Hey guys,

So I took my '03 protege for a cruise this weekend, just to take her out after i did some work.

About 100km away from home, my engine light started flashing. I hooked it up, and I got error code P0304 (Cylinder 4 misfire).
Note: Firing order is 1-3-4-2, in case you needed to look it up

I tried clearing the code, just in case, but it came right back. A rather violent (but not as bad as I've seen) shaking occurred.

I drove it home, I was able to baby the throttle in an effort to not engage that cylinder, and it's no worse than it was.

When I tried unplugging the ignition coil to cylinders 4 and 1, and at first the shaking ceased, but it returned later.
I tried unplugging the coil to cylinders 2 and 3, and it still shook.
I tried once again on 1 and 4, and the shaking was still there.

The above led me to believe that it's not an ignition coil problem (?) or a coil wire problem (?)

Three weeks ago, I had a Fuel Pressure up solenoid (0900) code, but it cleared itself - is it possible this is related?

What's left? Might it just be a plug? I didn't have the rest of my tools with me so I was unable to remove my coils to test the plugs, I will do that later tonight - if it was a plug though, why was the shaking still there when I disengaged the coil to that cylinder?

Could it be the fuel intake? Filter? PRC Solenoid? EGR valve? Fuel Pressure Regulator?

Car Specs:
2003 Mazda Protege LX
2.0L, 4 Cylinder engine /w turbo

when's the last time you changed plugs?

and it still shakes because it is off balance then if that cylinder is not firing..

This post was edited by DuckManParkey on Jul 7 2014 02:34pm

I still think an ignition coil went bad. How many miles and have you ever changed the coils?

200k km, so 125k miles - coils were done at 105k I believe, or so it says in the service manual (I've only had it since 115), so still quite probable that it went bad.

Again, would disabling one of the ignition coils not stop the shaking though?



I did the plugs 3-4 weeks ago - could it still be an issue?

How do you mean in regards to the shaking.

No, it would not stop the shaking. If you unplug the coil pack, the cylinder would still not be firing...What you should do is swap the coil to a different cylinder and see if the missfire code changes to the cylinder you moved the coil pack in question to.

Ah, and that will determine if it's the coil pack, plug or something else.

Makes sense, I'll grab my tools and do that now, I'll edit with results.

Swapped the coil pack and wire that was on cylinders 1-4 with the one that was on cylinders 2-3, cleared the code and took it for a spin to re-produce. P0304 came back, cylinder 4 misfire remains.

Is the spark plug still a possibility? Is it worth replacing? (It's only like 8 bucks for a new platinum tipped one anyways)

That's where I'd start. Maybe since so cheap just swap all four coils. If still getting a cylinder misfire it may be fatal.....



GL.

So check it out then. Swap the coil packs back, and try swapping wires with another cylinder. If still cyl 4, then try swapping plugs with another cyl. Report back with results.

Swapped coil packs and wires back to initial position, tried different combinations, none worked. Returned to second position (First swap, not original), no dice.

Cleaned fuel injectors and filled up with premium fuel, even if it wasn't a part of the solution, I probably needed it.

My Platinum bosche spark plug had failed, and stopped igniting completely, my mechanic left early, so I picked up a little $5 nickel one from walmart to diagnose. Replacing the spark plug in cylinder 4 restored full power to the car, took it for about a 20km drive to test, after clearing the code and scanning again, the code was gone, the shaking had ceased, and the check engine light was off, the car runs normal again. Very happy it wasn't anything more than what I'd tried.

However, P0900 returned, Fuel pressure up solenoid - my guess is it's either the Fuel Pressure Regulator (easy and cheap fix on this car), or the PRC Solenoid (Pressure regulator control), which is also a fairly cheap and easy fix - both located at the top of the engine and are very easily accessible. Has anyone else experienced this code, or a similar issue, or have any inclination as to what it is, so I don't go out and buy $100 worth of FPR parts?

Thanks so much for your help guys

This post was edited by Flawed on Jul 7 2014 08:52pm