Q&A

Read this article here for more info: http://repairpal.com/OBD-Code-P0300

To find a vacuum leak perform a visual inspection of the intake hoses and pipes, look for any that are disconnected, torn or have a hole in them. Let the engine idle and listen for a hissing noise from the intake system, this may indicate the source of a vacuum leak.
Any further diagnosis will best be performed with the proper test equipment and technical training, so you should consider taking it to a shop.

I have a 2003 Protege 5, with sport suspension (2.0L) I had been noticing several different symptoms including, a lurching (which felt like broken teeth on the clutch pressure plate, as the engagement of the clutch took place in a fraction of the standard travel) (ie. full clutch dump in 1/4" vs, smooth power transfer over 1.5") the occaisional valve rattle (under heavy throttle)a high pitched squeal (not too unlike a slipping belt), and more recently a random misfire that seemed to have no correlation to outside air temp or timing or weather or fuel conditions. I removed everything peripheral from the air box to the throttle body, and discovered that there was a bolt and a separate nut (that held the throttle body to the air intake plenum) that had come loose. And the result was a flood of unchecked air into my intake plenum. There had been a factory recall on the very same issue. (that I had Jim Ellis Mazda "do" I say that in quotes, because I'm pretty certain they didn't actually re-torque the stuff as they were supposed to. Lazy asses) But to my relief, I don't think any permanent damage has occurred. I cinched everything back up, replaced the missing hardware, and so far everything feels like the day I took delivery of the car (4 yrs ago) Hope this helps.

change the egr valve and no more rough idle ive had it happen 2 times already
its cost $80 or so