I had the same issue several months ago, I found mine to be the Vacuum line from the rear of the manifold that feeds the HVAC controls on the dash. A simple wire tie fixed it so it would not pop off.
1987 YJ, 258, Weber 32/36, HEI, Custom Sound bar, Hardtop/Sun-rider/bikini tops, 2" Lift, 1/2" Boomerangs, 1" Body & MM Lift. 32x11.5x15 Copper STT's on Black Rock 950 Lobos w/4" backspacing, Still need Front Bumper and Seats.
My build thread: [url]http://www.jeepforum.com/forum/f12/my-very-slow-87-yj-build-thread-1137667/[/url]