nodnal56

Hey guys I am having trouble with my 2008 jk. It has 120k miles on it and is throwing a PO304 code. Since figuring out the code I have replaced spark plug wires to all cylinders and replaced the spark plug to that cylinder (previous plugs had about 20k on them).

I'm thinking bad coil? But have noticed knock on startup (cold here in SD) which continues for about 5 minutes. Also just had manifold on drivers side replaced along with an injector cleaning service from dealership.

Any ideas? Thanks in advance.

nuke-blue

new plugs & wires is a great start. didn't fix, now it's time for a compression test. check all 3 on that bank, should be 150 psi or above. after that, I'd swap injectors w/ cyl 2 and see what happens. see if the misfire moves. that is if compression is good...

Spytec128

I just had this problem myself on cylinder 2 at 100,000 miles. I did plugs and wires, then did the ignition coil, and then did a block test with a kit from Autozone. It came back positive for exhaust gases so I knew I had a blown head gasket.

I first tried block sealer and it worked for a little while but clogged my thermostat and heater coil so I don't recommend using any of that stuff. It took me about 9 months before I got around to fixing it but after replacing the gasket it ran fine for about a week.

Then I got the misfire on the other side of the engine. I replaced that gasket and I'm running fine now with the exception that my catalytic converter went out because of the blown head gasket. Now I'm looking for a used y-pipe to fix that problem then I should be good to go.

nodnal56

I'm gonna take it to the shop this coming Monday and see what they say on the compression (fingers crossed). Hoping it is just the coil or injector as I'm guessing those aren't as dramatic? Any idea on the startup knock?

btadlock

Wow, I guess I got lucky, I had the exact same code and issue with cylinder 4, new plugs did the trick for me.
I was at ~110k miles, now @ 7 months later @ 121k, the issue has not returned.
I held off on the new plug wires, that was going to be the next step in process.

SaharaScott

Hi! New to this forum - hoping for some tips! Just got a 2008 4DR/4WD Wrangler Sahara last December with 116K miles on it.

Check Engine Light came on about a month ago and I took it into Jeep Dealership for work. The codes showed misfires on cylinders 3 and 4. They replaced all the plugs and wires, cleaned throttle body.

Few days later, CEL came back on. Took it back in. Only cylinder 4 is misfiring (P304), and now they find that cylinder 4 isn't holding pressure (compression test) and the injector is suspect. So they replace the #4 injector.

Few days later, CEL came back on. Dealer is stumped. They relay case to Chrysler - they say this isn't something they've seen before (guess they haven't checked these forums before!). Dealer said they do NOT think the head is bad - they see no evidence of blow-by.

So now I feel stuck. Jeep seems to run fine when while driving, but I have noticed 2 things:

(1) about every 3rd time I go to start the Jeep, it doesn't start on the first try, but always does on the second try.
(2) engine seems to run rough for a few seconds after starting, then smoothes out, and rest of driving seems well and good.

From reading these threads, I see some uncanny descriptions that match my 2 symptoms, which sounds like can lead to the misfires (i.e., the rough idle)...and those threads suggest a failed check valve on the fuel pump.

I've also seen some folks say they, even despite getting new plugs, that yet another set of plugs does the trick (better brands? proper gapping?). I would hope the dealer used the right plugs already.

I have also seen references to ignition coil being the culprit.

That's just what I've seen so far. Does anyone have any suggestions as to what my evidence seems to point to? Are the above named potential solutions valid for my case? Thanks for any help!!!

SaharaScott

I'm posting an update to my post on this thread, as I have just found resolution to my CEL/cyl 4 misfire engine code problem.
Of the 3 leading culprits I had found from these forums, 1 of them ended up being it! The "ignition coil"....actually, in my 2008 Wrangler, it's a "coil pack". Replacing the coil pack resolved the issue.

SaharaScott

after 10 days of driving with no Check Engine Light (P304) being thrown (a new record - previous resets and "fixes" always saw a re-trip within 3 days/20 mi/5 starts)...and 200 miles...and about 50 starts....that damn light came back on.
Despite the new coil pack.
Probably because i just posted here yesterday that the mysterious problem had been fixed.
Looks like not.

If anyone has an idea for me...please let me know.
The Jeep Dealership is stumped.

Ottawavalleylad

SaharaScott my 2008 just threw the cylinder 4 misfire code last weekend after going threw a deep water/mud crossing. Engine was saturated with water. I will be diagnosing it prior to the cooler temps setting in, so will post back here on any success that I have. So far:

- Plugs look good, no cracks. Swapped a couple plugs no change.
- Wires look fine, but are old, so will replace.
- Drivers side manifold has a small crack (been there for a couple years with no code) so will be replaced just to be thorough.

I have heard of others with the 3.8 getting water on the engine causing this code to come up. Others, I believe newer 3.6L models, end up replacing their heads. Stay tuned.

Ottawavalleylad

Update: Replaced drivers side exhaust manifold, gasket, heat shield and bolts with dorman parts. Unplugged the battery to reset code. Almost finished first tank of gas with about 10-12 drives. No check engine light. (knock on wood).

The 3.8l was running rough one morning, and when playing with the boots it evened out. I suspect when pulling the boots to check the plugs a couple boots may have slightly detached internally (wiring). Best to be careful when pulling your plug wires and use dialectic grease on the ceramic to ease removal.

Lastly, the dorman exhaust kit is the best priced exhaust replacement kit on the market (that I could find). However the dorman exhaust ports on my manifold were not well cast, and block some of the exhaust path. The factory unit matched the ports much better.