The mistery of the water system, no pressure

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SpaceDawg

SpaceDawg

Active Member
Posts
935
Location
Cowfold, West Sussex
Hi,

Ok just chucking this out there, not because I have a problem with it, but to see your views.

Its my 200TDI, it runs pretty cold just over the upper dot on the temperature gauge and on a motorway run after 30mins+ at 60+ it creeps the halfway up the gauge, no further.

I have no leaks. I can put my pressure tester on it at 1 bar pressure and it will stay there for hours at that pressure.

However, when I drive the car there is not really any pressure in the system. There are no leaks and its not using water.

It had an oem water pump 6 months ago, every hose, radiator and heater matrix replaced even pressure cap on the water reservoir.

No water is getting in the oil, no gases in the water, had gasket is good, head ok.

WHY DO I NOT GET ANY PRESSURE!!!!

Sorry about the capitals, I'm a little miffed, the only this I have not done is replace the thermostat I can't believe its that, oh I do have a spare.
 
It sounds like the radiator filler cap rubber might have gone, which then doesn't allow the pressure to build up.
They are dirt cheap and dead easy to replace ... or you could get a brand new cap.
 
If the pump wasn't working the water temp would get very hot, very soon, as it relies on the circulation through the radiator to keep it cool.
 
SpaceDawg said:
Okie's I'll replace that as well. How do you test if the pump is working, or do I assume it would have over heated.
Click to expand...

As stated above fit a new cap with the correct pressure rating for your vehicle.

You could check the circulating coolent by looking in the top of the rad when it's cold, other then that you will hear or see a faulty water pump and the pump has nothing to do with pressure, the coolent temp has... Google and you shall find out why. :)
 
As the temperature rises within the sealed system it increases the pressure, the pump simply circulates the fluid around the cooling circuit, including the radiator, which regulates the internal temperature via the thermostat.
Imagine how a steam engine works, where the pressure is generated by increasing the temperature of the water from cold, through boiling to supreheated, which then drives the pistons.
 
I had simialr after my new rad,

I tested the pressure on mine with a short length of hose on the overflow bit of the header tank, left the cap in place, clamped the small hose from the rad then fitted my gadget to the header and pumped it up with a bicycle pump until the pressure release valve in the cap hissed. Then I left it, but it only maintained pressure for a short while.

I used UV due to see if I had leaks, which i didn't. I assumed that the cap had a slow leak from the rubber seal but had no way to test it so injust swapped it out and now it's all fine, I get firm hoses.
 
As stated before, it's the increase in temperature that causes the water to expand and create pressure. The water expands into the expansion tank. It does this against the sealing pressure of the expansion tank lid, which is controlled by the spring in the lid. You should be able to see the pressure rating written on the lid "9lbs" or something similar. If the seal on the expansion tank lid is anything but good, you wont get pressure build up. If you're not losing water and the hoses go hard when hot, your system is working properly.
 
Zephod, yup got that, I'll check the cap its a plastic one on mine. It will be the 3rd one I have tried when I get one so I'll see if the original was duff the replacement was duff as well with another one.

Oldseadog said:
Isn't there a local motor shop anywhere near you?
Even a Halfrauds? http://www.halfords.com/webapp/wcs/...989&subset=2.5&fuel=Diesel&findPartsMMYS=Find
Click to expand...

Yup got one of them near the house.

After this little thread the only things I can thing it will be is the pressure cap or its just not getting warm enough.
 
Dont forget 200 takes an age to hit operating temperature and I have found if you get the stat open and then give the engine an easy time for a couple of miles the stat will close again as the lump cools down.
Both of mine take almost 4.5 miles until the stat opens fully, this is through town for 3 miles then 50 limit for 1 mile and as we get 1/2 mile into the 70 limit up a long hill away she goes the heater starts to work, obviously quicker if straight onto a fast road where you can make the engine work
 
Ok, update here, I replaced the old thermostat with a 88c bearmach one. The temperature of the engine has now gone up from 1/4 of the gauge to 3/4. The heating is really working and the engine is running fine. There is some pressure as well. Its the first time since having the car that the heating works and I mean its warm in the car. I'll run the car for a while tomorrow.

I'm not used to the temperature gauge sitting at 3/4 rather than 1/4 I have no idea if it should sit there. The pipe from the top of the rad to the engine is now hand hot as well.
 
Last edited:
Ok 30 miles on A roads done, temp guage stable at 70% heater working really well, like toasty, and no water loss, also pressure in the system at last. Lets see how it goes.
 
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