Troubleshooting Check Engine Light

BlacknRedJK

Member
As the title states I have been experiencing a temper mental MIL that comes on when I fall below a half tank of fuel. I have used the key cycling trick to read the DTC's and it comes back clear. The MIL shuts off when I refill the tank above half full. Unfortunately, I am at school and don't have a way to read the codes other than the key trick.

Any ideas what it could be or how to diagnose the problem?
 

WJCO

Meme King
See if parts store like Oreillys or Autozone can. It's free. My guess is you have an evap leak or pressure issue somewhere in the upper tank half or something similar.
 

BlacknRedJK

Member
See if parts store like Oreillys or Autozone can. It's free. My guess is you have an evap leak or pressure issue somewhere in the upper tank half or something similar.

Thank you for the help WJCO. I will see what we find with their scanner.
 

8POUND

New member
Do you happen to have a locking gas cap? They have been known to leak and cause the computer to throw a code.
 

8POUND

New member
Cool. Just wanted to ask. I swapped to a locking style on my GC and within a couple days, I had the MIL going off.
 

BlacknRedJK

Member
Cool. Just wanted to ask. I swapped to a locking style on my GC and within a couple days, I had the MIL going off.

Thank you though. I will run to Orielly this afternoon when I'm done with classes and report back with my findings. Hopefully it's an easy fix:thumb:
 

BlacknRedJK

Member
Okay you mechanical wizards.. throwing codes:

P0300-multiple cylinder misfires

P0302-cylinder 2 misfire

My next question would be why is it only throwing these codes when I'm below half a tank of gas? Is it coincidence or is there something I'm missing?
 

WJCO

Meme King
Okay you mechanical wizards.. throwing codes:

P0300-multiple cylinder misfires

P0302-cylinder 2 misfire

My next question would be why is it only throwing these codes when I'm below half a tank of gas? Is it coincidence or is there something I'm missing?

Very strange. I would honestly start with adding a fuel cleaning additive or injector cleaning service. Do you have any misfire symptoms like running rough or just the light? Your profile is correct, it's the 3.8 engine?
 

BlacknRedJK

Member
Very strange. I would honestly start with adding a fuel cleaning additive or injector cleaning service. Do you have any misfire symptoms like running rough or just the light? Your profile is correct, it's the 3.8 engine?

Sea foam or any others you would recommend? I have had a slight rough idle upon startup but it is not happening with every start. Seems to come and go.

3.8 is correct.
 

WJCO

Meme King
Sea foam or any others you would recommend? I have had a slight rough idle upon startup but it is not happening with every start. Seems to come and go.

3.8 is correct.

It's been awhile since I've used one. Hard to say. I've had good luck with BG 44k for pinging. There's just so many on the market now. I've heard good things about seafoam.
 

BlacknRedJK

Member
It's been awhile since I've used one. Hard to say. I've had good luck with BG 44k for pinging. There's just so many on the market now. I've heard good things about seafoam.

I will go grab some tonight and see if it makes any difference. I'm suspecting that it it's injector based as I haven't cleaned the fuel system as recommended at 60k yet. Thanks for your help.
 

BlacknRedJK

Member
Just want to shed some light on this again. I ended up not trying the fuel additive, but I did fuel up with 87 octane instead of 91 like I usually run and the CEL seems to have gone away. For those of your who are mechanically inclined does this make sense?


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jeeeep

Active Member
Just want to shed some light on this again. I ended up not trying the fuel additive, but I did fuel up with 87 octane instead of 91 like I usually run and the CEL seems to have gone away. For those of your who are mechanically inclined does this make sense?


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it ran ok to empty with 87 octane? are they both E10?
 

BlacknRedJK

Member
it ran ok to empty with 87 octane? are they both E10?

Missed this reply I apologize. Last two tanks have run fine with no CEL. Was at the dealership today to handle something else and had them pull the stored codes. P0302 was stored. Both fuels are E10 and it seems to like 87 octane. Could fuel really be the sole cause of the code?


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bigcale

Caught the Bug
What year JK is that on? That is the codes I had on mine before they replaced the bad head on my 2012.
 

WJCO

Meme King
Missed this reply I apologize. Last two tanks have run fine with no CEL. Was at the dealership today to handle something else and had them pull the stored codes. P0302 was stored. Both fuels are E10 and it seems to like 87 octane. Could fuel really be the sole cause of the code?


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Yep. When you get bad fuel, it can do a couple things. One is that it can plug the injector so it's not releasing the proper amount of fuel. Then when you have abnormal combustion, it can trigger a misfire code. The other thing that can happen, is bad fuel can cause varnish on valves and seats and cause them to stick. That usually will give you a P0316 code usually pretty quickly though. Either way, if you've been good with the higher octane gas, I would keep using it.
 

BlacknRedJK

Member
Yep. When you get bad fuel, it can do a couple things. One is that it can plug the injector so it's not releasing the proper amount of fuel. Then when you have abnormal combustion, it can trigger a misfire code. The other thing that can happen, is bad fuel can cause varnish on valves and seats and cause them to stick. That usually will give you a P0316 code usually pretty quickly though. Either way, if you've been good with the higher octane gas, I would keep using it.

I'm going to clear the code now and continue to use 87 octane. Time will tell if it will come back around and I have to go chasing further possibilities or if it was simply poor fuel.
Thanks for all the help.


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