Novak Conversions Jeep Wrangler TJ engine mounts

4.0 misfire codes and priming start

Gibson125t

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Original poster
Joined
Nov 29, 2024
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Florida
I will start by saying I have searched. I have read. I am posting to see if my specific combo of things might help with the diagnostics or a starting point. apologies in advance. i didnt see these specific things and it seems these codes have a ton of different causes.


now. for whats up. I just purchased this jeep wednesday night. looked at many and finally settled on this one. 2003 rubicon. 160k miles. manual trans. 4.0L. the only issue it seemed to have mechanically was axle lockers not working and you have to prime to fuel to start it up (use the key to get the fuel pump to run a couple times). after you do this it starts up fine. idles fine. runs fine. if you shut it off and start it right back up its fine. if you go in the store, shop around, come back. you need to prime again. PO said his thoughts where vacuum leak (he said why he thought this.... i remember it sounded reasonable... but I dont remember what it was... I just remembered the hilights... needs priming... check for vacuum leaks first to fix)

so anyway. I drove it home from the seller. about an hour drive. ran over something and got a flat. (this is my type of luck) so it stayed home yesterday. took it today for a patch and run a few errands. while out today check engine light came on (steady.. not blinking). so when I got home I connected my scanner and I got the following codes:

Current faults:
P0300 - Random/multiple cylinder misfire detected.
P0301 Cylinder 1 misfire detected.
P0455 - Evaporative emission control system leak detected.
and finally...

Pending fault:
P0302 cylinder 2 misfire detected.

my first wonder here was "did the previous owner just swindle me? clear the codes and let me ride off?" however. he was honest about everything else. even the axle locks not working which he could have easily kept to himself and most people wouldnt know until its too late... anyway... I tried to clear the codes myself to see how long until they pop up again. (idk if its right... but its my go to for a new code... my way of seeing if its some weird one off thing... or a reoccuring issue). and it basically doesnt clear. check engine light stays on. check codes again and they are there. so... i figure he couldnt have cleared them and kept them away while I test drove and then drove an hour home.

so... my question here is... could there be something that matches the misfire codes with the priming fuel issue that gives an indicator for first try? what about that evap code exactly? googling it seems like its kind of a vaque code (truth is they all do... wish I had an actual diagnostic scanner.. but thats a pretty penny). I figure i would ask because.. on one hand... I dont want to chase down a vacuum leak if that actually seems like its unlikely here. on the other.... no need to spend $200 replacing coil rail and plugs first if these signs lean towards vacuum leak ( which, BTW, the coil rail does look somewhat new. so at least its not the original and this has obviously had a tune up...... seemingly in recent history. though exact time would be unknown). seems like the priming and misfires could be related to a fuel delivery issue? but then... it would have me wondering why the first cyl is taking the brunt of the misfiring?

also. I wont say I have an ear for saying yes or no to a little difference in sound given that i am not familiar with this motor and only drive it for an hour prior (with the top back and other noise for good measure). but I can say... that driving it today after the light... there was no backfireing or poping or anything obvious going it. it idles steady at about 800rpm and if this light wasnt on... i wouldnt have thought any issue (but again... 1 cylinder could be completely dead and my ear wouldnt know without having a good reference for this motor prior... I noticed a misfire on a truck once before but I had driving it daily for a couple years and was very used to its sound. im no mechanic. I do most of my own vehicle repair work... but... there is plenty i dont know.
 
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the having to cycle your key a few times to get the fuel pressure up is either a bad check valve in the pump or a leaking injector. As for your misfire codes my 1st thing would be to replace the spark plugs with Autolite XP985'S. how many miles are on your RUBI ?
 
the having to cycle your key a few times to get the fuel pressure up is either a bad check valve in the pump or a leaking injector. As for your misfire codes my 1st thing would be to replace the spark plugs with Autolite XP985'S. how many miles are on your RUBI ?

160k. so your thinking here is that they are unrelated? not like a... pressure to the injectors issue for the misfire? I was about 3 seconds from going and getting plugs and a coil rail when I decided I should first ask here about it incase there is a common denominator for the causes.
 
160k. so your thinking here is that they are unrelated? not like a... pressure to the injectors issue for the misfire? I was about 3 seconds from going and getting plugs and a coil rail when I decided I should first ask here about it incase there is a common denominator for the causes.

Yes I'm thinking they're unrelated. but honestly with me I always like to change the plugs when I buy a used vehicle. pulling them can tell you a lot. my brother bought a 06 Rubi with 93k miles and had the same codes misfiring cylinders and it would skip enough we knew . Pulled the plugs to change them and no wonder it was throwing codes, the plugs were orig. and had about .100 gap ! get you a set of XP985's and the wire type gap tool, don't use the flat one's on Iridium tipped plugs.
 
Yes I'm thinking they're unrelated. but honestly with me I always like to change the plugs when I buy a used vehicle. pulling them can tell you a lot. my brother bought a 06 Rubi with 93k miles and had the same codes misfiring cylinders and it would skip enough we knew . Pulled the plugs to change them and no wonder it was throwing codes, the plugs were orig. and had about .100 gap ! get you a set of XP985's and the wire type gap tool, don't use the flat one's on Iridium tipped plugs.

you suggesting I replace just the plugs first? see if that fixes it before replacing coil rail? coil rail is a little pricey so I am honestly OK with that anyway.
 
you suggesting I replace just the plugs first? see if that fixes it before replacing coil rail? coil rail is a little pricey so I am honestly OK with that anyway.

I would wait on the rail myself. I know the XP985's are expensive compared to other plugs but they'll last you over 100k miles or more.
 
sorry for my late reply..... I changed the plugs saturday night. interestingly. these autolites are what was already in the jeep as well. eitherway.... it didnt let me clear the codes afterwords either the first attempt. but second attempt it did. I then drove about 100 miles with no check engine light on sunday verifying when I finished the day at was at home... still no light. however, monday morning when I cranked up the jeep I had a light. as if the misfires where from starting it up. speculation... as it was started up 3 times during the "no light" time without it coming back. eitherway... its back... sunday I visited a friend for non-jeep reason. but he was a mechanic for a bit years ago and has done some awesome builds in the past. not to say he is all knowing or anything. but... since I was there. I ran it by him to see if he hears the misfire or anything. he wanted to give the jeep a drive anyway. he also was confused by the codes as he also said it didnt seem to be running like it was having a consistant misfire. further adding to my new theory that its happening at startup only. (but... just a theory....)
 
i am following as similar problem.
have you cheked/replaced the ignition coil?

I have determined, with certainty I believe, that for me the misfires are at start up only because of the pressure valve needed replaced. I can sometimes here the misfire at start up, and it goes away once it gets going (within a few seconds) I never hear it otherwise. And it's not always throughing the codes. After a while the light goes off and it may take days (sometimes well over a week) for it to through a code again. Which I believe is because I have learned better how much to prime before starting so I don't grt the misfire as often, as it seems to be due to a lack of fuel at first. I have not yet replaced the pressure valve as it's on the top of the fuel pump so I am waiting until I have time to drop the tank. And so I can't be 100% until I do. But I feel pretty confident this is correct now.

For reference I replaced my spark plugs. Later on I checked them all for spark and seem good, and I checked compression in the cylinders and they seemed fine. And there is no indication at all that it's misfiring while driving (no sound, no power loss, etc).
 
Novak Conversions Jeep Wrangler TJ engine mounts