Dalek caps and pressure release

[ghost123uk]

Sorry, a bit long winded coming up but I reckon the answer might be worth finding out for all of us.

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OK, I just posted the following to "Kat&Dougal" re their problem =

"A bit worrying as if the system is pressurising enough to do the things that have happened, the dalek cap should be venting excess pressure into the top up tank (behind the number plate flap - this tank has vents to the outside world at the top).

I had this once and it turned out to be 2 things.

Firstly the thin metal coolant pipe that runs around the lip of the engine bay (on post 85 models) has "T" pieces in it.
On mine, the "T" piece that fed cooling water to the L/H head was totally blocked with crud.
This caused the head to overheat so much that it boiled whatever water was in there causing a big build up of pressure.
The Dalek cap did not vent the pressure and mine kept bursting hoses etc."



Now mine has been OK since then (2 years ago) but during a routine check I discovered that my genuine VW Dalek cap, bought for Brickwerks, was not right.
I could blow & suck the vent pipe in both directions.
As it was a genuine VW one I was a bit miffed.
I ordered another and it just arrived.
This one allows me to blow down the vent pipe, but not suck (as it should be).

Now to the question =

Many of us will be familiar with the older type "radiator cap" that blew off at 13 psi (iirc)
Do our Dalek caps have similar "blow off" system, I suspect they must but would just like confirmation.
Reason I ask is because over the years I have read many posts on here (such as Kat&Dougal's here = http://forum.club8090.co.uk/viewtopic.php?f=37&t=85079" onclick="window.open(this.href);return false; and an older one here = http://forum.club8090.co.uk/viewtopic.p ... 4&start=30" onclick="window.open(this.href);return false; ) where it seems that a build up of pressure from whatever reason has resulted in failed seals / pipes / radiators etc. I would have thought that in these circumstances the cap should "blow off" and vent pressure into coolant into the top up tank ? (making it obvious that there was excess pressure).



Call me paranoid, but I don't want to have this happen to me again and suffer the hassle of blown pipes (or MUCH worse, blown head to case seals ).
It happened once to me 2 years ago and cost me a HUGE amount of hassle and money, culminating in a new engine

Comments invited...

[Ian Hulley]

Do our Dalek caps have similar "blow off" system

Excess pressure is released into the top-up tank, this is why it may be over the 'max' mark when checked with the engine hot. As the system cools and pressure drops coolant is drawn back along the hose, through the cap back into the header tank.

Ian

[ghost123uk]

Do our Dalek caps have similar "blow off" system

Excess pressure is released into the top-up tank, this is why it may be over the 'max' mark when checked with the engine hot. As the system cools and pressure drops coolant is drawn back along the hose, through the cap back into the header tank.

Ian

Aye thanks Ian, I did understand the principal (though your words will no doubt help others who don't )

I am just puzzled by the fact that some owners seem to have suffered from a build up of excess pressure that has damaged things (rather than the pressure venting into the top up tank)

[Ian Hulley]

Classic symptom of a failed cap or a blockage elsewhere as you had.

Ian

[jamesc76]

or more likely cause for filed pipes or joints is due to the age of that part, and even if the cap is ok, it will all ways find the easiest part to blow!

[ghost123uk]

or more likely cause for filed pipes or joints is due to the age of that part, and even if the cap is ok, it will all ways find the easiest part to blow!

Yes I wondered about that, perhaps a perished coolant pipe or even a perished head to case seal might let go before the cap reaches it's blow off point ?

Do others agree with that hypothesis ?

[Ian Hulley]

Certainly agree with that.

[California Dreamin]

Talking in general, pressure caps typically blow off at around the 1 bar mark (14.5psi) which is a fair amount of pressure. So in practice any weakness in the system (a hose/seal/plastic pipe or housing) that can't withstand this nominal pressure will fail before the cap reaches it's blow off point.
And not wishing to teach granny to suck eggs but pressurizing the system does serve very important functions ie: to increase the coolants boiling point which enables the coolant to absorb more heat energy (because it can go over 100C without boiling)
Boiling coolant contains gas bubbles which do not conduct heat away from hot areas of the engine so you get localized hot spots which lead to seizing and engine damage generally.

Martin

[ghost123uk]

Talking in general, pressure caps typically blow off at around the 1 bar mark (14.5psi) which is a fair amount of pressure. So in practice any weakness in the system (a hose/seal/plastic pipe or housing) that can't withstand this nominal pressure will fail before the cap reaches it's blow off point.
And not wishing to teach granny to suck eggs but pressurizing the system does serve very important functions ie: to increase the coolants boiling point which enables the coolant to absorb more heat energy (because it can go over 100C without boiling)
Boiling coolant contains gas bubbles which do not conduct heat away from hot areas of the engine so you get localized hot spots which lead to seizing and engine damage generally.

Martin

That's good info Martin,

Whilst we are on the subject there is one thing that has puzzled me from way back, and that is, if as most vehicles do, an 88 degree C thermostat is fitted and assuming the rad and rad fan are working correctly, under what circumstances does the coolant in the system get to over that temperature ?

[Hacksawbob]

Talking in general, pressure caps typically blow off at around the 1 bar mark (14.5psi) which is a fair amount of pressure.

Thats puzzled me, 1 Bar is effectivley 1 atmosphere which means there is arguably no more pressure than the outside air pressure not a lot of pressure, no? Just its sealed system rather than open to air.

[ghost123uk]

Talking in general, pressure caps typically blow off at around the 1 bar mark (14.5psi) which is a fair amount of pressure.

Thats puzzled me, 1 Bar is effectivley 1 atmosphere which means there is arguably no more pressure than the outside air pressure not a lot of pressure, no? Just its sealed system rather than open to air.

I think it means 1 "bar" above atmospheric pressure

[jamesc76]

Talking in general, pressure caps typically blow off at around the 1 bar mark (14.5psi) which is a fair amount of pressure.

Thats puzzled me, 1 Bar is effectivley 1 atmosphere which means there is arguably no more pressure than the outside air pressure not a lot of pressure, no? Just its sealed system rather than open to air.

I think it means 1 "bar" above atmospheric pressure

all gauges used in the garage world, show it as 1 above 2 above its just a given!

[Plasticman]

Whilst we are on the subject there is one thing that has puzzled me from way back, and that is, if as most vehicles do, an 88 degree C thermostat is fitted and assuming the rad and rad fan are working correctly, under what circumstances does the coolant in the system get to over that temperature ?[/quote]


There are several reasons/circumstances , but say your going up a long long hill in a low gear in hot weather and laden well your engine will generate heat and the radiator and cooling system does not have the capacity to reduce the temperature of the coolant,
this excludes faults such as head gaskets, low coolant,blocked rad/pipes, partially stuck t'stat etc
mike

[Red Westie]

Whilst we are on the subject there is one thing that has puzzled me from way back, and that is, if as most vehicles do, an 88 degree C thermostat is fitted and assuming the rad and rad fan are working correctly, under what circumstances does the coolant in the system get to over that temperature ?
Think of the coolant as two seperate bodies of liquid, the first quantity circulating around the engine and the second in the radiator. Both are kept seperate by a thermo valve (thermostat) when the engine starts it begins to heat up the engine coolant until that coolant reaches the opening temperature of the thermo valve (thermostat).
At this point the thermostat doesn't just open and stay open, rather it opens and closes many times as the valve is cooled down by the cold coolant being introduced until a point when all the coolant is now at or above the opening temperature of the thermostat (82 -84C is common)
Now....if the heat energy from the engine exceeds the cooling capacity of the total body of coolant and the radiators capacity to impart that heat to the passing air the coolant temperature will keep on rising ie: driving in a low gears with full throttle, so the engine is producing lots of heat energy but the relative low speed means the air speed going through the radiator isn't sufficient to impart enough of the generated heat to the passing air.
This is where the electric fan kicks in on first speed to increase the air passing through the radiator in an attempt to improve the cooling capability of the radiator.
First speed may not be enough and despite the additional movement of air and improvement in heat disipation the temperature may continue to rise further as heat being generated exceeds heat being disipated (first speed clicks in at 87C) this is when the second much more powerful speed kicks in (93C) and if it keeps on rising now! well you've had it lol.
So in answer to your question....
under what circumstances does the coolant in the system get to over that temperature When the heat being generated exceeds the cooling ability of the radiator...and that would normally be at higher ambient temperatures (warm day) engine being worked hard but with reduced are flow passing through thr radiator (slow speed)

Note* 2.1 wasserboxers and 1.6TD's use a larger radiator than standard because these engines variants have the potential to produce more heat so need a radiator with greater cooling capacity.

Martin

[Plasticman]

Yep and dont forget that a/c's that have been converted to water (either oil or petrol) dont have the raised tunnel to give better airflow out of the rad.
mike