mine will stay on the big white mark at about 10-o-clock position, then after about 8 miles with go to just under have way when driving, as soon as revs drop/start driving sensibly/stop and leave at idle the gauge drops down to the 10-o-clock position, if i leave it at idle for twenty minutes approx it goes upto halfway and thats where it will stay, i've had it running at idle for about 4 hours and it won't go over halfway on the gauge.

p.s. mines a 04 tdci 130