A bad T. P. S. will do that too , some time you can pull it off and clean it , and it will clear up . Although it is pretty likely you disturbed a vacuum line while working on the exhaust , like Billy said . you can spray carb cleaner around the lines to try and fide it .
Vacuum leak , engine will run warmer with high idle . Spray carb cleaner around the suspicious hoses or any where air could be sneaking into the intake . When the cab cleaner goes in inted of air the engine will rev up .
The problem will be no doubt be related to something you did when changing the manifold. ( the `whatever u touched last rule`). Vacuum leak is the prime suspect as was pointed out earlier, so get the carb cleaner and find it the easy way. By the way, an UNLIT propane torch works also.
If it ain't broke, don't fix it - Don't worry, something will break soon enough for you to fix.