Rough idle, stalls in neutral, codes 300 and 171
 

For the past 6 months my '02 protege has had a very bad stumble while idling in Drive with my foot on the brake pedal. The rpms fluctuate between 400 and 850. It gets worse in stop and go traffic. If I put the car in neutral the idle would smooth out and idle normal.
Well today it got much worse. The rpms dropped below 200 and the car almost stalls. It stumbles really bad at idle in drive and the whole car shakes.
Today, now if I put the car in neutral from drive the car shutters and shuts off. Also the battery light came on.
Going to check charging system tomorrow.
The codes seem like the might be related. 171 bank 1 to lean. 300-multiple misfire, If I am running lean that might cause a misfire.
I am going to check for vacuum leaks, then plugs, then coils. Does anyone have any other suggestions?

Since it started with the brake pedal, I would check for internal leak in the brake booster. You would have to pinch the vaccuum line going from the engine to the booster and then see how it idles. If it idles better your booster is screwed. You should also have poor brakes once in a while (really have to step on pedal). This actually happened to me on my Mazda 6. Got a booster from the junkyard.
Misfires may be something else.

If it is not what Noggin said it could be the infamous air intake hose. It cracks at the rippled section and as the engine moves a little back and forth the crack may open and close causing loping idle. If in gear the it can make the whole car buck. That happend to me in a Ford Windstar minivan with a vacuum leak at the intake plenum.

Got a chance to take a look under the hood. After some research today and the suggestion from Tanprotege, I checked the intake hose and it was almost torn in half at the accordion. Ducked taped it, and everything ran great. Some times it is the simplest things
Thanks for the help Useyounoggin and Tanprotege.

I'm happy to hear it was an easy fix.
The accordian hose does crack after a while. Pretty common. I got rid of mine and put on an aftermarket K&N intake with silicone and aluminum tubing.
Mine is also isolated to cooler air---it does not get any hot air from the rad.
https://www.mazdaforum.com/forum/mem...hment-2265.jpghttps://www.mazdaforum.com/forum/mem...ddles-2135.jpg