hi have a 2009 1.9tdi, 210,000 miles it is a taxi but well maintained just recently

over past week I have noticed that mainly at night my engine idle jumps from 900rpm

to approx. 1100 rpm and my fans kick in ,in the 6 years i've had car never heard fans

on at all. temp sits consistently at 90 degrees its never overheated plenty water in tank

if I turn engine off and start up fans go off and it sits back at 900rpm, any ideas ?

thanks in advance..... mark 

Sounds like it's doing a DPF regen

Don't think the 1.9tdi ever got the dpf, so unlikely.