Although this is an older post, thanks for posting your experience on loosening the MerControl relieving the warm-up lever. I also wiggled and loosened mine, in frustration, and had the same positive result. I thought it was only a coincidence
until I saw your post reply. It seems these old worn controls have quirks and can use a good smack and maybe with a BFHammer now and then. Did some research on it saying it's designed to work only in neutral. Yes it works that way and if/when it pops straight up to 'take over' when your shifting from forward or reverse, like when you're pulling up on the trailer at the dock, you'll be creating some new choice 'words' when they're all staring at you! However, Contradictory to MFG Claims, it Does work In Forward gear going at idle, as you may have noticed and can be used to maintain Idle if necessary.
