Am I missing something?

[Andrew S]

Hi All
I have made up a water pressure and leak test home made jobby which consists of a pipe from the top of teh rad to a T piece and one pipe goes to a pressure gauge and the other goes to a pipe with a tyre valve on the. Idea being I can see what pressure my system is running at when engine is running or, when engine is off and cold say, I can use the tyre pump to pressure the system to to either 13lbns which I have or 20lbs and then look for leaks.
Had engine running with temp gauge on a 1/4 as normal, pressure gauge doesn't move :0 Turn engine off and put on type pump and gauge goes up to 13lbs and then the bottle cap lifts so all working well
So, I know the gauge works so why doesn't the gauge read at all when engine is running for 10 mins at least?
Sorry if I this is a stupid question, but I have been in the sun today
Cheers
Andrew

[mike@thenook]

Once the water stops circulating it gets hotter for a while and the pressure builds as well, after around 10/15 minutes it will gradually reduce in both degrees and temperature.
When the engine is running the fan is cooling the water too..

Makes sense?.

[Andrew S]

Sort of. So, if my engine is running too cool for sale 15 mins, there’s no pressure until I turn then engine off and then after standing for a short while the pressure drops

[Phil S]

If you have a header tank fitted there will be air space there to be compressed. The system is pressurised to raise the the boiling point in the system. If its not getting that hot the pressure will not raise enough to alter the gauge, you have possibly added even more air space to be compressed in the test pipework.
put a gauge onto a sealed rad cap and blank the overflow for a better test set up, add a tyre valve to the cap if you need to pressurise it as well

[mike@thenook]

Correct - as the temperature rises for that short period after you turn off so too does the pressure

That's the end of todays physics lesson - LOL!

[Andrew S]

Thanks.
So, if a head gasket was leaking compression air into the water system, it would show a pressure straight away. Good job I’m not getting a reading

[022Dave]

Thanks.
So, if a head gasket was leaking compression air into the water system, it would show a pressure straight away. Good job I’m not getting a reading

Andrew, if you’re suspicious of the head gasket, there are ‘sniffer’ kits on the market that give an indication of exhaust gases present in the cooling system, which is probably a better/faster indicator of HGF. If the indicator doesn’t change colour, then you can start looking for other (cheaper?) gaskets that may be the cause, such as inlet manifold. However, as an exercise in pressure measurement devices, yours is up there with the best of them!

Dave

[Andrew S]

Hi Dave
Thanks, no problems with the car fortunately but I just wanted to rig up something that would detect water leaks under pressure on any other Stag and curious to see as to what pressure the system runs under.
Hope alls well in sunny Cornwall

[022Dave]

Hope alls well in sunny Cornwall

Yup; I’m still shelling out on Mr Nikon’s products….

[Andrew S]

Shareholder

[Wheelz]

Pressure depends on the airspace, and the temperature rise. The smaller the air volume, the greater will be the pressure rise.

[Andrew S]

Thanks Chris