After close to a year since the problems first came creeping I have finally solved the problem (or so I sincerely believe with my whole heart etc.... =8-o )

Patient: Defender 1999 TD5, 10p engine. multi-oil conversion for running on whatever liquid fat (Elsbett, DE), 275 000 km, no major rebuild.

june 2015:
Went to Sweden on holiday, daughter got sick, return home on motorways (speed limit 110 km/h, for 1 hour). 10 km from home engine boils through coolant cap. Stopped, cooled engine down, refilled water, got home w/o problems.

Enter: New thermostat. New cooling fan.

july 2015:
Went on holiday. After "cruising" (with a Defender...) @ 90 km/h for 2h, same problem. Boiling engine. This time there is a hissing noise from around the thermostat. After cool-down the problem reappears after ca 30 minutes @ 80 km/h.

Workshop reports: Radiator has uneven temp. distribution. Aulro.wisdom: replace coolant cap.

Both done. Intercooler severely corroded - replaced by myself.
Workshop reports: Have run car on mountain hillways for 200km - no problem. Myself; motorway test for 1hour/ 100km/h - OK.

Running the vehicle home (250 km) at motorway speeds (80 - 110 km/h) for 4 h. After 3.5 h, problem reappears. Engine boils through the coolant cap. Hissing noise from around the thermostat.

Limps home (30 km)...

I finally stopped listening to others and pulled the whole external cooling system down. It appeared that the bottom coolant hose had been sligthly moving against the bottom plastic clamp keeping it away from the bottom pulley, eroding material. The hose had become thinner and thinner at a point the size of 2mm and eventually gave way to a leak when the cooling curcuit became highly pressurized - read: motorway speeds for extended periods.

I changed the hose - problem solved (I hope). Moral: ?

Espen Olsen
Norway