P0304: cylinder 4 misfire

Jackflack44

Jackflack44

Member
Joined
May 16, 2019
Messages
28
Location
DFW
Hey guys,

So excited to be back in the Jeep community. Just picked up a rust-free 2000 Sport 4.0L with 161k. Big improvement from the '97 2.5L I had in high school! Well, hopefully :unsure:.

It was running fine, maybe had a slightly rough idle. Took it on an hour test drive and found no glaring issues. My buddy and I could find no leaks. They wanted $7500, I ended up at $6,000. I was ecstatic. Cut to 3 days later, and it seems to be running a little rough. BAM. CEL. Went to Autozone to pull the code, only pulled P0304, cylinder 4 misfire (bought a scan tool from Amazon as I'm sure I'll need it again). I've been doing my best to try to figure out the best way to diagnose this issue without just swapping in new parts until it resolves itself.

I went ahead and pulled the Carfax before buying. Noted a longggg time of no use on the engine, so this may just be a symptom of a clogged injector or something similar. The Jeep passed emissions 7/6/15 with 160,839 miles, and was re-registered 7/28/19 with 160,926 where it had an alignment, new battery, front shocks, trackbar, and emissions checked and passed again. It now has about 161,300 on it. On the test drive we took it down to the gas light, so it has new gas in the system.

My first thoughts are to change the fuel filter, and swap the injector to a different cylinder to see if it just needs a new injector. Can any of you offer some of your expertise as to the steps I should take to accurately diagnose the issue? I had the top off yesterday and was just so freakin' excited to be in a Jeep again, I really want this engine to last quite a while and want to ensure I don't foul up the converter by driving around with the misfire occurring.

Thanks for your help in advance, hopefully I'll see some of you on the Texas beaches this summer!
 
Chase the misfire with the injector before you do anything else. Try swapping it for #1, clearing all of the codes, and waiting for it to show back up. Any other symptoms, aside from the misfire code? Any other conditions coinciding with it?
 
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It doesn't seem happy to rev over about 3000 RPM. But I mean less than 1000 miles over 4 years... I think I'm going to pull the injectors and have them cleaned and tested by a shop after swapping them to see if the code changes. One of the big injector shops is here in Houston so hopefully it's not too long of a turnaround. Also seems to have a more noticeable vibration at idle than my buddy's old 4.0.
 
Injectors rarely cause problems in our TJs but the wrong technology spark plugs commonly cause misfires and bad engine performance on the newer 4.0 engines without a distributor cap. Can you pull a plug and let us know what brand and, especially, what its exact model # is?

Known wrong/bad plug types for newer distributorless 4.0L engines include Autolite's AP-985 (but APP-985 is fine) and Champion's 3034.

Also, does it run ok when cold but develops that problem once the engine starts to warm up?
 
Jerry Bransford said:
Injectors rarely cause problems in our TJs but the wrong technology spark plugs commonly cause misfires and bad engine performance on the newer 4.0 engines without a distributor cap. Can you pull a plug and let us know what brand and, especially, what its exact model # is?

Known wrong/bad plug types for newer distributorless 4.0L engines include Autolite's AP-985 (but APP-985 is fine) and Champion's 3034.

Also, does it run ok when cold but develops that problem once the engine starts to warm up?
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You think even with the ~1000 miles in 4 years the injectors arent an issue? I figured maybe they had gummed up a bit. I'm going to my buddies about 5 miles away on Saturday, pulling all of the spark plugs, and swapping the injector to a different cylinder. I was thinking coil pack, but they fire in pairs if I recall, so I should have two misfiring cylinders. The spark plugs are a great place to start though. Also just filled it up with fresh gas from empty so maybe I should add a fuel treatment/seafoam?

Really appreciate y'all's help. Making a list of everything to check for on Saturday (and nonstop googling/hoping I didn't make a stupid decision).
 
Also because I'm still so damn excited here's some pictures!

20190515_152421.jpg


20190514_184549.jpg
 
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Jackflack44 said:
You think even with the ~1000 miles in 4 years the injectors arent an issue? I figured maybe they had gummed up a bit. I'm going to my buddies about 5 miles away on Saturday, pulling all of the spark plugs, and swapping the injector to a different cylinder. I was thinking coil pack, but they fire in pairs if I recall, so I should have two misfiring cylinders. The spark plugs are a great place to start though. Also just filled it up with fresh gas from empty so maybe I should add a fuel treatment/seafoam?
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Anything is possible. If you really feel the need for a fuel treatment I'd go for a focused fuel injector cleaner, not a general cleaning product like Sea Foam. Not to mention most bottles of pure fuel injector cleaner are half the price of Sea Foam.
 
When troubleshooting your misfire do 1 thing at a time. See what works or not after each change.

The spark plug swap is a good idea.

Save your money on sea foam, imo, keep it in ur wallet for future parts.

When you get a chance check out Texas Jeepers thread.

Nice jeep btw
 
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Incase it hasn't already been said... DO NOT use any replacement sensor other than an OE Mopar sensor. Any of the sensors Autozone or your local auto part store sells are junk and will not work 99% of the time.
 
Jerry Bransford said:
Injectors rarely cause problems in our TJs but the wrong technology spark plugs commonly cause misfires and bad engine performance on the newer 4.0 engines without a distributor cap.
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That's a good call. Most of the plug issues I've personally seen have shown up in multiple cylinders at once, which was why I started thinking that it was an injector issue (that and the fact that it hasn't run in awhile) I guess either one is a good place to start; if it's not one, it's 95% likely to be the other!

T Crawford said:
...change the plugs and wires rotor and cap
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Change the what?
 
If all else fails. 2000 yr. - 0331 head, prone to cracking between cylinders 3 and 4. This is worst case scenario only after all other checks and repairs have been addressed. Not trying to scare you. You can pull the valve cover and there should be stamp on the head "tupy" is the newer better casting not as prone to cracking as the non- tupy.
94207

This is more than likely not your problem, but is a possibility after all else fails.
 
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Quick update:

Going to pull spark plugs tomorrow, determine if they were the correct ones in the first place, then replace with new Autolite XP985's. After that, I think I'm going to go ahead and pull the injectors and drop them off for cleaning/testing at the local shop. 24 hour turnaround, so I'm hoping to have everything put together again by Tuesday and drive it back and forth to work for a few days. See if that can fix the problem. Also my Amazon OBDII scanner just got delivered so I'm gonna start it up again and determine if P0304 is still the only code it's pulling.

Really appreciate everyone's help and advice and I'll continue with updates until she's running smooth again!
 
Chris said:
Incase it hasn't already been said... DO NOT use any replacement sensor other than an OE Mopar sensor. Any of the sensors Autozone or your local auto part store sells are junk and will not work 99% of the time.
Click to expand...

Wouldn't I pull an O2 sensor error code if that was one of the issues?
 
Are you losing any coolant? As the above poster mentioned, the cracked 0331 head can sometimes throw misfire codes in 3 or 4.
 
bromel said:
Are you losing any coolant? As the above poster mentioned, the cracked 0331 head can sometimes throw misfire codes in 3 or 4.
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No coolant leaking or anything that I can see. In the week since I first test drove until today I've probably only put about ~75-100 miles on it. Coolant has remained at same level, didn't see any oil in the coolant when I popped the cap off yesterday.
 
Jerry Bransford said:
Not necessarily. The PCM is not always smart enough to know when an O2 sensor is putting out bad/faulty data.
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Fair point. Do you think I should continue on with cleaning/testing injectors and swapping plugs first before I begin replacing the O2 sensors? If it was coil pack I would have two misfires correct? Or even just put some fuel injector cleaner in it since it's been sitting so long? Just don't want to hurt the engine/cat.

Do you all think I can continue driving it on short 10-15 minute trips around town or should I stick with my other car?
 
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Hook_62 said:
If all else fails. 2000 yr. - 0331 head, prone to cracking between cylinders 3 and 4. This is worst case scenario only after all other checks and repairs have been addressed. Not trying to scare you. You can pull the valve cover and there should be stamp on the head "tupy" is the newer better casting not as prone to cracking as the non- tupy.
View attachment 94207
This is more than likely not your problem, but is a possibility after all else fails.
Click to expand...
Please god don't let it be that hahaha
 
Welcome to the forum, great looking TJ, love the gold. Great advise above and like others stated if you start having to replace sensors check on here first for suggestions. OEM are really what these engines love.
 
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