after driving the car for a while and letting it cool back down, the top hose is flat. after opening the coolant cap, it returns to normal. any ideas?

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'92 300D 2.5
'99 E300TD

Replace the cap is first thing to try.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Also make sure the overflow tube isn't blocked by bugs.

Could also be a bad hose. When the system cools the water contracts and any steam in the system will condense. If the upper hose is degraded it will not remain round under vacuum

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!