For starters I'm new to this forum and a first time poster. I've typically been a browser for information (great forum for it) to fix my JK problems but this time things may be past my knowledge. Hopefully I can find some help here.

2008 JK Unlimited X, Automatic, almost 200K miles

Driving down the highway this past weekend felt a huge knock under the hood and then it started behaving abnormal. I've learned to live with the tranny shudder as I've read it's pretty typical with Wranglers but this was something different. I continued driving and the CEL light came on and started blinking. Limped it to Autozone (2 miles) and had the codes pulled (P0300 and P0339). Decided to throw some money at the problem and bought new crank position sensor, plugs and wires (realize it's getting older and some of these things need to be replaced regardless). Cylinder plugs 1 and 2 (especially 2) looked more fouled than the others. I also pulled the battery cables off and cleaned the connections. I also thought maybe running some high quality fuel through the engine might help. After replacing these items and adding the upgraded fuel the car ran "better" but I was still getting the blinking CEL and acceleration shuddering so it's obviously something else.

Things I've replaced or had replaced:
-2014 car had a PCM fire under the hood while driving; PCM, relay box and wiring harness replaced; Ran fine after repair.
-2017 CATS were replaced.
-2017 Battery
-All O2 sensors have been replaced in last 4 years.
-Coil pack within the last 2 years.
-2018 crank position sensor, plugs, wires.

I was considering replacing the fuel injectors (read this on another forum) since they are almost 10 years old and they've never been replaced. Is that a horrible idea considering the age? Any thoughts or ideas on what else it could be? I don't mind throwing some money at these older parts with the hopes of getting a couple more years out of the vehicle. Thanks in advance for any help or suggestions!

Update: After changing plugs, wires and crank position sensor it is now throwing code P0302 (using key on,off,on,off,on method) and the CEL is still blinking.

 

Did you clear the codes to see if the same codes come back?

 

The CEL light (and I'm assuming the codes) cleared when I disconnected the battery to clean the terminals. I drove it around yesterday for a bit because the CEL was off but when I started it up to head to work this morning it came back on and started blinking again. So I parked it and took my wife's car to work. I'll check the codes again when I get home this evening.

 

Driving with the blinking CEL will rapidly ruin your catalytic converters. I mean like in minutes, if not seconds.

The crank position sensor was a good call, can cause the P0300 and of course P0339. What code does it have now? Get a bluetooth code reader and know for sure without having to go to Autozone every time.

There are a number of things that can cause a P0300. Since you fiddled with plug wires I'd say that's actually a likely cause. Also check the rest of the ignition system (coil packs, if it has them, etc.). Could also be worn valves/seats, a burnt valve, jumped time, etc.

 

Unfortunately it's a little too late to not have driven it but I have parked it until I can figure out what's wrong with it. I checked the codes again tonight using the key method and it threw a P0302 code (I guess this singles out the second cylinder which I previously said was the most fouled spark plug). At this point I'm guessing it's because the cylinder isn't getting fuel or there is something mechanically wrong with cylinder 2. Advice?

 

Can I test whether the coil pack is working by removing the bank two spark plug (not unplugging it from coil) to see if it's getting fire without damaging the engine?

 

It is now throwing code P0302 with the check engine light still blinking.

 

P0302 with the CEL blinking means you have a consistent hard misfire on cylinder #2. There are three major causes of this:

1. no ignition on that cylinder -- if this is the case, then the engine will continue dumping unburnt fuel into the exhaust and rapidly fry the catalytic converters
2. poor or no compression on that cylinder -- this would be due to a bad ring, burned or bent valve, receding valve, or mistimed engine (timing belt off a tooth etc.), maybe bad head gasket, a few other causes. In this case you are getting combustion so you are not dumping fuel into the exhaust but you just don't get enough of a pulse from the cylinder to keep the misfire from triggering
3. bad fuel injector on that cylinder -- could be clogged or just bad. In this case you aren't dumping fuel at all.

IDK how hard it is to swap fuel injectors or whether you can swap coils on your engine. I don't really have much experience working on Jeeps at all and certainly none on the 3.8. But it's reasonably common to try and diagnose misfire this way:
1. test the compression on the cylinder in question. if it is bad, well you have your cause. Need to figure out what is causing bad compression.
2. swap the coil (especially with coils-on-plugs) to a different cylinder, see if the failure follows the coil, if so it's a bad coil. Does the JK 3.8 have coils on plugs? I figured all cars did by then...
3. swap the fuel injector from that cylinder to another one, see if the problem follows the fuel injector. If so, replace the bad injector.

If it was an ignition problem then you may have put the hurt on your catalytic converter. If so then you will get an oxygen sensor low code pretty much right after you fix the bad coil. While the coil-on-plug would be a cheap fix, the collateral damage is expensive.

 

This is from the JK service manual:
NOTE: Anything that affects the speed of the crankshaft can cause a misfire DTC.
NOTE: When a Misfire is detected for a particular cylinder, the PCM will shut down that cylinder's Injector Control circuit.

 

I'll stop and get a spark tester tonight to see if the coil is bad first (although it's relatively new I'll still check it). On this model of Jeep the coil is separate from the plugs. If the coil checks out I'll do a compression test on the second cylinder (after I get a compression testing kit). Although if it gets down to the internal parts of the engine being worn out or broken that's above my mechanical know how pay grade. If worse comes to worse I'll toss a couple fuel injectors in there and see if it improves. Thanks for the help I'll report back with anything that helps!