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Discussion Starter #1
Ok, on to my next issue. I have an engine code 300 - all cylinder misfire and codes 301,302,303,304,305,306 - individual cylinder misfires.

Changed all tune up stuff - plugs (champion), wires, cap, rotor, air filter.
They all needed changed so that was my first shot. Cleared the light with it coming back on within a day.

Did some digging here and all over the net for a common problem to these codes for a 98 tj with a 4.0 auto and found out there were a bad batch of valve springs that were not letting the valves shut fast enough at higher rpms, thus throwing the misfire codes.

Changed the springs along with new valve seals and was surprised to see the top end was very clean for 184k. Cleared the codes with hopes of this being the problem. Jeep seemed to run smoother (at least in my head). Light came back on after about 40 miles.

Which direction should I go now? It runs & idles great, you would never know it had a "misfire" by driving it. I still need to do a fuel pressure check, change the fuel filter/regulator and a compression test. I don't think a coil would be the culprit with it running so good. Computer?
 

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Discussion Starter #2
Anyone?

Checked the fuel pressure, it's sitting right at 55psi.
Haven't had a chance to do anything else.

Someone told me the 4.0's are prone to high carbon build up around the exhaust vales & that some sea foam in the gas, oil and intake should help.
 

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I had a similar code in my F250 and it was a broken line on my charcoal canister. Fixed it and never got the code again.
 

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Moduhrater
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First I have to ask, did you clear the codes after the new springs/valve guides/seals etc?

Can you get realtime information with your OBDII scanner? Might check your O2 readings, they can cause problems when they're going bad, or are bad, including multiple misfire conditions.
Check for vacuum leaks as well (get a can of starting fluid and spray around vacuum lines, intake manifold gasket etc, listen for the idle to kick up)
 

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King Thread De-Railer
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i would try the sea foam. clear the code check it again. what type of scanner are you using? have you checked the o2s?
 

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Discussion Starter #6
I did clear the codes but I only have a code reader.

For the amount of wrenching I do, there should be better tool in my box for theses situations.

Anyone have suggestions on a good scan tool?
 
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