I've been struggling with random misfire codes since 2009. I've performed the following to resolve it... but to no avail.

- Replaced spark plugs
- Valve adjustment
- New air filter
- MAP wack/reset
- Swapped ignition coils
- Compression test
- Leakdown test
- Techron fuel cleaner

I read this on another forum:

Can someone point me to some resources on how to troubleshoot a vacuum leak, a leaky EGR valve, and low fuel pressure? I can't seem to find any.

 

try cleaning your injectors first.

 

doesn't the techron fuel injector work good enough? that's what i use and i still have random misfires. i pretty much did everything the op did as well. however, changing out the coil packs did help a little bit and my flashing cel doesn't come on as much but it still comes on, just randomly.

the problem usually occurs between 3-4k rpm. but sometimes i don't even get a flashing cel just a loud popping noise coming out of the exhaust. i don't think it's a bent valve coz it would happen pretty much all the time wouldn't it? btw, my idling is smooth no hiccups no misfires whatsoever.

could it be...
knock sensor?
fuel pump?
fuel injectors?
or what?

 

i'm no professional but, if the vale(s) are bent or slightly the motor would have trouble with compression in lower rpms as you mentioned 3k-4k rpms seems low enough. low compression could cause misfire since there is not an efficient combustion. Although I could be 100% wrong here!
so try the compression and leak down test.

 

Is p0300 the only code showing up on the scanner? Do you feel a misfire at all? If so is it more when the engine is cold or warm?

 

I doubt the problem is with the injectors. I swapped them around and the error code is always the same.

P0302
P0304
P0300
P1399

Besides the CEL being on, the car feels completely normal. There is a fuel smell which means it's running rich.

 

i do feel the misfires and it's usually when the car is all warmed up. it doesn't misfire at all when idling or when accelerating. when i cruise at around 3-4k rpm that's when it misfires but not as much as before when i swapped out the coil packs. sounds something like an electrical problem to me but i can't yet to figure that out for sure. btw, i have no cel that's turned on at all, just flashing ones every now and then. thanks

 

Spray the coils with a mist if water while the engine is running, look for arcing.

 

Hirev, did you really just hijack my post?? How about you let me get some help with my problem before cutting in?

 

So JOHNNY

Can you get the freeze data on those codes? Particularly the fuel trim for the P1399.

Then I can help you fix your misfire.