Hi All,

I'm new to the boards because we are at our wits end with this stupid P0303 code (cylinder 3 misfire). 2017 Jeep Wrangler JK Unlimited, 133000 miles. I am out of extended warranty and dont know where else to turn, figured I'd ask the experts! Got the code originally in May, so replaced all the spark plugs with NGK laser iridiums, Code went away for 2 months, life was good. August comes and the lights comes back, rough idle only in park, same code, of course , checked #3 replaced it because maybe it was a bad plug, at the same time replaced the coils as well with TRQ coils. YAY...check engine light went away again for about 3 weeks then lo and behold, "bling" check engine light again while sitting in traffic. Proceeded to replace all 6 fuel injectors and respective gaskets, running beautiful, purring for about another 2 weeks...check engine fixed! Today, check engine light comes on again, rough idle now both in park and at a stop! I'm so over this! Someone please help, what else could it be, what should we have checked? It's very worrisome since I no longer have warranty to take it in.

 

Sorry, just realized this got posted in the introductions, can a mod please move for me to the correct board??

 

thread is moved to Stock JK Tech.

So, i know this is for P0306, but all the cylinder misfires are going to be the same. I just happen to have these pages on hand cuz posted for someone else that was having cylinder 6 issue. pages from service manual attached.

If that were my jeep, I'd be doing a compression test on cylinder 1, 3, and 5 for a good comparison. You could have a head issue, or a head gasket issue. the good news is at least you're on the passenger side there and super easy to access the plugs and do a compression test vs. the driver's side.

 

Thank you! It turns out now we think its a vacuum leak in one of the intake manifolds. Cross my fingers I hope its that! Turns out the middle bolt was stripped, bought a new larger bolt but still having the same issue. Looks like we are going to try replacing the upper and lower manifolds now. My husband wants to go with aluminum but I'm hesitant hearing that the aluminum makes it hold heat and my jeep already runs hot.

 

if you think you have a vacuum leak, you could try to spray starter fluid around the manifold ports best you can. if you have a vacuum leak that starter fluid is gonna get sucked in and the engine is going to rev up.

 

He did, thats how we know just dont know if its from the thread being stripped on the center bolt or a crack in the lower manifold, doesnt look like its anything with the upper but who knows! Somewhere its not sitting and sealing correctly. All the hoses in that area look fine.

 

Well, I'd agree then, I'd much rather deal with that then something in the head! best of luck. please post a follup when you get it resolved!

 

Hi, just wanted to update incase it helps anyone else. Turns out there was a vacuum leak and it was the lower intake manifold. Bought one on amazon for about $60 and fixed the issue. Also, my husband had no idea that you have to do a bolt pattern with torque specs when putting everything back together which i think messed everything up in the first place when he did the tune up.