I think that is a fancy name for radiator cap or in this case, coolant tank fill cap.

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Jim

I changed my coolant several years ago and decided to change tstat for assurance. The replacement was a behr. Same identical one I replaced. The filling procedure was through the radiator hose manner. Drove the car and temp was about 95c. I did try to purge air but no result. I finally ordered another tstat and repeated. Almost the same result. Same manf same temp. I finally took the three tstats and did the thermometer hot water test and the two new tstats opened 10c higher than the old one. I cleaned that baby up and it is still in the car and doing about 75-80c. Test the tstat first.
Bud

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1987 300D Turbo, 175k mi., 1998 BMW 323i Convertible, 1997 F250 4x4 7.3L PSD

I'm still trying to track down the cause of my overheating. Before I began any of this the car would only run hot on long steep hills in 1 and 2 gears. The problem was manageable by turning on the heat. I decided I want to fix it and now the car overhears in normal driving conditions. This is what I have done thus far: remove old thermostat replace with new (checked both in boiling water, old was ok but new was more responsive to temp changes). Next I refilled radiator through expansion tank, ran car with heat on and added more fluid, then I disconnected the return line to expansion tank and filled head with fluid to bleed. Then I replace expansion tank cap. Drove car and it was overheating worse than when I started. Next I replaced the fan clutch. No result. Then I ran the car until it was hot, poped the hood, hit the kill switch and the fan just spun, clutch was not engaged. I put my hand on the radiator and it is cool to the touch. There is pressure on upper hose and it is hot to the touch, lower is not. The cabin heat blows really hot, so I'm assuming my water pump is working. One odd thing I noticed was that with the car warm and running if I disconnect the return line to the expansion tank no water is coming out of it, this makes me wonder about the water pump. Also I broke the prong off the sensor that attaches to my water pump, what does this sensor do? Is it possible that my water pump is bad but still have heat in the cabin? Does anyone have any answers?

I haven't followed your saga and I don't know what car you have but I'll try:

1) Buy a MB brand clutch. Nothing else seems to last more than a few revs, Even ACM and eespacially Uro brand.

2) If you have an 87 300D, TD or SDL, try bypassing the oxidation catalyst.

3) The reservoir is part of the pressurized system but it isn't part of the priimary cooling loop. The small line is more of a balance tube than a flow tube. Most of the transfer occurs through the connection at the lower radiator hose.

4) Your radiator might be clogged. If you have an 87 300D, TD or SDL and the radiator is 10 years old, some would say to replace it as the plastic necks are ripe to break off.

5) If you have an 87 300D, TD or SDL, the switch by the water pump is for the 50*C switchover valve for the vacuum amplifier (blue flying saucer) it has no influence on engine cooling.

Sixto
87 300D

have you thought about running a flush through the system? if your rad is plugged or gunged up it may not be cooling as well as it could. also, I dont know if it's possible but check to see if your water pump is pushing enough coolant it might be on it's way out or internally damaged.

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

It's a 87 300d w124. What is the oxidation catalyst? Does having cabin heat mean my water pump is working?

US spec 87 300Ds came with a trap oxidizer between the exhaust manifold and turbo. It's an emissions device that traps soot and whatnot. The trap was prone to failure sending debris into the turbo. MB issued a recall to remove the trap and replace it with a downstream oxidation catalyst. It looks like any other catalytic converter aft of the transmission. The oxidation catalyst can clog causing lack of power, overheating and in the extreme a no start condition.

Sixto
87 300D

no. it means your AUX pump is working though. I'd do a Heavy flush of your cooling system and go from there.

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Going to do the citric acid flush next week, quick question

Consider doing this--

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Oh yea, I've removed the trap ox. The cooling system as an axuilary water pump for cabin heat? Can my water pump be working at only 10 percent? I figure it would work or not work.

you would have to check and see, what coolant are you running? the green stuff or the MB stuff? I would reeaalllly consider doing the flush, see what comes out and see if replacing the coolant helps use the MB stuff or Zerex G-05 Coolant (the MB Equivalent)

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

I'm using peak brand, not mb brand coolant

I would Flush it. Go from there.

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed