3.4L Cylinder 3 Misfire

[3100]

disconnect your maf sensor see if shaking stops

[FromTheInside]

I had swapped it out. My misfire is in cylinder 3 and it is slight

[DanTel]

I've had an intermittent P0300 random misfire and rough idle on startup for going on 2 years, on our 3.4L 2004 Impala, with 250,000 miles. Been testing and replacing parts for most of that to no avail.

Letting it warm up for a couple of minutes, turning it off for 15 seconds and then restarting always makes the code and the rough idle stop and she purrs like a kitten (getting about 27 MPG) till the next random rough start.

On monday night, at about 1 ¼ hours into a 1 ½ hour drive, she started running really rough, and developed a bit of a knock. No change to oil pressure, temp, or anything, so I took it on home.

On the firewall side, 2nd and 3rd rocker arms from the left (passenger side) are moving in unison - up and down at the exact same time. One of these is an intake and the other an exhaust. Shouldn't be happening.

One of the mechanics I work with said the camshaft on these is a hollow tube with the lobes pressed on. It seems that a cam lobe has rotated putting one cylinder (#3 we think) radically out of time. Probably a dimple in the piston or bent valve head.

Apparently it has been "loose" for quite a while, and maybe moving back and forth a bit to generate the intermittent random misfire code.

Might want to inspect you camshaft.

[Tech II]

DanTel, check the date of the post.....

[Mavila]

Did you ever find out what it was ?

[Robert B]

Hello, I have a 1999 3.4L Venture also misfiring that seems like half the cylinders are being deactivated after a period of highway driving for about 40 miles.
Is there a way to easily find out if this vehicle has a cylinder deactivation feature that could be malfunctioning or coming on due to old engine not in perfect condition, that I could turn off?
If gas pedal is pushed to floor to engage passing gear it doesn't misfire at all, when pedal is let up it runs on half cylinders, back and forth to stay near 70 MPH on highway.
Thank you.