I diagnosed sludge on my radiator after the temp gauge started running higher in stop / start traffic.
I bought an inexpensive infra red monitor from Maplins to check the temperatures across the radiator and that the thermostat was opening correctly. On the basis that ice melts at O'c, water boils at 100'c and the 82'c thermostat opened at an indicated 83'c on the housing it was reasonably accurate and Not bad for £8.00
http://www.maplin.co.uk/p/pocket-infrar ... eter-n19fr With the engine up to temperature on the drive I took readings from the bottom of the rad upwards. (Electric pusher fan, no viscous fan) The bottom rows on the exit side of my rad were about 18'c below those at the top indicating a partial blockage. After a thorough clean top, middle and bottom were within a couple of degrees of each other.