are a renowned failure point, so need renewing every now and then, so it has been said.
    On some French cars, if coolant ever gets through an expansionn tank cap you have to replace it, FFS! Happened to us once.
    Some I do think they ought to let air back into the system, but i am hampered by thinking about things in old skool ways.
    Frinstance I have owned many cars with no expansion tank. Those always had a cap with a spring. The spring pressurised the system and when the pressure in the system overcame the spring pressure, the coolant went through the base of the cap and flowed out of an overflow pipe. when cool, the valve in the cap allowed the air back into the system. But you lost coolant that way, which is why the expansion tank was invented.
     
  11. sierrafery

    sierrafery Well-Known Member

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    The cap has definitely a one way valve in it, which IMO is logical as it's there to release pressure and as it's sealed it will not let coolant leak out from the system without any pressure... here's the description from the WSM

    " ....The expansion tank is fitted with a sealed pressure cap. The cap contains a pressure relief valve which opens to allow
    excessive pressure and coolant to vent through the overflow pipe. The relief valve is open at a pressure of 1.4 bar (20
    lbf.in2) and above..."
     
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  12. Stanleysteamer

    Stanleysteamer Well-Known Member

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    So it does contain what I thought it should.
    Don't see why you think it is illogical. :(
     
  13. sierrafery

    sierrafery Well-Known Member

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    Maybe i misunderstood your other post then sorry
    English is my third language and i might not get subtilities :rolleyes:
     
  14. Stanleysteamer

    Stanleysteamer Well-Known Member

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    No, not at all, I can understand it having either a one way or a two way valve. As I gave the explanation about the two way valve and also what happened when a more modern car (French) was fitted with just a one way valve.
    I think, in the end, if there is enough air space in the expansion tank, there is much less need for a two way valve, as the air space gives a kind of safety margin. In the old systems with no real airspace the two way was needed to ensure rads etc didn't collapse under atmospheric pressure after an overheat and coolant having escaped via the tank cap and valve. But that led to air moving back into the rad instead of coolant, so cars were driving around with less and less coolant in, if there was an overheating problem.
    With the expansion tank, if an overheat happened, air, more than coolant would leave the expansion tank and when things cooled down there would be coolant in the tank which would move back into the rest of the cooling system. It would take quite a massive overheat for the expansion tank to empty of coolant and thus for only air to get back to the rad etc.
    I cannot really say when the change to an expansion tank happened all I know is that it must have been in about the 1970s as I rebuilt a kit car in the late 80s and converted the rad and system to one with an expansion tank using bits from 1970s vehicles. But Minis right into the 90s never had expansion tanks.
    Interesting!:):):):):)
     
  15. Stanleysteamer

    Stanleysteamer Well-Known Member

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    Also my post #50 is a bit clearer.;)
     
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  16. NPG

    NPG Well-Known Member

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    That is clear and I expect that RAVE would be 100% accurate on something like this. But the question remains - what happens when the engine cools down after an overflow discharge and the combined volume of the remaining air in the expansion tank and coolant in the system is less than it was before the overflow. Since it appears that the cap won't allow air in to replace the fluid loss, two things come to mind - either the system operates with negative pressure due to the partial vacuum until the air and coolant heat up again and expand. Or else hoses will deform inwards so the system can achieve pressure equilibrium in line with the reduced volume.
     
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  17. sierrafery

    sierrafery Well-Known Member

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    That's what happens, i've seen it myself
     
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  18. Stanleysteamer

    Stanleysteamer Well-Known Member

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    I didn't really think I would be interested enough to look any further into this but you have now intrigued me!
    Tubes deforming is better than the the rad collapsing inwards!
    But nonetheless shows the need for a two way valve! It can't do tubes any good to be collapsing and reinflating. Shows the old fashioned weekly checks of coolant level still have a place in one's maintenance!
    It's all Boyle's Law in the end. Hardly rocket science.
    In the old systems with brass rads and steel caps with springs and rubber seals, the fit of the cap to the rad was crucial to avoid coolant boiling if pressure was not maintained in the system. Have spent many happy times filing the tops of rad openings and changing caps to get pressures right, and seals of course.:)
     
  19. Stanleysteamer

    Stanleysteamer Well-Known Member

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    Wonder if this is some of the reason why tanks crack?:oops:
     
  20. sierrafery

    sierrafery Well-Known Member

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    As long as the system is OK there's no reason for coolant nor air to be released through that valve so once it worked as to make the hoses to shrink there is a problem already. In normal conditions the pressure in the system doesnt exceed 1.2 bar to activate that valve
     
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