jorjio_k

jorjio_k

Active Member
hello gents;
Please i am inquiring about that cap,what i know is that at 14 or 15 psi or so,the valve inside of it will open to reduce the pressure,i was wondering that when the car is turned off after a long drive,the valve must open to vent the pressure?or not at all the pressure is reduced somehow?and when it throws coolant (not in my case) it means coolant has rised and it is ejected or the over pressurizing will throw the coolant and air from pressure to the outside? :confused:
thank you gents
 
jorjio_k said:
hello gents;
Please i am inquiring about that cap,what i know is that at 14 or 15 psi or so,the valve inside of it will open to reduce the pressure,i was wondering that when the car is turned off after a long drive,the valve must open to vent the pressure?or not at all the pressure is reduced somehow?and when it throws coolant (not in my case) it means coolant has rised and it is ejected or the over pressurizing will throw the coolant and air from pressure to the outside? :confused:
thank you gents
Click to expand...

Erm..

The valve as you say opens when the pressure reaches 14 or 15 psi. Pressure will icnrease after a drive because
a, the pump will have pressurised the coolant and
b, air at the top of the header/expansion tank whatever you want to call it will expand due to being heated. It is that pressue that is release if the expansion tank cap is removed before the vehicle has cooled.

Coolant gets ejected when either the expansion cap is remove before sufficient cooling or the vehicle has overheated and boiled the coolant.

See a kettle boiling if you need to see where the extra air then comes from ;)
 
okay so,as per boiling of the water in the kettle vapors starts no doubt about that,but then these vapors do they get released from the system through the expansion tank at a 15 psi,what if the system reaches 13 psi for example how the vapor will be released afterwards in case this valve wont open,how the system will depressurize when the engine is off for 10 hours after a drive (lets say from evening till morning)
 
the coolant cools down and the steam condenses back to coolant..... its a closed system if it is working as it should
 
okay so you think at this temperature the vapors can be condensated again to coolant?that is IF it is working as it should,with p38 we are lucky to have the system just work lol.
 
When a liquid (or any other meterial for that matter) gets hot it expands.

As the liquid then cools it contracts.Water is fairly unique in that when it then gets colder to below freezing it expands again (hence why pipes burst)

So the internal coolant pressure increases as the coolant temp rises, then the pressure reduces as the coolant cools again.

This is eplained in the Combined Gas Laws (P1V1)/T1 = (P2V2)/T2

As temperature increases in a fixed volume, then Pressure has to increase to balance the equation.

There should be no steam generation in the closed system that is the coolant circuit as this would lead to hot spots due to steam pockets which would not allow for efficent heat transfer.

The reason the coolant boils over when you release the expansion cap before it is cool is because as the pressure increases with the coolant system, the boiling point increases (the same principle is why a keetle boils at a lower temeperature at altitude - the lower the pressure the lower the boiling point - I could bore you with why the bodys blood system will boil above a certain height!!) so when you release the cap, the pressure is reduced immediatly causing the coolant which is above it's natural boiling point to boil immediatly and expand rapidly and shot out of the tank.
 
Last edited:
so here comes a new question,forgive me but me and physics never got along well,why with my old cap coolant was still under pressure in the morning,and with my new one it is not anymore?is it :
a.the old cap wasn't releasing the pressure in case it got above 15
b.something is still fooked in the coolant system after replacing the h/g
c.Just check the coolant everyday and drive the feker?
 
I believe the answer would be (A)

If the cap was faulty the pressure could increase beyond its contraction pressure....i.e: The System increased to 20psi, over night it contracted to 5psi, the increased to 22psi, then cooled to 7psi, the heated to 25psi then contracted to 9psi and so on....the rate of expansion and contraction will vary due to a million and one factors including contaminents, ambient temperature etc etc....

The system is designed to reach 15psi then reduce to close to atmosphere again, any over pressure may not be fully reduced.

If you are concerned, best to get the engine up to temp, then open the bonnet and listen for leaking pressure from the new cap, if it is still over 15psi and venting, there could be additional issues.
 
ok professor nice feedback.
there is no hissing when engine is running,all what i know now is that i feel good but dunno till when,because everything was changed in the cooling system from hoses to gaskets to water pump tstat,etc...,except one tiny hose that sits under the plenum it is leaking a tiny little bit,but i am anxious to replace it.
cheers sir
 
I like the Professor title...may have to request a name change..!!

It is a pleasure good Sir to be of some help, sounds like you have changed most of the culprits....change the little plenum heater pipe when you can for true peace of mind....

Relax and enjoy the ride, we all know something is going to go wrong with a P38, so try to just enjoy it until the next bridge to cross arrives.

I wish you fair seas and following winds....
 
Hat down for you professor,i will simply just enjoy driving it till it blows apart,then what the heck a new block so be it,i will keep it,but believe me driving it with one eye on the road and the other on the temp gauge is a pain in the back,lucky camelions each eye moves in a separate direction,guess we have to look at things like them with the p38 lol
 
jorjio_k said:
so here comes a new question,forgive me but me and physics never got along well,why with my old cap coolant was still under pressure in the morning,and with my new one it is not anymore is it?

The last time I had pressure in a (3.5efi) system after an overnight stand it was a failing head-gasket.
Click to expand...
 
Head gasket was done and the heads skimmed,the liners have a marginal slipp barely noticed,guess having some pressure is normal after skimming the heads,the important thing is there is no overheating but adding up some coolant once per week a tiny bit is happening,because i believe that the plenum hose is leaking a bit.dunno
 
Jorjio,

Have you checked that the little pipe that goes from the radiator to the expansion tank is clear ? If it is clogged it will pressurize the system because the pressure never reaches the expansion tank to be released by the cap.
 
mcapinha said:
Jorjio,

yep it is checked,sometimes tiny particles are blocking the smal hole inside the tank, the coolant needs flushing,i keep on poking it every morning,it is a must do.the funny things is that something deep inside of me tells me,the block must be replaced i keep on hearing that voice,make it stop,make it go away lol
Click to expand...
 
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