Mitsibushi L200 (4D56) 2005 - Just had new short engine fitted (3 months ago) and now it has started overheating (at least 1litre of water for a 20 mile trip...water bubbles in header tank...Heater is under performing...Have removed thermostat but no different...

please state the type of engine fitted to this vehicle ,is it the K series engine or a BMW engine

Sounds like your thermostat isn't opening properly

You may have a blown engine head gasket. Check your oil to see if it is milky because water is getting into the oil, and check the water to see if oil is getting into the coolant.
If you have either of these, you may need to have a new head gasket for the engine, and hopefully you do not have a cracked engine head.
This will require a fair bit of work to fix correctly. Your local garage or mechanic may be your next call.
There are some products you can use in the coolant for either of these, but it is only a temporary fix.

My first question is make model and year.
Second, are you saying the engine is boiling over into the overflow tank and onto the ground ? Is the tank about half full when the engine is cold ?
Have you run the engine with the thermostat removed to test ?
Usually when you can hear the coolant moving around, you have air in the system and it is not full of liquid coolant.

For a coolant leak you need to check all the hoses and fittings by radiator, heater core, expansion tank for leaks and/or integrity. Also make sure that your water pump is not leaking along with radiator cap and thermostat body. Overheat condition has no effect on battery - if it does not hld charge it is because it is chemically worn out and needs replacement. Until you correct water loss, have extra water in car at all times and run your heater full blast - windows open!!!!

Have you checked the catalytic converter? After 20 years they do pug up causing some of the problems you describe

make sure all the air induction is in place to the radiator behind the front bumper if its not there that maybe your problem, or blown headgasket

Did you ever find a solution to this? I have a 2000 Hyundai Galloper (basically the same car) and I am having the exact same problem. I have replaced the glow plugs 3 times in the past 12 months.

Replace thermostats.