I have an 05 4.7L with 50k on it, that started throwing the p0300 random misfire code a couple weeks ago. Plugged a scantool into it, and the misfires are only on one side, #'s 2,4,6, and 8. o2 readings are jumping around on one side, but that's expected because it's leaning out to cause the misfires. Long term and short term fuel trims are showing random negative percentages on one side, and all exactly even at about 65% on the other side. o2 sensors seem ok to me because the heater readings are normal, but i admittedly don't know enough about that. Cat temp readings are even and normal.

Engine is idling at about 650. Problem is worse in open loop, still bad in closed loop.

So has anyone had a similar problem or have any ideas? I haven't touched anything in the engine compartment other than the air filter and plugs, but that was about 5k ago.

 

Have you checked your vacuum lines? If you have ruled out the O2 sensors for each bank and the coolant temp sensor I'm thinking maybe a vacuum leak or even a clogged or faulty injector.

 

Fixed the problem a while ago, but it was a bad injector. Injector was pouring into one of the cylinders (I think #4) so it was leaning that side out to "fix" the problem. I just pulled the connectors on the injectors one by one until the mixture went back to normal. One easy $30 injector swap and problem solved.

 

Glad you got it fixed and thanks for sharing. Hopefully you won't have anymore issues.

 

Good info to know. Mine is at 58k now and has a bit of a rough idle. Could mine be injectors too? Dont have any codes yet. Just vibrates more than id like it too.

 

that's how mine was before it threw a code. randomly when I would stop at a light the idle would bog down a bit. then about month or two before it threw a code the idle was pretty shaky and it was doing about 20 miles worse on a tank. Then it got real bad and the light came on after a couple days. i actually should hook it up to a scanner again because i suspect i might have another one starting to go.

 

I will need a full list of symptoms. Simple bad gas can give you a rough idle. There are other things as well.

Bad EGR valve will eventually throw a code but not before you experience
Rough idle
Throttle surge
Horrible gas mileage
Engine might try to die as you come out of RPMs (like when turning)
Exhaust popping.

Throttle position sensor going out can also give some of those.