that would be your turbo boost line which runs from the back of the manifold to the overboost protection valve on the firewall. you need to reconnect it. that 'red' thingy would be the top cap of your trans fluid dipstick.

for further info on the boost line you can search the diesel forum for 'banjo bolt' and 'alda' . plenty of reading there.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP