Multiple Cylinder Misfire - Code P0300

Colorado4x4

Colorado4x4

Active Member
My 2016 Jeep JKU with 65,000 miles threw a code yesterday morning for P0300 - multiple cylinder misfire. This was while pulling out of the neighborhood on my commute to downtown Denver. I was idling down a small hill when the CEL came on. I cleared the code and made the 35 minute drive and it the check engine light did not come back on. The CEL did not appear on the commute back home at the end of the day yesterday (45 minute drive).

There were no symptoms that I could tell. No popcorn sound from the exhaust, no loss of power or engine shaking at all. I checked the resting battery voltage this morning and it was 12.7 V. I fired her up and checked idle charging voltage and it was 14.1 V. It appears the battery and alternator are ok. I then went on a 5 mile drive and did not get a CEL.

This is the first time that I have had this code P0300. It has been raining here a lot and we hit Spring Creek on Sunday and it rained the whole 2nd half of the trail as well as raining hard on the way home. Maybe the rain had something to do with ECM power/wiring?

I read online and usually if it was a serious misfire the computer will tell you which cylinder is misfiring. Also, if it is serious the CEL flashes, which it did not.

Here is what I am thinking
- wait to see if the code comes back and not jump to conclusions. Could it be bad fuel?
- check spark plug wires and coils. I am still running original spark plugs. How many miles have you guys gotten out of the OEM plugs and coils?
- check fuel pressure
- check for vacuum leaks
- check EGR - I have no idea how to do this so will look for a write up here. I think I saw one at one point

Thanks!
 
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Sweet. Thanks Eddie! Will it throw a code for bad O2 sensor or if there is an issue with the catalytic converter, or are those under P0300?
 
WJCO said:
I have seen P0300 codes set from poor gas. Did this occur right after a fill up? Just a thought.
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That makes me feel better. I filled up by the trailhead Sunday and then wheeled all day and drove 1 hour home on Sunday. Didn’t drive it at all on Monday and then it threw the code first thing Tuesday morning.
 
Colorado4x4 said:
Sweet. Thanks Eddie! Will it throw a code for bad O2 sensor or if there is an issue with the catalytic converter, or are those under P0300?
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If the O2 sensor gets a bad reading, it will cause a cylinder misfire. They should last at least 100k but you're getting close to it so it's something to look at. I would wait for another code to pop up before doing anything. You might get better info.
 
wayoflife said:
If the O2 sensor gets a bad reading, it will cause a cylinder misfire. They should last at least 100k but you're getting close to it so it's something to look at. I would wait for another code to pop up before doing anything. You might get better info.
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Thanks!!!
 
Mine had the same code around 60,000 miles. Cleared it and it kept coming back. Turned out to be a bad injector. Only happened at idle and was really not noticeable except the code being displayed. Had the injector replaced and had the shop do the plugs and new manifold gaskets. Runs great ever since.
 
Jkratt said:
Mine had the same code around 60,000 miles. Cleared it and it kept coming back. Turned out to be a bad injector. Only happened at idle and was really not noticeable except the code being displayed. Had the injector replaced and had the shop do the plugs and new manifold gaskets. Runs great ever since.
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Thanks! Good info. So my code did come on while idiling down the small hill leaving my neighborhood. How quickly would your code return?
 
Wifes pickup threw the same code the day I was supposed to leave for a trip. Turned out to be severe corrosion on the spark plugs and inside the plug caps. The little resistor pills in the plug caps were all corroded so bad I couldn't believe it.
 
Ddays said:
Wifes pickup threw the same code the day I was supposed to leave for a trip. Turned out to be severe corrosion on the spark plugs and inside the plug caps. The little resistor pills in the plug caps were all corroded so bad I couldn't believe it.
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Wow. That is crazy. Well, I made the commute to work today (40 minute drive) and no CEL so we will see what happens this weekend with a long drive!
 
Update: I have driven just about 1,000 miles since I started this thread a week ago. Commutes to work and a road trip to Ouray this weekend and not another check engine light has popped up.

I do have the JScan app and selected “cylinder misfire count” for each cylinder and logged most of this weekends road trip and a couple of the cylinders logged a misfire but I’m not concerned. What does the computer consider a misfire? Either way, it wasn’t enough for the check engine light to come on so I’m going to Jeep on and not worry about it. If it pops back up then I will update this thread. Thanks for all of your help.
 
I know it's a minor PIA on 2011 an newer Jeeps to pull the manifold but I would check the coil paks and replace the plugs.

Our Toyota had the same code off and on then ran great for a few months until it didn't.
When I pulled the coil paks to inspect the plugs, I could see a couple of the coil pak surfaces were discolored indicating failure, the plugs also showed signs of arcing from the burn marks on them.

better to check and know than getting stranded...
 
jeeeep said:
I know it's a minor PIA on 2011 an newer Jeeps to pull the manifold but I would check the coil paks and replace the plugs.

Our Toyota had the same code off and on then ran great for a few months until it didn't.
When I pulled the coil paks to inspect the plugs, I could see a couple of the coil pak surfaces were discolored indicating failure, the plugs also showed signs of arcing from the burn marks on them.

better to check and know than getting stranded...
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Not a bad idea. I should just go ahead and do it. Is there any other PM work that you recommend while replacing plugs?
 
Colorado4x4 said:
Not a bad idea. I should just go ahead and do it. Is there any other PM work that you recommend while replacing plugs?
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i'd get an intake manifold gasket kit since it's apart, replace all plugs.

It's an expense but i'd get all the coil paks and return what you don't use.

ask me what a PIA it is to make multiple trips to the auto parts store lol
 
jeeeep said:
i'd get an intake manifold gasket kit since it's apart, replace all plugs.

It's an expense but i'd get all the coil paks and return what you don't use.

ask me what a PIA it is to make multiple trips to the auto parts store lol
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Thanks buddy!!! I will probably just replace the coil packs while I am in there. I haven’t even looked at cost yet but I might just do it and be done with it.
 
On Random cylinder misfires I usually start with the simple and cheaper stuff like spark plugs. I replaced mine on my 2011 3.8 at 70k.

If it happened right after a fill up of gas at a station you don’t normally visit then the likely hood of bad gas is very possible. You could start by adding an octane booster to your tank or top off your tank with high octane fuel from a top tier fuel station like Sunoco.
 
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