I have an 05 pacifica With the 3.5l and it has 167,000 miles I've replaced the timing belt and water pump and spark plugs and I keep getting the p0300 code and every so often the p0420 and p0304. Any suggestions on what could be causing it to misfire randomly? Also it seems to run worse with the A.C. on and the p0300 code comes on faster. If I don't have the A.C. on it takes awhile for it to send the fault code. And driving down the road it is fine no misfires or signs there is anything wrong. Any help would be greatly appreciated because I can't afford to just start replacing parts hoping that it will fix it.

 

coil, Carbon on the valves or pistons. Fuel injector ,

 

thank you. What product do u recommend for cleaning the carbon and injectors? Keep in mind I'm on a small budget.

 

sea foam lucuas a professional fuel injection cleaning service

 

There is Tech bulletin 09-010-06 says the valve rotators on these engines do not spin the valves fast enough to prevent carbon from building up on the valves, especially if you never or rarely break 3500 rpm or car is used in short hops, like in town shopping and kids to school driving.

There is a news valve spring head that allows valves to rotate enough to prevent this, but is a bit complicated if you haven't done engine work before.

You can try reving to 5k rpm which will rotate valves, but I think the Seafoam treatment through brake boost line ti UIM could be easiest fix to try.

If that fixes it, then make sure you get 3500 plus revs in every few days and use Techron cleaner or other in gas as well to keep carbon build from happening

see aa1car.com has good write up on this:
Misfire Diagnosis Chrysler 3.5L V6

similar write up at:
Part 1 -How to Troubleshoot a Misfire (Chrysler 2.7L, 3.2L, 3.5L)

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This probably isn't your problem but file it under mfg fault causes multi problems you might never find. This will cause a vacuum leak and mis fire on #6 and also affect fuel trims and air delivery to all cylinders so a miss fire could occur on other cylinders.

The intake manifold tuning solenoid thats sits on UIM at #6 has 2 problems:
1. The internal rod controlling the shutters exits the UIM at #6 and the rod can move
closer in to the manifold or further out from it, breaking seal and causing vacuum
leak.
2. A very flimsy plastic rod, 3-4 inches long connects to the end of the shutter valve to
the intake manifold tuning solenoid, which moves the 3-4 inch rod to position
the long rod and shutters internal to UIM. Problem is this flimsy and worst designed
component I've ever seen easily comes off at the long shutter rod so there is nothing
to tune or control shutters, which wacks fuel trim and I'm not enough of an expert
to tell you what else.

Fix for vacuum leak is new 500$ UIM or you can try placing o-rings (fuel injector o-ring size works) over the end of the wiggling shutter actuator rod to seal it and prevent in-out wiggle.

To fix the 3-4 inch solenoid actuator rod I had to use wire in figure 8 pattern to keep
it on the shutter actuator rod, this could be broke and you wouldn't know it, its easy to check looking at the solenoid.

For a better visual on the vacuum leak part and fix see this video