Misfire / Rough Idle

[markram]

2004 Aveo

Code P0300 - random misfire.
Changed Spark plugs and wires, ran seafoam (cleaner) through. Problem was fixed for about a month, started happening again.

Since i have changed the Coil, Oxygen sensors, and the fuel filter/pump. I've had a compression test and the compression is apparently good.

None of these have fixed the problem. I dont hear or feel the misfire, but the CEL flashes after about a minute of resetting the code. The engine idles very rough.

Anybody have any more suggestions? I'm considering swapping the ECU but am not sure where it is. Also considered swapping the Throttle position sensor. Is the TPS just part of the throttle body?

I basically just need to get the CEL off so I can sell the thing... but Im not a fan of just disconnecting the light!

Thanks a lot!

[petrified.rabbit]

do the idle relearn, see if that helps. (i'm not sure 2004 needs it done).

mist your wires and coil pack with water while running and look for a light show. Also verify you put the wires on the plugs all the way and did you gap the plugs?

[whitefish]

(i'm not sure 2004 needs it done).

mist your wires and coil pack with water while running and look for a light show. Also verify you put the wires on the plugs all the way and did you gap the plugs?

To make it easier to see any shorts in the spark plug/coil wires, shut the lights off in the garage, or wait until night time and do in the dark. I did this with a sunfire once and you could easily see which wires were toast.

[markram]

Misted the wires, no light show. Pulled the plugs, it looks like only 2 have been firing. They are all pretty charred. Changing the plugs seemed to make a bit of a difference, but still not fixed.

Possibly bad valve seals?

[FyrFytr]

<bump> having a similar issue (havent tried the light show yet though)

[thehunterooo]

Definitely do what Rabbit said above, that is your best bet.

[FyrFytr]

what does "mist your wires and coil pack with water while running and look for a light show" mean? I am in the fire service so I am much better at disassembling cars then I am with engine work, although I do know my way around an engine bay

[petrified.rabbit]

take a spray bottle like an old 409 cleaner bottle, that was empty. Fill it with water, set it to spray not stream. Take the cover off your engine so you can see the wires and coil pack. Start the car, let it run a minute or two. Then using the spray bottle "soak" the plug wires and coil pack. If there is a short (causing a spark plug to miss) then the water will ease the resistance and the sparks will fly, creating a light show. Even if there isn't a visible light show if the water being sprayed makes the car chug, stall or ___ the part you are spraying most likely needs replaced. Doing this at night or in the dark makes it very easy to see the sparks. Do not touch the wires or coil park while its running, 40000 volts (or whatever our specific voltage is) kind of hurts, a lot.

[charlieb]

wish i read that that before i pulled that plug wire while it was running,lol.i was 21 (1981)was trying to figure out which cylinder was not firing,so reached in pulled first plug wire and WHAAAMMM didnt relize dc current had that much power.no ill effects that i know of but wouldnt try again with out some good thick leather gloves on.

[pistonbroke]

My 2007 Aveo was running ok but thought it needed maintainence. Changed the oil and the plugs now runs rough. The Actron tells me #4 is misfiring, checked everything a dozen times all seems ok. Does the system reset after the battery has been disconnected.