So when you say the MPG has dropped, you are saying you can only do 90-100 on a tank of gas, right?
Take the PC3 out of circuit, and see what happens.
I'd have to think that if your vacuum lines are so bad that zip ties make a difference, you have lots of little issues adding up to a bigger problem. Changing one thing at a time, and making log entries so you cna track what actually happens with the results of your experiments will lead to a solution more quickly than just 'stabbing' at it.