On a 240d (no fan clutch), if the belt is loose, should the fan be able to spin freely in both directions? There is absolutely no resistance when I spin it. I just installed this pump about a month ago, wondering if there is a problem and that is why I am still seeing very high temps.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

The only resistance—if the belt is loose—would be from the vanes of the water pump against the coolant. The vanes are not very impressive so the resistance is not very high.

The other possibility is that the fan is slipping on the water pump axle or that the water pump wheel with the vanes is slipping, but it very unlikely.

What are very high temperatures? Where they different before and after you changed the water pump? Why did you change the water pump?

Looks like every thing is ok now. My alternator bracket broke (it's complicated, but the bolt in the upper part of the mount froze, so I couldn't tighten it down). That led to a wiggle in the alternatorthat actually destroyed the mounting tab on the alternator and must have also impacted the water pump spin.

I replaced the bracket with one from a junkyard, and installed a new alternator and everything is nice and tight.

Left the car at idle for 20-30 min in full sun (it's 100*F here today), and it just barely ticked above 100* on the temp gage. Before it had been getting up to 120* at idle.

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

100ºC at idle, even at an outside temperature of 100F, is too high. It should be in the 80º–85ºC range.