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Discussion Starter · #1 ·
I have a 1997 Jeep TJ 4.0 liter Automatic. I went down to Autozone and got some codes read once my check engine came on. I got 5 codes:

P0300 - Cylinder misfire detected - Random cylinders

P0301 - Cylinder misfire detected - Cylinder Number 1

P0302 - Cylinder misfire detected - Cylinder Number 2

P0304 - Cylinder misfire detected - Cylinder Number 4

P0128 - Circuit condition (Heated oxygen sensor bank, sensor 2)

Some of the recent history on the jeep is a full tune up 2500 miles ago. Change ignition coil and crankshaft position sensor about 500 miles ago for a previous code.

I do feel a slight rumble at a stop light when I am just sitting still. I do not feel any significant power loss.

Any help is much appreciated!
 

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Discussion Starter · #4 ·
Changed em and still have the light....

Anyone have any other ideas before I start forking over cash?
 

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Some distributors have minimal adjustment in them after the initial installation. You can turn them a few degrees CW an CCW. From what your describing it sounds like your timing is just a little bit out. Id take a look and if theres any way to adjust and turn your distributor, Id try that first. Just start your jeep, and slightly turn it CW or CCW and see if the idle levels out. Good luck
 

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Discussion Starter · #9 ·
Not sure. But I am planning on doing a full tune up soon. The P0138 code is what is making me think its noy plugs or wires
 

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First, you don't need to take your '97 to Autozone to read its codes. Simply turning your ignition switch on-off-on three times within five seconds and leaving it in the on position after the last cycle will start the built-in diagnostic routine. Give it several seconds, the codes won't show up right way.

Once you do that, you'll see several '43' codes which are the older equivalents to the misfire codes Autozone gave you. You'lll also probably get a 31 or close code for the 02 sensor, and the final code will be a '55' which means no more codes follow.

Ok... on early TJs including '97 and '98, many received a bad/weak set of valve springs that cause cause random engine misfires. The fix is to replace the valve springs with the current p/n which no longer has that problem. That will either cure the misfires or reduce them substantially. If it doesn't eliminate them, then the bad valve springs caused the valve seats to burn/go bad which requires a complete valve job to finish fixing the misfire problem. Replacing spark plugs, distributor cap, fuel injectors, the rotor, ignition wires, etc. won't likely cure this particular problem.

Once you get the codes generated on your TJ using its internal diagnostic tool, let us know what all of the codes are which will help pinpoint the potentially bad 02 sensor the P0128 code indicated. Your TJ only has two 02 sensors and if one is bad, it's likely the one just below the exhaust header... the exact code will confirm whether it's that one or the one on the catalytic converter which is far less likely.
 

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Discussion Starter · #11 ·
Wow thanks, I did not know about the internal fault system. This is what I got:

12, 43, 43, 43, 43, 21, 55

The 12 is just the battery being disconnected on last 50 starts. 43 are the misfires. 21 is a bad O2 sensor.

So, do you think its valve springs?
 

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Sounds like it to me... a common issue for 97/98 TJs... enough that Jeep issued a TSB (tech service bulletin) for it that advises changing the valve springs.
 

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new to the jeep forum but was wondering about the automatic diagnostic system. Where does it show the codes.
We don't know what year TJ you have so that procedure may not work. Doesn't work on my 98 for example. Uses the the one spot on your gauge cluster than can flash numbers... :) What year is your TJ?
 

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Discussion Starter · #17 ·
As an update, I pulled the head off and changed the valve springs. All codes disappeared after about 20 cold starts. Thanks Jerry Bransford!
 

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As an update, I pulled the head off and changed the valve springs. All codes disappeared after about 20 cold starts. Thanks Jerry Bransford!
Congratulations. Not to burst your bubble but the head doesn't need to come off to change the springs. You can just pressurized the cylinder's with compressed air and use a spring removal tool. Did you change your valve seals too? Anyway, great job. Jerry is huge asset here.
 
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