I bled the air out of the cooling system using the method suggested below and the running water sound behind the dash stopped. I topped off the tank and drove for a full 40 mins in stop and start city traffic. According to Torque Pro, the coolant temp rose to 91 degrees C and stayed there for the rest of the trip. As it had been consistently hovering around 105, that was a welcome change.

I will replace the O2 sensors and hopefully that will clear the codes.

And yet . . . if I'm getting service engine soon lean codes, have a slow coolant leak and the vehicle idles rough does that mean I have an intake vacuum leak? Do I need a new intake manifold gasket?

Thanks again.

 

you still need to find that slow coolant leak, the number one killer of LR engines is Small coolant leaks. Beccause they are ussually a sigh of a large problem

 

If you must keep driving it, put a bottle of Barr's leaks into it.
This will slow down the leak.
If the leak is going into your oil - this may help a lot.

If the leak is going into the cylinders, the Barr's will do nothing.

Best not to add Barr's and not drive it.
But, if you must drive, use Barr's.