Check radiator top tank/hose and bottom hose temperatures and also run it up from cold with the cap off. The bottom should be cooler than the top, but not a whole lot. I got something like a 25 degree (F) difference at idle after a few mile drive last time I checked. If the top is blazing while the bottom is still cold this is usually a sign of a blocked radiator. However, read on.

Usually a bad water pump will leak, but in some instances the impeller fins erode to the point of the pump being ineffective without developing an external leak. You should be able to see coolant moving in the radiator after the thermostat opens. Also, it seems from Pelican that a '77 300D does have a fan clutch. Verify the fan is operating. If not this will lead to eventual overheating at idle even with everything else working right.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.