Help cylinder 3 misfire

I bought a kJ a couple weeks ago. Don’t really know much about it. When I got it it was throwing a P0303 code. It has a misfire on the #3 cylinder. We changed the spark plug and the coil. It still has a misfire on the 3rd cylinder. She drives great other than the misfire and it won’t go up past 3,000 RPM. It starts bouncing right at 3. Drove it yesterday and the CEL went off and then the battery light came on.(has brand new battery bought it same day I bought the Jeep) The P0303 code went away and now it’s throwing P1682. Any ideas on what could be throwing these codes? And what’s the fix?!

What year is your KJ?

Battery light usually means alternator not charging...you need to put a digital voltmeter across the battery.
Should show 12.5 volts with engine OFF.
Should not drop below about 10 volts while cranking.
Should show about 13.4 volts while running.

Misfire could be the fuel injector.

Let us know the above information please then we can give you further advice.

You can download the Jeep KJ Service Manual for your year KJ...except 2004....below.

I cannot find P1682...need to know year of the Jeep but P1684 is "Battery Was Disconnected". Could be related to the Battery light coming ON.

With a meter across the battery while running voltage should be about 13.4 volts.
If not check the Grounding Straps grounding engine to chassis....a strap from the rear of each bank I believe. If 13.4 volts is not at the battery, move the lead to the large lug on the alternator....if 13.4 Volts is there with respect to ground then the thick wire going from the alternator to the positive lug of the battery has blown!

This thick wire is called a Fusable Link ie. it has fuse built inside it that will blow if there is a major short. If blown, a new correct rated Fusable Link has to be fitted.

Early KJs had a Battery Temp. Sensor located under the battery which measures the temp of the battery and gets the PCM to slightly adjust the charging rate.
What the Owner's Manual does not mention is that this temp. sensor can also cause the Charge Light to come on...I had this due to Ground going missing on the one side of the Temp Sensor!

www.colorado4wheel.com/manuals/Jeep/KJ/

It’s 2002 Jeep Liberty limited

CarLee said:

It’s 2002 Jeep Liberty limited

Click to expand...

OK then download the 2002 Jeep KJ Service Manual from the link and check the items I listed.
Section 8W has the wiring diagrams.

Make sure battery terminals are clean and tight both ends and the engine grounding straps are clean and tight all ends.

While the engine is running the large terminal on the alternator should be pushing out about 13.4 volts which should find its way to the battery unless the Fusable Link has blown.
There is a small plug on the alternator which is there to excite the field coil of the alternator so as to provide the power coming out of the alternator.
The signal on this small plug is a variable Pulse Width Modulation wave which can only be see by use of an Oscilloscope but you can make sure that the plug is firmly plugged into the alternator and make sure the plugs on the PCM are firmly in place...re-plug them a few times to clean up the contacts

If you are still not getting 13.4 Volts from the alternator then the internal brushes may be worn or dirty and the commutator rings may be dirty...opening up the alternator may show worn brushes or commutator rings...you can get the alternator tested to see if it is OK.

If the alternator tests out OK then the Battery Temperature Sensor located under the battery may be faulty. You can remove it and measure the resistance of it at room temperature then heat it up with a hairdryer or incandescent lamp to see if the resistance changes.

In my case the one side of this sensor is supposed to go to ground via a string of splices within the main harness. This ground was missing so I added my own ground from the sensor to chassis.

Hopefully this will clear your Charge lamp on the cluster otherwise you have a wiring error somewhere which is not fun!

Well the alternator was bad I replaced it and had the codes cleared it wouldn’t go past 3,000 rpms bc it was stuck in limp mode due to the coil went bad one the 3rd cylinder. I replaced the coil and spark plug and it stopped missing. Ran great and started running over 3 grand after the codes were cleared. I drove it like 6 miles and the damn check engine light came on again. It’s saying I have another misfire on cylinder 3 again. Any ideas on what I should do next?

The fact that after replacing the coil on cyl. #3 it worked for a while probably means a loose wire, bad connector or shorting wire somewhere around the area that you were working around. In other words you disturbed something while working there....works for a while then fault comes back.

Problem is probably the wiring for coil #3 making bad contact at the PCM or shorting to chassis somewhere, I will have a closer look at the 2002 KJ Wiring diagrams!