I recently bought a used (obviously) '02 Mazda Protege5. It drove perfectly fine for the first two weeks, then one day the check engine light came on and the car began jerking/hesitating while driving and would idle rough while stopped. (I did not buy a warranty from the dealership and they will not help me with any repairs "should've bought the warranty"...). Anyways the car quickly got worse but only after it has been driven until it warms up. If the engine's cold, it drives fine, but after it gets warm it begins jerking very very badly. I took it to the dealer and they said it had a random misfire code and needed new plugs, wires and coils. Paid for that, but nothing changed. Still ran the very same way. They then said it was the egr valve...new egr valve didn't change a thing. Actually the car would hardly go...and actually stalled on me while driving after the new egr valve was installed. So now I don't know what to do. I've read it could be the mass air flow, 02 sensor, computer...i honestly don't have the money to keep doing all these repairs hoping this time it will be fixed. Anyone out there that has had the same thing happen and what fixed it? Thanks.

 

Unfortunately with random misfires changing the plugs, wires, etc is the first step.
Missfires are one of the worst things to diagnose. For me, they changed plug, coil and fuel injector on the cylinder that gave the missfire code. Then they changed the fuel pump. It was actually the poor exhaust flow that caused it. Ended up changing pre-catalytic converter and main cat and re-doing poor crossover y-pipe up front on my 2006 V6 Mazda 6.

The following is a list made up:

Missfires Can Be Caused By a Million and One Things,
Vac Leaks
Wiring Issues
Spark Plugs(Even Fresh Spark Plugs That Are Crappy Quality.... E3 And Such)
Ignition Coils
Alternators (Search EMI)
MAFS
Injectors
Injector O-Rings
Intake Gaskets
TB Gaskets
PCV Hoses
PCV’s
Malfunctioning EGR
Malfunctioning CAS
O2 Sensors
Aftermarket Intakes
Intake Hoses
Vac Lines
Timing Chains
Timing Chain Tensioner
Broken Cam Gears
Malfunctioning Cam Sensors
Incorrect Crank Gears(The I4 Has Like 40 Different Configurations For Whatever Reason)
Incorrect PCM Flash
Issues Within The PCM
PCM Wiring
Bad Diaphram Within The FPR
Issues Within The Fuel Pump
Dirty Fuel Filter
Dirty Fuel Lines
Pre-Cats/Poor Exhaust Flow

If the above gaskets/seals have not been changed, change them. Clean your MAF sensor, throttle body.
Change your PCV valve and probably that short hose as well.
Inspect the intake accordian tube for cracks. Inspect for vaccuum leaks.
Put in a good Fuel system cleaner: http://www.redlineoil.com/product.aspx?pid=80&pcid=12
Inspect wires. Re-set ECU after doing above.

GOOD LUCK!

 

Sheila: the first thing you do is not to go to the dealer anymore. Find a good independent mom and pop shop. Get your oil changes and tires there and establish a business relationship.
2nd. Before you go deep into noggin's list (great list but kind of long) go to a parts store and let them read the trouble codes for you. Jot them down and report them here. We then have a better chance to narrow down the possible causes.

 

Got that list from a so called alpha male on another forum. I copied and pasted it. Don't if all is valid, but I added pre-cat/exhaust flow because it happened to me. I had a misfire on 1 cylinder. After changing all plugs, 1 coil, 1 injector (on cylinder causing missfire), fuel pump and so called warped gas tank, it was finally the exhaust pre-cat/manifold on 1 side. I had less than 25,000 miles on it. I also re-designed the crossover pipe after the pre-cat and haven't had any more misfire issues and great exhaust flow and more power!