My friend has a 99 discovery td5 he can drive 100 miles and undo the header tank cap with no hissing or water loss ie no pressure. where as mine 20 miles you undo the cap slowly to release pressure too quick you lose a bit of water and he insists I have a head gasket going. he says his is normal.

Layman view - the td5 cooling system (at least on my 53 plate) is designed to work under a degree of pressure. The pressure ensures the water does not boil at temperatures in excess of 100 deg, thus retaining it in liquid form and happily flowing around the block and ancillaries to be cooled via the radiator. I understand the header tank cap has a pressure release valve, allowing excessive (i.e. not "normal" conditions / component failure) pressure to be released to avoid damage to the system in an "over pressure" situation. So, it works up to about 1.5bar I believe (happy to stand corrected), then vents.....

Thing is, that what constitutes too much pressure can often be a very subjective view, based upon an individual's view. I have seen several references to a squeeze test on the top hose - to identify a normal operating condition - it should have a degree of resistance to squeezing after running up to temperature (i.e. thermostat open etc). This would indicate that the cooling system is not being over-pressured, say in the event of a head gasket failure. It should not be "rock-hard".

If, after a run of a good few miles, and the engine is up to temperature, thermostat open etc, you then (carefully - hot, pressurised fluid!) open the header tank cap, the pressure will be released. If you do this slowly, then it does not "erupt" spilling coolant everywhere, you just get the hiss as the pressure is released.

There are others on here that have a far better insight into such things, but simply put, if you have moderate pressure in the cooling system, and you have no leaks, and you are not loosing water, then I'd say you are probably OK. Equally, I'm no trained mechanic, so I'd look for a second opinion from this forum! There are a number of tell-tale head gasket failure tests - loss of coolant with no external visible leaks, "sniff test" on the coolant, foaming in the oil etc - again, I'd look to others for a definitive view on these.....

(If your mate's has no pressure - perhaps the problem is with his and not yours!)

If hes got no hissing at all I would say his header tank cap is duff.

At 1 bar pressure IMO hissing is not mandatory depends on how fast you undo the cap, i dont remember mine hissing and my cooling system is certainly OK

Just checked my bold statements against RAVE.

The pressure relief valve on the header tank is designed to vent at 1.4bar (20 lbf.in2), not 1.5bar as I suggested. So, given there is a pressure relief designed to operate at this point, and that there is an expansion / header tank in place, I believe the system is designed to operate under a level of pressure (up to 1.4bar), in order to reduce the boiling point of the water contained therein.

As I said, the level of "pressure" "hissing" "firmness of hoses" is all rather subjective. I think I'm happy that mine hisses a little after a run when opened, as it suggests to me that all is well and there is a level of pressure in the system, as designed.

[QUOTE="GJ1, post: 4045284, member: 138105"

I think I'm happy that mine hisses a little after a run when opened, as it suggests to me that all is well and there is a level of pressure in the system, as designed.[/QUOTE]
But that's just me, and maybe once the new alternator is fitted, I'll find that my head gasket goes next and there should not have been any pressure in the system, and my understanding of the laws of physics and LR design were all wrong!!

GJ1 said:

Just checked my bold statements against RAVE.

The pressure relief valve on the header tank is designed to vent at 1.4bar (20 lbf.in2), not 1.5bar as I suggested. So, given there is a pressure relief designed to operate at this point, and that there is an expansion / header tank in place, I believe the system is designed to operate under a level of pressure (up to 1.4bar), in order to reduce the boiling point of the water contained therein.

As I said, the level of "pressure" "hissing" "firmness of hoses" is all rather subjective. I think I'm happy that mine hisses a little after a run when opened, as it suggests to me that all is well and there is a level of pressure in the system, as designed.

Click to expand...

Yours is ok. Mate has a problem. Ask how often and how much he tops up. If the header level is correct, when hot it will be near full. If his isnt he has a leak somewhere in the system. Might not see it as it could be venting as a bit of steam.

GJ1 said:

Just checked my bold statements against RAVE.

The pressure relief valve on the header tank is designed to vent at 1.4bar (20 lbf.in2), not 1.5bar as I suggested. So, given there is a pressure relief designed to operate at this point, and that there is an expansion / header tank in place, I believe the system is designed to operate under a level of pressure (up to 1.4bar), in order to reduce the boiling point of the water contained therein.

As I said, the level of "pressure" "hissing" "firmness of hoses" is all rather subjective. I think I'm happy that mine hisses a little after a run when opened, as it suggests to me that all is well and there is a level of pressure in the system, as designed.

Click to expand...

Of course, by "reduce" I obviously mean "increase". Doh!

Thanks all. I did have some water loss but that turned out to be the fuel cooler. done around 600 miles since repair and no water loss. I have new radiator,waterpump (airtex) new genuine thermostat and new genuine filler cap.

lynall said:

If hes got no hissing at all I would say his header tank cap is duff.

Click to expand...

Either that or ironically, his mate has the head gasket problem - coolant to air leak, letting out the pressure from the system. Or a pinhole leak in a hose. There should be at least some pressure after a 100 mile drive!

For what it is worth, mine hisses when I open it and my cooling is most certainly working correctly.

cambridgecockney said:

My friend has a 99 discovery td5 he can drive 100 miles and undo the header tank cap with no hissing or water loss ie no pressure. where as mine 20 miles you undo the cap slowly to release pressure too quick you lose a bit of water and he insists I have a head gasket going. he says his is normal.

Click to expand...

You may have the beginnings of a small compression to coolant leak, but I think your friend also may have a problem, without pressure the cooling system contents will boil at a little over 100 deg C, that actual temperature depends on the ratio of coolant additive to water, the 1.4 bar pressure elevates the boiling point of correct ratio coolant to around 116 deg C ,unless you are using "waterless coolant", in which case the "unpressurised" boiling point of your coolant is around, I think, 180 deg C.
A friend of mine has a Disco 1 on waterless coolant, and with the engine stinking hot after a thrash you can squeeze the hoses and remove the coolant reservoir cap with no pressure at all.
I cannot say if running an engine at those elevated temperatures afforded by waterless coolant will have long term effects to the detriment of the engine, but it certainly saves the old boil up and head gasket failure problems usual to most 300Tdi engines.