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Odd Misfire - P0305

NewJeeper2020

NAXJA Forum User
Location
Michigan
Hey everybody!!

This is my first post here and the first time I've posted anywhere about my Jeep! I am usually able to find a solution to the problem I'm facing but this one is a bit of a doozy. I am by no means mechanically inclined and have been self-taught through several forums and Youtube, so if the answer is painfully obvious please tell me! This post may be a bit long but I want to give you all the details I can. I have a 2001 Jeep Cherokee Classic, 4.0L, 180k miles, 160A upgraded alternator, Big 3 upgrade from JeepCables.com, new 4-hole injectors from a reputable source on eBay that I found through NAXJA, and all other changes have been more cosmetic, new bracket for the center console, stuff like that. ANYWAY!

So the situation:

The jeep starts up just fine. It has a little bit of a rough idle whenever I come to a stoplight but nothing concerning where it's really hesitating when I step on the gas again. Driving down the road is the same story, it handles beautifully and it doesn't feel like anything is wrong. Once it gets up to operating temp and I come to an extended stop and put it in park, say like in the garage and I'm talking on the phone with someone, I get the same rough idle I do at stop lights, but now i'm in park. Then after maybe 5-10 mins of idling and the tach is just barely bouncing directly under the second small hashmark, and without feeling anything buck or jump, the CEL comes on and it reads as a cylinder 5 misfire (P0305).

Things I've done to address it:
- New plugs (NGK), properly gapped at .035
- New coil pack
- Ran seafoam through the intake
- Added a heat shield between the injectors and the intake, as well as heat wrap on the fuel rail, the injector wiring, and the injectors themselves (probably overkill but *shrug*)
- Ran a full bottle of Seafoam through the gas tank with a full tank of gas
- I just had the head replaced with a clearwater head which I've read positive reviews on both here and other forums
- Head also came with new rockers, springs, and valves
- The new head came with new exhaust, head, and intake gaskets
- New high output alternator (not done to fix the problem but I want to be thorough)
- Just swapped the injectors 456, with 123 to see if the misfire would follow but it stayed on 5

Next steps:

I am a bit at a loss and I am running out of money to throw at this thing, especially with a pandemic on our hands obviously food comes before the jeep (at least for me it does!!), but with that being said I think the following steps will be to replace the CPS (from what I've heard online), look into replacing the O2 sensors, and possibly replace the cat? But if I can avoid firing the parts cannon at this thing that's the route I'd like to take.

Thank you for taking the time to read!
 
What's your cylinder compression look like? (Note: to PROPERLY do a compression test, you need the engine warm-ish, with the throttle butterfly open, and all spark plugs removed.)

Try also swapping your spark plugs around. You don't necessarily need to swap all of them, just #5 and whatever one's easiest to get at.
 
Got ya! I'll move the spark plugs and look into a compression test. The shop that replaced the head redid the compression once it was off and said it was good to go, but I don't know the exact numbers they were seeing.

Head was replaced about 5 months ago, the misfire was MUCH worse before the head was replaced. I would get a blinking CEL and then brought it to the shop and they swapped the head, now the light only comes on when I've been driving and I come to an extended stop and have it in park, and the CEL doesn't flash it just pops up and stays there.

This may be a stupid question, but what is blowby?
 
I had the same issue as you only mine was P0306. Didn't do the seafoam or wrap the fuel rail, but everything else. Turned out it was the wire harness to #6 injector. Replaced all the injector pigtails with a kit I got from Amazon. Hasn't come back yet.. 3 months.
 
This may be a stupid question, but what is blowby?

Combustion gases that make their way past the piston rings. Happens on ALL piston engines to varying degrees (hence part of the reason we do oil changes- those blowby gases and contaminants get trapped in the oil), but an old, shadetree mechanic trick to check for EXCESSIVE blow-by is to pull the oil dipstick and/or fill cap and check for smoke and gases coming out.

Bad piston rings can be diagnosed during a compression test. If a cylinder has a compression reading below spec, a squirt of oil from a can can be shot into the cylinder through the spark plug hole and the test repeated. If the compression reading than significantly improves, this would indicate bad piston rings.
 
Fantastic!

Thank you all for the great advice on next steps! I'm going to go through and check as best I can for all the things you've mentioned above and I'll report back with any issues or results!

jimbar, would you mind linking that kit you used on amazon?

Thank you again!
 
The catalyst should be fine, leave them alone. Make sure you mark the components from cyl 5 when you swap them. Give the #5 plug boot a close look as well.
 
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