1. Car heats up faster than normal
2. Temp fluctuates between 85 - 100 deg. C.
3. Heater works fine. when on, temp drops.
4. no system leaks.
5. Upper radiator hose is quite warm upon reaching operating temp. Lower hose is
cool to the touch. Upper hose is quite
taut, but can be squeezed when engine
6. Both sides of thermostat housing are hot
but connection to lower radiator hose is cool.
7. Thermostat was replaced 1-14-00 with MB
8. Expansion tank cap was replaced 1-14-00.
9. Expansion tank was replaced 9 months ago.
10. Water pump was replaced 9 months ago.
1. When you say faster than normal. What is normal, ie. at what ambient outside temp. Is it now different in character from when you had the recent work done.
2. When the temp is fluctuating, is this at a standstill, or while driving? 85-100 C. is near the normal range for a diesel. If it is bouncing around at a standstill without anyother changes going on, this usually indicates cycling of the thermostat. While driving, it should be a bit more stable.
3. Normal, temp should drop a bit when the heat is turned on.
5. Your upper hose will usually be hot. Taut indicates pressure in the system, but not necessarily any flow of coolant.
6. Thermostat housing temp is not a reliable indicator of function. It's metal and metal will conduct heat.
Here's what I would do to troubleshoot this problem.
Make sure there is no air in the cooling system. If you have a bleed capscrew on the stat housing get the right sized wrench and open it slowly with care. If you get steam, hissing but no coolant, leave it open until it starts to dribble coolant, then retighten. If no bleed screw, remove cap from expansion tank prior to starting engine and watch what the coolant level is doing while the engine is warming up. If your coolant level is correct and you see some bubbles or hear gurgling, squeeze the upper hose and see if this causes the coolant level to fluctuate. If it does, and the coolant level stays on the mark, then you've probably got air in the system. A Cool lower hose can mean a couple of things, but basically there is no flow through the radiator. With the engine at operating temp, shut it off and see if you can literally feel your way across the radiator from the top hose to the bottom hose. With no flow, the rad will be generally cool. With restricted flow, it will have some warm areas near the upper hose.
Bottom line here, Just because the thermostat is new doesn't mean it has to work. It either the stat or air probably as you just had this work done a couple of days ago and it's really not likely your radiator suddenly plugged itself spontaneously. Hope this helps.