2006 4.0 issues misfire and catalytic converter codes

fourwheelinj1

Well-Known Member
Joined
Apr 11, 2011
Location
Raleigh
I bought a 06 tj with 202k miles on it a few months ago. It had several small issues that needed fixing like valve cover leak, axle bearing, needed tune up and a check engine light for a cylinder 1 misfire. I fixed all the small problems and replaced the spark plugs which the PO had never replaced. The old plugs were toast. I also did a compression test. Cylinder one was a little over 150 PSI and the other 5 were in the high 170's. I didn't like the variation but seemed to be with tolerance. I also had to replace the exhaust due to the PO welding up his own exhaust in the wrong place which caused the front driveshaft to hit the exhaust. I replaced the exhaust with a set from a 04 TJ with 60k miles. After getting all that done I took it out for a drive and it ran good with no codes. I took it to get a state inspection and it passed just fine. Then on the way home a had a brief shutter but then smoothed back out. Now after putting about 20 more miles on it I have a check engine light and three codes and it runs crappy at idle but seems fine above idle. I am also getting a rattling sound at idle. I don't remember that sound from before. The three codes I am getting are.

P0301 Cylinder 1 missfire

P0431 Warm Up Catalyst Efficiency Below Threshold (Bank 2)

P0627 Fuel Pump "A" Control Circuit / Open ( I may have caused this one when trying to cycle the key to check codes)

Engine cranks good and runs good above idle. Everything under valve cover looked really good and very clean no visible issues. I am thinking that the used exhaust I bought might of had bad CATS in it. I am hoping the rattle is coming from a bad front CAT and not the engine itself. I am just not sure what to think about the misfire. The fact that the misfire is on the same cylinder as the lower compression has me concerned. I know lots of things can cause the misfire.I am thinking I need to fix this before I bother messing with replacing the CATS. Thoughts on what to try first? I know it could be a coil, injector, short in wiring, vacuum leak, burnt valve, carbon build up etc. I don't want to waste money throwing parts at it so what would the most likely causes be or what would you try first given your experience with 4.0 engines?
 
The P0431 code could be the downstream O2 sensor. It's what is compared to the upstream wideband O2 to determine cat function. If the O2 sensor goes bad, you can get a false "cat bad" reading.
 
Yeah I know about the o2 sensor part. I am really not too worried about fixing that code until after I get the misfire fixed. Anyone have any suggestions as to fixing the misfire?
 
What type of plugs did you use, I've found those engines like the OE style champions, I've also seen the coil pack break down.
 
Monitor the bank 2 downstream oxygen sensor. If the voltage fluctuates more than a tenth of a volt, the cat is bad. End of discussion. The upstream sensor should bounce around as the reading is pre-cat but the downstream should remain very constant. If the voltage does not fluctuate, the sensor is bad.
For the misfire code, fist thing to do ithe old dollar bill test. At idle with it running poorly, hold a dollar bill at the end of the tailpipe. See if it ever gets sucked in as it's running. If so, you have a poorly sealing valve.
Like was said, 4.0 run best on old school Champions. RC12LYC FTW!
 
Last edited:
I did install the oem champion copper plugs. I will try a leak down test next, hopefully today or tomorrow. I am puzzled by the fact that it ran good so good when I first took it for a drive and the. After 20 min or do then changed to running crappy. I was thinking that if it is a valve or ring problem it would have never run good.
 
Well I did the Dollar bill test and it appears to be sucking back in. I then did a leak down test on cylinder 1 and got 60% leakage and air coming out of the exhaust. So I am guessing I have a exhaust valve problem. I am surprised I got 150+ psi on a compression test and then 60% leakage on the leak down. Is there anything else for me to look at before I bite the bullet on pulling the head to get it rebuilt?
 
fourwheelinj1 said:
Well I did the Dollar bill test and it appears to be sucking back in. I then did a leak down test on cylinder 1 and got 60% leakage and air coming out of the exhaust. So I am guessing I have a exhaust valve problem. I am surprised I got 150+ psi on a compression test and then 60% leakage on the leak down. Is there anything else for me to look at before I bite the bullet on pulling the head to get it rebuilt?
Click to expand...

Unfortunately no. The heads gotta come off regardless at this point.
 
I guess a weak valve spring is not likely? If the valve is bad I can’t understand why it ran so good for a short period.
 
Last edited:
Back
Top