The problem turned out to be a cracked vacuum hose that hides under the upper intake plenum. The plenum curves to the drivers side and the hose was under it, quite hard to see. The hose was a rubber hose, not one of the smaller plastic hoses. Spray some carb cleaner under the upper intake plenum if you suspect the same problem on your Mustang, if the idle changes, you probably found the leak. Hope this helps someone else.
- posted
20 years ago