PO300 - Anybody good at narrowing down a misfire?

[guysnydr]

Anybody good at chasing down a PO300 misfire code? Got this code a few months back, turned out to be very old/bad spark plugs. Been running perfectly fine since new plugs, well until yesterday, now the PO300 is back again, seems to run fine going down the road but at low idle starts running very rough and starts missing. Any ideas? I hesitate this think it is fuel related because it seems like the symptom would be opposite (runs poorly under acceleration). Liberty is an 04, and I don't think it has an EGR valve. No other codes so far to help narrow down which cylinder.

Possibly worth of noting is that I changed the upstream and downstream O2 sensors the other day (was getting check engine light on those), is there any chance that could be my culprit, or is the PO300 happening around the same exact time just an uncanny coincidence? I can't see how the o2 sensors would be related, but I am also old and much more comfortable around small block chevy's and big block pontiacs! I am not ignorant with engines, but these new emissions engines are a little outside of my wheelhouse.

Any help/guidance is appreciated!

 

[uss2defiant]

it could be the coils as well.

 

[LibertyTC]

Welcome to the forums.
What brand of o2 sensors did you buy? The jeep will not like anything other than NGK/NTK.

You have a multiple cylinder misfires situation.
Only use the correct spark plugs for the KJ. Are these what you have installed?

You must be registered for see images attach

If not install the above^ make sure gap is set correctly as well.
Change spark plugs every 30k.

Coil packs, should consider replacing all 6 if they have never been changed before. They're normally good for 100k.
Mopar coil pack are best. I used Standard brand, & so far so good.

What is the idle speed in rpm?

 

[guysnydr]

The Jeep is actually my daughters. She ordered these sensors: https://www.amazon.com/gp/product/B...n_title_o06_s00?ie=UTF8&psc=1#customerReviews

Could bad o2 sensors cause this? That was my first thought since it started this the day after the 02 sensors were changed, but I am not sure how o2 sensors would cause a misfire situation like this. But as mentioned before, this emissions stuff is not my area of expertise.

The misfire seems somewhat random, and at lower RPM's. It did it to her after about a 30 minute drive where the Jeep ran fine until her and her friend got into a parking lot, then started misfiring terribly. At some point they started it back up and it ran okay, then drove it home and it ran okay. Then once they got home the misfire returned. I went for about a 25 minute drive with her next day and ran smoothly - until we got home, then ran rough again and bad misfire.

I did replace all plugs with NGK (same part number that came out of it) but they were not the V power as you have noted. I gapped them at .040. It is just very odd that all of this began after 02 sensors were changed out. Could be a coincidence I suppose.... ?? But I don't know either. Coil packs are probably originals I would guess.

 

[guysnydr]

I believe it is idling at around 700 RPMS's but I can double check that.

The plugs were changed about 4,000 miles ago BTW.