skaterdude888

iTrader: (1)

Alright, I drive an 03 VIII, with about 75k miles on it. Only 'power' mod is a 3in cat back. Otherwise she is stock.
I've been having a P0300 random/multiple misfire code for a while now. The check engine light will come on generally when cruising at highway speeds (60mph) and generally in 5th gear. It has come on at slower speeds and lower gears as well though. Oddly though the code will go away on its own after a few days, only to show up once again. I was also getting a P0420 Catalyst efficiency below threshold code but after changing out the O2 sensor that hasn't come back on.
When the check engine is on, as I'm cruising on the highway, the engine will run smooth, then randomly sound like its running very roughly and will barely accelerate. I have to let off the gas and then get back on it to smooth it out. and then repeat the process every minute or so. This only happens at speeds over 60 when in 5th gear. No problems with Idle, and no problems with accelerating hard from low speeds.
I've already replaced the spark plugs and checked the gaps. I was wondering if anyone has had a similar problem. I've heard that it could be a problem with the ECU and a new Flash could correct this problem. I know it could possibly be bad coils, but it seems like more of a computer issue than anything else. Any input would be greatly appreciated.

RockmanX

iTrader: (78)

does the car shake and vibrate kind of violently and makes a loud sound until you let off the gas and reapply and it comes back randomly but mostly in 5 gear when it happens if it does then that sounds like my exact issue i had with my OEM motor long story short i had a slightly cracked head causing the madness had to get anew head. since thats happening check for pinging detonation on your pistons tops

Hendrixmx

iTrader: (35)

https://www.evolutionm.net/forums/ec...0300-test.html

skaterdude888

iTrader: (1)

Well I did a compression test, and had no leaks. So I would say its probably not a cracked head. Anyone else have similar problems with other solutions? I'm still trying to narrow it down.
As far as just disabling the code, I don't want to just mask a problem that could potentially get worse. Unless it is just throwing the code with no reason, but until I've narrowed it down to that I probably won't disable it. Could it just be a bad engine map?

barneyb

They made changes to the clutch and other drivetrain parts in years after 03 to get rid of the problem. The dealers should have just offered a reflash that kills the P0300 code. But, most likely it is a Federal requirement. You still have codes for individual cylinders misfiring. So, to make a long story short, it is probably doing it for no reason other than that is how the car was made.

skaterdude888

iTrader: (1)

I'm still having this issue. My evo will need to be able to pass emissions so I don't want to over write the code yet. I'm using autolite iridium tip spark plugs and it has the original coils/wires still. Is it possible that the spark plugs are just not agreeing with the car? I've heard to go with NGK's, does it matter copper or iridium? and if I replace the coils I'd like to go with the spooling up kit. Will I need a tune after that? Thanks in advance, and sorry if these are noob questions.

barneyb

The great thing about having your tuner do away with the p300 code is that the inspectors can't tell. The ECU lies to them.

As for why the p0300 code started happening, you probably finally have enough wear and slop in the drive train. Most '03 Evos had this way before 75K.

evol etg-zj2

iTrader: (13)

I have all the same codes in my 04, but no shaking at any speed and I hear no misfire under idle whatsoever. It's just something that sucks. I'm going have my codes written out in my new tune (o2 catalysts and random misfire) cause I don't have cats and there's no misfire.

It certainly sounds like your car has an issue of some kind due to the shaking. I wish I could help you, but I've only owned an evo for a week.