I used dark gray packing foam from computer shipments stuffed between the two center vents to keep the center flap door open. Removing the door is definately a permanent fix. The center vents can be manually shut off with the thumbwheel at the vent opening as well and it's 100% reliable.

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'96 C220 138,000mi, '95 E300D 239,000 mi., '87 300TD 214,000mi '88 6.2 Turbo Diesel Chevy Conversion Van 253,000 mi.