Have an infrared heat gun to do spot temperature measurements? You'll have to remove the electric fan to do this, but take spot measurements all over the radiator to see if it has hot and cold spots. should be almost uniform in temperature. If you have colder spots in the middle or scattered around it, then its the radiator.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i