I recently swapped an engine (with the turbo) and without the intake manifold from a 1985 740 turbo into a 1988 760 turbo. I kept the original intake manifold in the 1988 760 but I disconnected more hoses than I needed to in order to easily move the manifold out of the way. I now wish I had documented things a little better.

I have one left over vacuum hose. There are 3 vacuum hoses attached to fittings at the center of the intake manifold located one on top of the other. These fittings point towards the engine. The hose on the top fitting was never disconnected and leads to a T and then through a couple of check valves to some plastic hoses. The hose on the middle fitting goes to a red and gray check valve and then on to a diaphragm bolted to the head that appears to operate a valve between the turbo intake and outlet.


I have no idea what the bottom hose attaches to. On the 1985 740 it went to the firewall on the drivers side However on the 1988 760 there is nothing there for it to connect to there.



Arrow points to mystery hose - bolt is plugging the free end of the hose

Where does the mystery hose go?

 

Let's see if I remember the correctly;

That top fatty hose feeds vacuum to a vacuum reservoir and your climate control panel. The next one down goes to the bypass valve in the turbo. The bottom goes into the dash for your turbo gauge. Then one just a little toward the firewall goes to the crankcase ventilator below the intake maifold.

Okay, I took a few pics. The view of the hoses is from the top looking down and a little towards the firewall. In order from left to right: climate supply, bypass valve, turbo gauge/over pressure switch and PCV. There's a pic of the chart on the passenger shock tower in my car. It does not show the climate control line.

So you need vacuum going to all those places. It sounds like you need to find the one that goes to the gauge. It should come out of the fire wall near the top on the driver's side.

Good luck.