2018 JL Wrangler 3.6 Multi-cylinder misfires, fuel soaked spark plug, poor fuel economy. Missing coolant. Root causes?

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Hey guys, this is not my Jeep but it belongs to someone important to me. She bought it new and has 44k on it.
Naturally, the first check engine light came on while I was messing with the manual mode for the auto trans... Miraculously, I was not blamed for the check engine light.

It's just out of bumper to bumper but still under powertrain. Hoping someone on here knows about what's usually wrong with these Pentastars.

It throws a intermittent multi-cylinder misfire code that can easily be induced by holding a gear under load. Fuel economy is down, according to her, about 3mpg and it feels like it's lost power. I can confirm that it does feel like there's something up with the engine on a hill and the idle seems off to me but I don't know what's normal for a 3.6. In the past few thousand miles it's also lost about 20 oz of coolant from the reservoir that I filled up a couple months ago when I replaced the air filter.

On my scan tool I can see the long-term fuel trims while driving are off about 8-9% between the two banks. Catalytic converter temperatures are the same though. I took out one spark plug after it had been sitting for over 24 hours and it was totally soaked in fuel. More than I've seen on any fuel injected car or even carbureted car.

I could already tell from the phone call she had with the dealer that they were trying to brush off the problems. I want to be informed so I can blast anyone if they try to downplay these issues. I saw a ton of complaints about multi-cylinder misfires on the Jeep forums but never any resolutions.

When you cold start this Jeep you can see a light haze of smoke in the xenon bulbs. If it is an injector problem I think they should do a compression test at FCAs expense. Kind of wondering if a cylinder has been washed out enough to hurt the rings.
 
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doesn't that thing have a 5 year 60k warranty? Take it in don't mess with it unless you've been told you absolutely do have to pay for it.
 
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Gasbuggy

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doesn't that thing have a 5 year 60k warranty? Take it in don't mess with it unless you've been told you absolutely do have to pay for it.

It's out of the bumper to bumper warranty. It's under powertrain but FCA has a pretty cleverly worded powertrain warranty that won't cover things like say a bad coil pack boot. However I can't purchase coil pack boots, only a whole coil assembly.
 
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The catalytic converter temperatures are calculated, not measured. If it's dumping excess fuel the calculations are probably wrong and the cat is much hotter than reported.

The good news is, that Federal 80K emissions warranty covers the cat...
 

4WD

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Bad O2 sensors can cause symptoms or even clogging of cats
If it’s a head gasket as mentioned - coolant might reach a cat …
 

4WD

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They usually don't go bad in only 44K and they should set a o2 sensor code.
Temp gun is how I’d start - but to pressure test won’t it be that same inlet ?
I know how cats last when things are normal … but coolant, fuel, and oil change that …

OP: does it use oil ?
 
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Temp gun is how I’d start - but to pressure test won’t it be that same inlet ?

I would pressure test the cooling system and leave the pressure on for as long as possible. Then, if it won't start because a cylinder is filled with coolant, get it towed to the dealer!
 

4WD

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I would pressure test the cooling system and leave the pressure on for as long as possible. Then, if it won't start because a cylinder is filled with coolant, get it towed to the dealer!
Sad part is a good dealership would be doing all this …
Think I need to read up more on warranty for my 2020 …
 
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What is the actual code? P0300?

Seems to be a lot of concern for something that is most likely a plug or coil. I am not sure on the JL but on the JK the overflow bottle is vented to atmosphere and some evaporation over time from the cooling system is normal.
 

Gasbuggy

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Temp gun is how I’d start - but to pressure test won’t it be that same inlet ?
I know how cats last when things are normal … but coolant, fuel, and oil change that …

OP: does it use oil ?


It does not use oil.
 

Gasbuggy

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What is the actual code? P0300?

Seems to be a lot of concern for something that is most likely a plug or coil. I am not sure on the JL but on the JK the overflow bottle is vented to atmosphere and some evaporation over time from the cooling system is normal.

Correct P0300

There do seem to be some changes for this 2018+ JL. I'm almost positive it has an integrated coolant tank, like what I'm used to seeing on GM vehicles.
 

Gasbuggy

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I would pressure test the cooling system and leave the pressure on for as long as possible. Then, if it won't start because a cylinder is filled with coolant, get it towed to the dealer!


That is a clever suggestion. Thank you
 
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