I just took apart a good monovalve and the spring is quite strong. The range of motion on the plunger is about 1/2 inch. I can't imagine that spring can stretch that much and loose effectiveness. Easy to check. take it out and pull on the plunger and feel how much tension is on that spring.

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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