So I recently bought a 2003 mazda protoge and it has been giving me a lot of misfiring problems. The car usually misfires when idling or at low RPM. Sometime when I just start it it ideals at 2000rpm and when I start driving it drops to the regular 600-700 rpm (its summer so its warm outside). I brought it to a few mechanics and they haven't been able to figure the problem out. They checked the computer and the error code is P0300. I changed the EGR valves and it didn't solve the problem. I am thinking of changing the ignition coils. I know almost nothing about cars and this is my first car so I'm extremely disappointed. Any suggestions guys?

 

Mileage? service history?
ie: when were plugs changed last?
PCV valve ever changed?
Automatic or standard?
Engine size!

A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

A code P0300 may mean that one or more of the following has happened:
Faulty spark plugs or wires Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve Faulty
catalytic converter(s)
Stuck/blocked EGR valve / passages
Faulty camshaft position sensor
Defective computer

Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back. If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors. Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

put 2-4 quarts of lacquer thinner in the gas tank and take a drive on the highway for at least 50 miles not revving it over 5,000 rpm, that should help clean things up, especially the catalytic converter and O2 sensors.

 

I changed the EGR valve and it didn't solve the problem. I am thinking of changing the ignition coils. I know almost nothing about cars and this is my first car so I'm extremely disappointed. Any suggestions guys?

Your car is 10 years old, even if it is new to you. Change the spark plugs, wires (if your car has them) and coils together. That's the first step with a very high probability of success. and Noggins suggestion of using the Lacquer Thinner has made my engines run smooooth!
Look for oil in the park plug tubes. If there is oil you need a new valve cover gasket set.

 

I had the same issues with same code. I had one spark plug that fused. No gap = no spark.

 

Just a note ... I replaced coils, plugs, etc... but still had a misfire happening. The plugs I used were very good quality. I went back to the original Mazda plugs and the problem stopped. 10 years old and almost 180K and runs beautifully.


When I bought the car in 2006 the plugs were fusing or cracking. That was a manifold issue under a recall/warranty and Mazda fixed it.

 

I just posted about having this same problem. In my '02 protege, the misfire was caused by a bad fuel pump relay.

 

so I bought a used 2003 mazda protege 5 speed with known oil leak...so after replacing the valve cover gasket and changing the oil no more oil is noticed but heres the issue car ran well but smoked horribly so we took the exhaust cover off to find that it looked like something tried to escape out the cat converter so took into shop and had a new one put on..ran very very badly after it was installed missing of power ect...same code as everyone else p300 changed both o2 sensors and ran lucas treatment through it ran better but still smoked alot when first started then goes away... took it back to same shop to smog...well we passed smog but the new cat converter had busted in two is what I got from shop owner...and after smogging the car started the whole misfiring again drove it home to await the new cat converter less than five miles and caught fire enough to melt fans...took car back to shop they are fixing the fans but now says the car burned out the cat converter and now its not warrantied...he said that car seems to run well but there is so much smoke when starting Im wondering what I can do bout all this smoke now

 

White smoke is coolant. Blue smoke is oil. Black smoke is excessive fuel not being burned. Which one you got? Does it leak coolant? Clogged converters can cause a backfire in the intake which can blow carbon backwards through the throttlebody and onto the MAF and MAP and through all vacuum ports. Clean the MAF and MAP. I assume the ignition system has been checked out? Maybe try a different shop?

 

check the air intake tube that goes from the air intake box into the intake manifold......
these are notorious for getting a very big split in the bottom side of them where you are unable to see this when looking at the air intake tube from above.
mine went bad and caused a bad random misfire......took that and replaced it with one off of ebay ( $25 i think ) and my misfire was gone never to be heard from again.....

also, i did change my plugs back to the factory plugs ( ngk plugs,but i don't recall the part numbers,just search on the forum for the part number.....the proteges don't care much for those expensive plugs.....

i would do those things first,and also check for any vacuum leaks ( listen for a hissing noise when the car is running with the hood up.....)

 

also forgot to mention this in my above post,but if this isn't resolved fairly quickly,if yours has the same air intake problem like mine did,it will burn a hole through the front cat converter.

how do i know???

that is what happened to my front converter.....and they aren't either cheap or easy to get on......front converter is $100-$180ish part,just the converter alone......

just my advice on what to check on.....