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Is the check engine light on?
Did you change the PCV valve by chance? Was there any running conditions that occurred before the vehcile started to run very rough? Do you have access to a scan tool?
How many miles?
YES THE CHECK ENGINE LIGHT IS ON. NO ON THE PCV VALVE. NO SCAN TOOL. ENGINE WOULD OCASIONALLY BLOW OFF THE VACCUME LINE FROM THE EVAP PURGE VALVE TO THE AIR INTAKE. 140,000 MILES.
Did this start suddenly? The first thing that comes to mind is that the timing chain has skipped. Have you had the car since new, and how frequently did the oil get changed?
Compression test results would also be handy. I would get a spark tester, and make sure you have good spark at all the cylinders - a blue spark. I would also get a noisd light, and check for injector pulse at all the injectors
Another trick is to unplug the mass airflow sensor, and it will run off the MAP sensor. If there is no change in the way it runs, this would rule out the MAF. You can also do the opposite - unplug the MAP and have it run off the MAF, to rule out the MAP as a cause