E46 325i cooling problem only in standing traffic

[Herman Toothrot]

Discovered my up until now totally reliable 325i can't cope with traffic jams, I found this out on the M4 about 100 yards off the M25 tonight in the standstill traffic. Luckily traffic was so bad it was safe to open the bonnet on the hard shoulder and let it cool until traffic started moving again. I then got off the M25 and into a retail park to investigate, coolant level low, took a litre - presume as it got hot spitting some out, Halfords to the rescue and topped it up, read how to bleed it online.

Decided not to continue the journey until traffic thinned, did the remaining 100 miles as if there was never a problem - but then there was no standstill traffic.

I presumed it's a lack of a fan kicking in issue as only happened when at a standstill but the big fan on the rad seemed to be spinning happily.

Anyone got any suggestions for how my investigations should begin? Never had to lift its bonnet until tonight.

[Herman Toothrot]

Yes it's an auto, I shall start with the fan.

[Herman Toothrot]

Cheers - just seen a guide and looks an ok job ECP has an EIS fan coupling for £36. I take it the rolled up paper test is try and stop the fan?

[Herman Toothrot]

Yes it's got AC, I've ordered the vicous fan coupling as £36 may as well do it and looks an easy job, away for the weekend so can't test it.