TB48 idles up when hot

[jaysee]

I've had this problem for years. When I'm doing low range work and its hot (greater than 35C) or its a hot day and I'm is slow traffic, the engine idle creeps up to approx 2000rpm (difficult driving is peak hour traffic) . It drops to normal if I drive at normal speeds. The temp guage does not register the engine is hot, infact it always sits at about half way at ALL times. I've flushed the radiator and used new coolant without change. There is no indication the engine is overheating etc. The viscous fan works well as does the two electric fans. I dont know if the electric fans are working when it idles up. Any ideas

Its a 2002 4.8 GU manual patrol.