P0304 cylinder 4 misfire 1997 wrangler 2.5L - JeepForum.com
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Unread 06-03-2010, 08:28 PM   #1
actionpaintball
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P0304 cylinder 4 misfire 1997 wrangler 2.5L

This one has me stumped:

New plugs and wires, spark is good on all cylinders. Compression is great 132 +/- 5 PSI on all four. Jeep has 140K miles. Plug wires rechecked and ohmed, spark checked when cranking and all looked good. Injectors 3 and 4 swapped and misfire stayed on cylinder 4. Injector wires swapped and misfire stayed on 4. When idling, unplugging injector #4-which makes no difference in choppy engine, but all other 3 cylinders will stall engine. Fuel rail removed shows similar good spray pattern on all four injectors. One can of seafoam ran through is desperate failed attempt.

No other problems except erratic oil pressure (will be checking will manual gauge this weekend). No soft (pending) codes, but will throw the P0304 within 5 minutes after clearing.

Only notable potential issue may be PO has a less than clean 02 sensor transplant, but no 02 codes are thrown and my current scanner is not reading 02 frames.

Throw me a bone. I promise to followup with fix

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Unread 06-04-2010, 01:26 AM   #2
bruteboy
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does it misfire while driving or only at idle?
does it pop or backfire from intake/exhaust under load?
have you had it scope checked?
any valve train noise?
if ignition is ok,check for worn cam lobe.
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Unread 06-04-2010, 06:49 AM   #3
actionpaintball
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No popping or backfire, just a little rough but will hold idle. I have not had the injector pulse scoped yet.
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Unread 06-04-2010, 12:16 PM   #4
bruteboy
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scope test ignition system,if ok check for proper valve lift(ie worn cam lobe,broken/bent pushrod/rocker arm)
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Unread 06-04-2010, 12:40 PM   #5
wheelin
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you havent sucked any water in the engine have you. cause a very slight bent connecting rod could cause piston to not travel high enough and not ignite the fuel. just a thought, im working on a very simular problem p0301.
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Unread 06-04-2010, 06:48 PM   #6
actionpaintball
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No water fording...found my box of old 2.5L parts, will start swapping parts as the one tool I do not own is a scope. My MAC, snapon and laptop scanners are telling me zip. I will also bench test the 02 sensor and swap the CPK sensor, and NOID test the injector pulse(s). I also have a spare distributor just for the hell of it; if it gets to that point. I am not a big fan of throwing parts at a problem, but none of the normal diagnostics if working.
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Unread 06-04-2010, 09:32 PM   #7
bruteboy
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you have already swapped the injectors with no improvement,rules out injector.
injector control problem is possible.listen for injector click while eng running(stethescope or screwdriver to ear)
o2 sensor will not cause single cyl missfire.
ignition secondary is most common cause,dist cap and rotor?
comp test is good,rules out valves and rings.
valve train is next,cam lobe,push rod,rocker
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Unread 06-05-2010, 06:44 AM   #8
actionpaintball
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There are confirmed 02 and crank sensor malfunctions on alldata throwing same or other individual misfire codes, as well as ecu issues. Noid test (injector pulse) looked good. I will be taking the valve cover and rockers off tonight.
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Unread 06-05-2010, 11:57 AM   #9
bruteboy
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yes,a failure of the o2 sensor will throw missfire codes as certain failures will drive the computer lean to the point of missfire on all cylinders,and the computer may detect one or all cyl. missfires and throw codes for one or more cyls.however,the o2 sensor can not cause a single cylinder missfire(ie constant dead miss).
the crank sensor may also throw codes,but again will not cause single cyl. missfire(ie constant dead miss)
any o2 codes-ie lean or rich condition detected/o2 sensor voltage high or low/crosscounts low?
any other codes?
check/change dist cap first,then pull valve cover,rotate engine by hand and compare valve lift with the other cyls.
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Unread 06-05-2010, 08:26 PM   #10
actionpaintball
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Quote:
Originally Posted by bruteboy View Post
scope test ignition system,if ok check for proper valve lift(ie worn cam lobe,broken/bent pushrod/rocker arm)
One beer to you...bent pushrod.

Makes perfect sense with the diagnosis prior to pulling the valve cover.

Wish I knew why it bent? I will pull and inspect lifters and throw some new or spare rods in tomorrow and cross my fingers.

Now onto the oil pressure fluctuations...looks like a newer sending unit.
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Unread 06-13-2010, 08:24 PM   #11
actionpaintball
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...frozen lifter...back in action
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