P38A Water lose.

Hi all my p38 RR is losing water from the expansion tank , ive noticed a white powderyish substance around the cap and when it leaks its not a lot could this be a air lock or even a head gasket going but theres no water in the oil its nice clean oil, any help would be appreciated. ohh its a petrol 4.6 on lpg and its a 1999 reg on a t plate.

The tank caps are designed to release at a certain pressure so the tank or system doesn't blow. Could be as simple as a faulty cap, I'd start by replacing that.

Just found my leak (Disco 2) and it turned out to be a hose.
I initially thought expansion tank cap but it wasn't.
Have you tried putting a plastic bottle on the end of the blow off pipe from the expansion tank and running to temp. If it doesn't catch any water then its not the cap.
If it is the cap then happy days but if not then lets hope its a hose (time to get a torch and asbestos gloves out).

One part to check with an lpg car is the vaporiser seal is ok and not split letting gas pressure into the coolant, and pull the spark plugs one by one and see if any look like they have been steam cleaned as that will mean gasket.

Check the throttle body heater,they seem prone to leaks. They can be bypassed with a piece of pipe connesting the hoses

Plus your heater matrix O Rings check for wet/damp carpet driversside , all hoses mine seems to like dripping from the thermo stat hoses think iv cured that one at last time will tell easiest way to find a coolant leak is to put some fluro dye in the cooling system run it a bit then use the UV light the dye comes with to spot the leak.