I have a 96 4.0L EL falcon that is holding pressure in the radiator overnight, when I open the overflow cap the next day(motor is stone cold) I get a bit of a whoosh out of the overflow bottle, any idea whats causing this? I did replace the cap recently, and I was assured the cap is correct pressure 120kpa I think it is, have never ever come across this problem in 30 odd years of working on my own vehicles..

PS...I should add I was having overheating issues with the vehicle, hence why I changed the cap the other day.

Anyone is welcome to chime in

If it has pressure and you take the cap off you're letting the pressure out and you'll probably feel/hear/see the pressure leaving by the unscrewed cap?

If it is over heating, have you had the radiator checked for blockages both in the air and water directions?
Is the condenser blocked stopping air from passing through the lot?
Have changed the thermostat?
Have you flushed the cooling system?
Then you have things like the water pump and that to consider...

Cheers,
Tim

Hi Tim, yes Ive replaced new radiator thermostat water pump all 3 rad hoses rad cap flushed system with heater on and new coolant. Also replaced coolant sensor and check thermo fan operation. The cooling system still holds pressure over night, If it was a BHG wouldn't the pressure subside after the car was turned off??

What's a BHG?

I thought you meant you were hooking a pressure tester up and pressurising it overnight and then finding it still had pressure in the morning... I guess it's possible for the system to retain some pressure after it cools down... It is meant to be a sealed system, so if you have a pressure differential then it's sealed...

Sounds like you've gotten everything... Only other thing I can think off is blocked air con condenser not letting air get to the rad...

If it's not that then you could have an internal blockage in the motor... You could running it without the thermostat... Maybe it isn't opening far enough or is faulty and not letting it cool enough...

Go in diag mode in the cluster and see what digital temperature is it reading, vs the needle position... Maybe it is reading right and needle is out of calibration... To get into diag mode you can reference the tech docs in the navi bar...

Cheers,
Tim

Is the car losing coolant ?

My guess would be that he means Blown Head Gasket.

Which is all I can put the problem down to also - I have an EB with the same problem, and has an entire new cooling system.

BHG into a cylinder, thereby pressurising the system. Been there, done that, got the t - shirt

Ok Ive made some progress with the cooling system holding pressure overnight. I found the new overflow cap is in fact a 135kpa one so I put the old genuine ford 120kpa cap back on and its now no longer holding pressure overnight. Also changed out the new thermostat for another new one, thought I might of had a faulty one. Ive put the dash into diagnostic mode and can now read the temp in digital form..took the car for a 15 min drive and the temp sat around 90c, highest i saw was 93.5c. Engine appears to be using a little coolant, cant see any leaks at all, did a leak down test and all six cylinders come up very good, all about 10% of each other. The coolant looks very clean so does the oil so I'm not getting coolant into the oil, no bubbles in overflow bottle either. Got a feeling I might have a very small coolant leak into one of the cylinders and its being burnt off thru the exhaust. Strange though the leak down test didn't show anything more conclusive. Have a few more tests to do.

Got a feeling I might have a very small coolant leak into one of the cylinders and its being burnt off thru the exhaust.

Most likely. Only definite way to know is a coolant exhaust gas test (CO) - a mechanic can do it, or buy an Ebay kit.
Isn't always the gasket itself, a headbolt can shear the head.