Hose pressure on 1.9TD

XantiaMan · Post by **XantiaMan** » 25 Jun 2008, 20:31

How much pressure can i expect at normal running temperature? Mine hasnt got any pressure at all, which is pointing at the expansion tank cap being faulty which should help ease the temperature gauge going up this time of year.... seen it hit 90 but only after severe thrashing and most of the time it sits just below 80, and lower with the A/C. Doesnt use any water or oil either and stat is not that old.

Gareth

AndersDK · Post by **AndersDK** » 25 Jun 2008, 21:51

Ehhrrr ...

How would you read the pressure ????
You should have a pressure pretty close to 1bar, which is the standard for almost any watercooled engine.

The radiator usually still holds the pressure cap, which then leads off excess coolant to the header tank when the coolant heats up and pressurises. Since you then have no header tank UNDER the radiator cap, there is no compressed air that gives you the wellknown "hiss" when releasing the radiator cap. Only pressurised coolant which will ease pressure when 1 (one) single drop (or even less) is released !
The only thing that can make the coolant "flexible" to flow out - is the flexible coolant hoses.

The pressure cap is double acting - such that vacuum generated when the coolant cools off and retracts instantly opens the cap and lets the coolant seep back from the header tank.
Thats the whole idea with a separate header tank after all.

citronut · Post by **citronut** » 26 Jun 2008, 00:02

Anders if this car is a XANTIA it dose not have a rad cap at all, it only has the cap on the header tank, i think the type of system you explain is one with an expansion tank not a header tank, but as you did say i would usualy expect to have a very slight escape of presure when removing the header tank cap,

with this car i would not be unduley concerned,
regards malcolm

XantiaMan · Post by **XantiaMan** » 26 Jun 2008, 07:57

AndersDK wrote:Ehhrrr ...

How would you read the pressure ????

Very simple Anders. Removing the expansion tank cap after running should give a healthy hiss noise but not excess pressure that would cause the water to overflow. The top rad hose should also feel hot and have some resistance if squeezed, rock hard could indicate potential HG failure.

As mine has no pressure at all ( and i dont need anything to actually measure it), i needed to know what is considered normal behaviour for the system. I suspect on a healthy engine there should only be minimal pressure in the hoses and a small hiss when cap released.

I'll find out later when i pop to Citroen and get another cap. My car seems to get hot quickly, by hot i mean it runs above its normal temperature very quickly if sat in traffic, or if pressing on when overtaking. I'm hoping the cap replacement will solve this, as without any pressure my cooling system could quite easily boil over and have localised hot spots.

Gareth

citronut · Post by **citronut** » 26 Jun 2008, 13:24

if the cap was loosing presure you woul;d also loose coolant
regards malcolm

AndersDK · Post by **AndersDK** » 26 Jun 2008, 18:40

Well I certainly do mess up things from time to time dont I

You are certainly right Malcolm.
The TD radiator has no build-on sidetanks, but is connected to a separate header tank, which made me think of it as an expansion tank system.
This separate header tank also functions as a de-aereator.

Still it functions much the same way as I explained - except the cap only vents air in or out according to temperature (and thus pressure).
Only after topping up too high a level in the separate tank, any excess coolant will initially be expelled first time the engine is heated up.

The problems I could think of with this type of system are :
1) a leaking or defunct pressure cap
2) missing rubber seal ring under cap
3) a leaking header tank (cracks in the plastic moulding)

There would be no leaking rubber hoses or Headgasket as this would evidently lead to very noticeable coolant fluid loss.

On a final note :
Any healthy cooling system will have no noticeable "hiss" from releasing pressure - if it is left for some time to safely cool off before trying.
Meaning : only wait some 5mins before you test for a "hiss" on a ready stopped hot engine, as from then on the retracted air (by cooling) will cause vacuum in the header tank and cooling system.

If you are interested in whats really going on - then any standard engine oil pressure sensor switch will give an open contact switch at 0.6-0.8bar of pressure, which is very close to the approx 0.9-1.0bar of cooling system pressure.
The engine oil switch works well in coolant no problem and is a cheap standard part.

Post by **Peter.N.** » 27 Jun 2008, 10:49

The pressure builds up as the temperature rises, once at normal, it will stay constant (unless you have a leaky head gasket) if there is a small leak above the water level, leaky cap for instance, the pressure will drop to practically nothing. The reason for the system being pressurised is to raise the boiling point of the water thus making the system more effective but providing you dont thrash it and get the temperature above 100C it won't boil.

XantiaMan · Post by **XantiaMan** » 27 Jun 2008, 22:50

New cap fitted, and much more consistant gauge behaviour.

I've seen the gauge hit 90 before but back off at that point. Are the fans meant to run at full pelt at that temp? They only ever seem to run at half speed, ive heard the 'about to take off noise' only twice!

Gareth

XantiaMan · Post by **XantiaMan** » 01 Jul 2008, 17:29

Confirmed on the hose pressure now, you can still press them a little but they definatlly have pressure, and i've not lost a drop of water, so all promising.