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Q: Code P0300 Rough Idle on 2003 Mazda Protege5

Engine is idling low (650 rpm)-- rough and stalls.
Engine check light on with code P0300 only.
Changed the plugs and air-filter to no effect.
Even when the idle is held at 1000 rpm it feels a bit rough, but pulls cleanly and strong to red line.
From what I've read so far there are several other components that can cause rough idle (CPS, TPS, EGR).

What next and if a vacuum leak check is needed what method )of detection is best?
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Read this article here for more info:

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.
Thanks for getting back to us with the solution to the problem. A stuck EGR Valve will sure make a car run poorly, I bet it is much better now.
It was the EGR valve.
When removed it was stuck open.
Cleaned it up and a;; good to go.
Cheap repair...
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.
Just as a clarification, the recall on these engines involved the 2003 Protégé Intake Manifold Shutter and Variable Tumble Control Valve and was called Recall 3005C, issued in December 2005. You can look for it on line. It involved a group of vehicles built during a specific time period (although there have been people with cars outside that group which ran into the same issues) and involved factory under torquing of the screws holding the butterfly plates of the intake manifold and other similar internal parts together. In the cases of failure, the screws loosened causing the butterfly valves to flap, sometimes damaging the intake manifold valve body, which in those cases needed to be replaced. Otherwise, new screws were installed with proper torquing. In the worst case scenario, the screws fell out and got sucked into the cylinders, and in some cases damaged or embedded into the cylinder heads, scored the cylinder bores and collided with the spark plugs causing compression failures or extreme misfiring when the plug firing tip was smashed together with the electrode by the screws. I do not believe what you reported is the same defect that the recall addressed.

I am not suggesting whether your issue was a factory defect, a fluke, or a bad mechanic who didn't tighten it down properly after some work, but it doesn't seem to be part of the recall you were referencing.
change the egr valve and no more rough idle ive had it happen 2 times already
its cost $80 or so
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