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Misfire

XJensen

XJensen

NAXJA Forum User
Location
Sacramento
Back in July my Jeep had a misfire in two of the cyl and ran rough for a few seconds then smoothed out and everything was fine. This was while the jeep was running. It also threw a code for the evap circuit.

Two days ago on a snow run the I started the jeep after stopping and it ran rough, very low idle. I gave it a little gas and everything smoothed back out.

About 15 minutes later we stopped and I shut it down again. When I started it up it misfired and ran a low rough idle. The oil pressure read zero for about five seconds. gave it a little gas and the pressure shot up to normal and everything smoothed back out. This is when the engine light came on with the misfire codes and I smelt a slight burnt oil smell so I would assume the gauge was right and it did not get oil for a few seconds. I left it running the rest of the day until I got home.

I was going to change the oil, needed it anyway, and add some more engine restore to it as well as check and change the spark plugs. What else should I check and/or replace before trying to fire it back up?

Edit: It's an '01 with a completely stock engine BTW

Josh
 
Last edited:
Get those new spark plugs in there and see how it runs. Make sure you don't use BOSCH plugs, the 4.0 doesn't like em. NGK is a nice plug for the 01 and others use Champions.

Keep an eye on that oil pressure gauge.

And as a matter of course, with an 01 and the infamous 0331 head which can crack, always monitor your coolant level in the reservoir, as the #1 symptom of a cracked 0331 head is coolant consumption.

Post the new exact codes here for comment; want to be make sure you're not missing anything that the OBD system is picking up.
 
birchlakeXJ said:
Get those new spark plugs in there and see how it runs. Make sure you don't use BOSCH plugs, the 4.0 doesn't like em. NGK is a nice plug for the 01 and others use Champions.

Keep an eye on that oil pressure gauge.

And as a matter of course, with an 01 and the infamous 0331 head which can crack, always monitor your coolant level in the reservoir, as the #1 symptom of a cracked 0331 head is coolant consumption.

Post the new exact codes here for comment; want to be make sure you're not missing anything that the OBD system is picking up.
Click to expand...

Always post up the code numbers.

NGK V-Fire plugs for the 00-01 4.0 distributorless ignitions.
 
The code to key in on here is the P0300 (random misfire). The other cylinders misfiring are just along for the ride.

If you find a P0300 random misfire code, it means the misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine's air/fuel mixture, such as a major vacuum leak, or unusually low fuel pressure (weak pump or faulty pressure regulator).

Job #1 is to test your fuel pressure WITH A GAUGE. If you don't have access to one, they can be rented cheaply from a parts store for a few bucks.

For an 01, you should have 49 psi at idle, plus or minus 5 psi.

I'd start there, along with that search for a vacuum leak.
 
I agree with birchlakeXJ.

One other thing to consider is a possible synch sensor alignment issue.
 
Well the fuel pressure was 45 lbs with the ignition on and 48 lbs with the engine running.

I replaced the spark plugs and the old ones looked like crap. I will try to post a pick later. I figured this was the cause of the issue.

I fired it back up and the oil pressure continued to climb and I shut it down as it was passing up 60 lbs and quickly heading to 80 lbs.

I disconnected the battery for five minutes, reconnected and tried it again. Same thing happend with the exception of the oil pressure gauge pausing and shaking for a second just before it hit 40 lbs. It the quick shot up toward 80 lbs before I shut it down.
 
after reading up on it a bit it sounds like it is most likely the sending unit. Someone suggested this to me on the trail when the pressure would not climb until I gave it gas.

I also read something about a bad ground to the engine or fire wall. I just hooked up my winch and replaced the negative leads. All Napa had was 4 gauge, much larger diameter than what I had replaced. Could the issue be the body and engine grounds I replaced?

Thanks
Josh
 
You may very well have two separate things going on.

1. A flaky oil pressure sending unit (very common, replace it)

2. Whatever is causing your misfire codes. An oil pressure sending unit isn't related to your misfires.

**Keep an eye on your coolant level in the reservoir just as a matter of course as the 00-01 has that infamous 0331 head

**These 01's are sensitive to the camshaft position sensor indexing as Joe mentioned earlier. I'd try that.
 
birchlakeXJ said:
You may very well have two separate things going on.

1. A flaky oil pressure sending unit (very common, replace it)

2. Whatever is causing your misfire codes. An oil pressure sending unit isn't related to your misfires.

**Keep an eye on your coolant level in the reservoir just as a matter of course as the 00-01 has that infamous 0331 head

**These 01's are sensitive to the camshaft position sensor indexing as Joe mentioned earlier. I'd try that.
Click to expand...

Coolant is just below the add line now. :gag:

I will fill it back up and try to drive it around a little so I can watch it.

I will post up when I know more.
 
Just figured out that my jeep managed to suck down half a tank on the 66 mile trip to the meeting place of our snow run. It was all highway driving with about 1/4 of the trip being in the hills. Still should not have used half of what I did.

I also noticed a significant loss in power going up those hills.
 
If you have multiple misfires, a significant drop in gas mileage isn't unusual. As well as power loss.

Need to isolate that misfire issue. Then see where you are at .......................
 
Well i tried to fix the oil pressure issue before checking the cam sensor for the misfire issue. I figured the oil pressure would be the easier of the two problems to fix.

I swapped in a new oil sending unit, took 5 minutes, and did an oil change. The oil pressure gauge now reads 40 psi at idle, not 80, but still very high. There is also a fairly loud ticking coming from the rear of the valve cover, at least that's where it sounds like it is coming from. I can hear it better from the pass side but is plenty loud to hear from in the cab.

The oil pressure and engine temp gauges shutter sometimes when the clicking goes off. The clicking kind of sounds like the ratcheting of a gas cap or oil cap. I can post a recording later.

I don't have a clue and would appreciate any help.
 
Hey guys i was reading about this and thought i would see if you could help me out with my problem haha
So cylinder has a misfire tried plugs tried seafoam etc well now my jeep is throwing 7dif codes and no matter if i fix them the codes are still there
misfire code
o2 sensor 1 and 2
and torque converter code
and tps
i did a full trans flush on it and put all new shift sensors in the trans
tps is brand new
o2 sensor brand new
i was told it could be the computer going haywire
any ideas guys????
 
xjpride.........would be best if you started a brand new thread as this one is not resolved yet.

New threads for new problems / different owners makes for easier reads and less confusion. Which results in more replies for you.
 
Pulled the #6 and 5 plugs to see if that is where the noise is coming from. The #6 plug was fouled with a little oil. Everything is running smooth now.

I will take it for a drive to check if there was an improvement in gas mileage and watch the coolant level. I will post a pic of the old plugs later but they were very bad and I think may be the cause of the misfires or at least a contributing factor.

The oil pressure remains at 40 during idle and 45 when I bring the rpms up. This is after replacing the sending unit with a Napa Auto part. I have since read the OEM unit is worth the extra coin to ensure proper readings from the guage.

Josh
 
oil pressure seems to be all running fine, oil sending unit fixed it. I took it for a test drive and it reads oil pressure at 50, normal for my jeep, when under power and at 20 when at idle, was around 10 but does not seem to be a big deal.

Now I can focus on the big problem, the misfire.
 
You could be having a heat soak issue, thats what it sounds like to me. If your engine heat soaks and the fuel in the rail is all airy it will run rough and have a hard time idling. If your Jeep has a low idle and is miss firing until it clears out then goes away your computer could set the check engine light because of how it was running for that moment in time.
 
Alex E said:
You could be having a heat soak issue, thats what it sounds like to me. If your engine heat soaks and the fuel in the rail is all airy it will run rough and have a hard time idling. If your Jeep has a low idle and is miss firing until it clears out then goes away your computer could set the check engine light because of how it was running for that moment in time.
Click to expand...

Yeah I think that may be the case as well but it still does not explain why I got 6 mpg on the way to the trail! I didn't even have any gear.

Josh
 
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