Thanks for the replies everyone. It's an 87 with the m103. A little more detail about the problem: it runs hot ONLY with the a/c on and stuck in traffic (idling). The coolant reservoir, cap and level sensor are brand new. Sensors in cylinder head are original (how do I unscrew the green three pole one, there's no room for a wrench or crow foot?). Fan clutch is new and works perfect. water pump has about 70K on it. How do I know if I have the right thermostat? it normally runs just slightly above the 80 degree celsius mark. Currently running Prestone 50/50 prediluted mix (I know, I know!!) Coolant flush and refill with the mercedes stuff coming soon... Did I mention it has 212,000 miles on it?