If you have 19 inches HG at the distributor and 5 inches HG at the manifold pressure sensor on the firewall it sounds like you have a pretty bif vac leak. What condition is the line to the sensor? It runs from the back side of the left head up to the sensor. On my 4.5 the line was the original braided line and appeared to be good, that is until I removed it a looked closely. The line was severely cracked all along it's length. I couldn't find an original replacement so I used 5/16 fuel injector line. It fits good on the barb at the sensor but loose at the fitting on the head. I had to use a small hose clamp...it is a tight fit but doable. Check out that line.

__________________
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.