Scotty,
I have a 1976 Cadillac Deville with 82,000 miles in very good mechanical shape. At some point the vacuum lines have been rerouted. It runs okay, but when I hook up the vacuum lines correctly (using the 1976 factory shop manual for reference) it idles much smoother. However, there is a slight miss, very high idle, and once warm the engine acts like it doesn't want to shut off when the key is turned off. It has new vacuum lines and EGR valve. What should I next?
Adjust the carb and ignition timing. It's also possible the carb would need to be rebuilt due to age and today's ethanol-laced fuel despite the low mileage. The dieseling problem (running after shutoff) will likely disappear once the idle speed is down to normal. (I have the same problem with my '71 Ambassador if the idle speed is too high when turning the engine off.)
I would definitely consider rebuilding or replacing your carburetor if your car's sat for any significant amount of time. I'm currently working on figuring out why my 1979 Pontiac Catalina is running very rich, it sat for 5 years, and had the carburetor replaced about a year before it was parked. It was just bolted on in place of the old one by the previous owner, and was not fine tuned, so I'm hoping this is the issue. Rebuilding it could be on the table, even though it's relatively new. Check your ignition timing with a timing gun as well.
I would double or triple check your vacuum hose routing, and check for hidden vacuum leaks if you haven't already. Cross check the shop diagram with your under-the-hood diagram. Like your Deville, my Catalina's vacuum hoses were all over the place and half were dry rotted or missing. I used GM's official service manual over the summer to replace and reconnect all of the hoses, and I found that none of the official GM book diagrams fully matched my under-hood label. Chuck was very helpful with figuring this out.
One issue I ran into with my Catalina is what I believe was a large vacuum leak from the EFE valve (Early Fuel Evaporation Valve) If I understood its purpose correctly, it routes some exhaust gas back under the fuel bowl when the engine is cold to help evaporate fuel more quickly, then sends the exhaust back out the exhaust manifold. I noticed once I connected that particular emissions device to vacuum, my engine would race. Spraying the EFE valve with carb cleaner slowed the engine quite a bit. I didn't like doing it, as the EFE valve is on the hot exhaust. I couldn't find anything to replace the valve, so I capped that line. You didn't mention which engine you have in your Deville, but on my Pontiac V8, that device is the rearmost line, right behind the vacuum advance on the distributor. Check that out if you have one, it's exposed to moisture, heat and road debris, so it could easily be damaged.
