P0301 Cylinder 1 Misfire - JeepForum.com
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post #1 of 30 Old 10-12-2015, 09:32 PM Thread Starter
KSBoilermaker
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P0301 Cylinder 1 Misfire

Hey everyone,

I'm a long time reader/lurker but I usually don't have much to contribute compared to many of you until now. For the past several months I have been chasing the dreaded P0301 Cylinder 1 Misfire code that many others have encountered. After going through almost every thread remotely related to this topic I have seemingly tried almost everything to get this issue resolved. Hopefully this will help others that have been chasing similar issues. What I found most frustrating was that oftentimes people would never post what actually fixed their issue so myself, and others I'm sure, have been lost/misguided as a result. Anyways, below is a list of all of the things I've tried in order to correct this code. Unfortunately, I'm still chasing it but hopefully this will help 80% of the people who get thrown this CEL code.

This is on a 2000 Jeep Wrangler TJ Sport with 101k miles on it. 4.0L I-6 engine with a manual transmission.

A little backstory - this problem started with a P0340 code (no CEL) which prompted me to replace my camshaft position sensor, spark plugs and clean out my throttle body before a long road trip from Chicago to San Francisco. It fixed the rough idle/misfire for a few months (unfortunately, it did not fix the clutch that went out in Wyoming during the drive out here but I digress). Eventually the P0301 code came back and, after the long road trip, it finally threw the CEL. The CEL would come on, turn itself off, come on again a couple weeks later, etc. The problem was basically intermittent which made it especially difficult to pinpoint. Addtionally, my problem only occurred at idle. When I'd rev the engine or be driving at speed, I had no issues with misfires. I also never had issues getting the Jeep to start. I finally broke down, bought a scan tool (California has some silly law regarding Autozone not being able to loan you a scanner) and just cleared the code. It'd be fine for a little while, then you'd hear the misfire a little bit at idle and, eventually, it'd throw the CEL again.

List of what I've already done/replaced:
- Replaced spark plugs twice - once with platinum plugs, again with copper plugs. I ran platinum plugs for over 70k miles and never had issues, I figured when I replaced them that I'd get platinum plugs again. In the process of chasing this misfire, I ended up putting in regular Champion Copper Plus plugs (thinking that the better conductive properties of copper might resolve my issue). When doing so (and every subsequent time that I pulled the spark plugs out) I made sure to use compressed air so no dirt would cause a bad seal on a cylinder.
Write-up link: http://www.4x4xplor.com/sparkplug.html
- Replaced ignition coil rail - I also added dielectric grease to the boots to ensure a good connection. I wasn't going to do this but I found the part on Ebay for ~$50. I hadn't replaced it in over 50k miles so it was probably time in terms of preventative maintenance.
Write-up link: http://www.4x4xplor.com/coil.html
- Replaced camshaft positions sensor - I did this back before the road trip in relation to the P0340 code it was throwing for the CPS sensor.
- Replaced the throttle position sensor - afterwards I started throwing two codes (P0121 and P0123) that were related to the TPS. I also had problems with the Jeep jerking and erratic/high idle. I put the old sensor back in and, after clearing the codes, these two errors went away. The TPS I put in must've been a bad part.
- Cleaned the throttle body / air intake manifold and valve - I even used a toothbrush and rags to get it spotless.
- Ran multiple bottles of fuel injector cleaner through the gas tank (I used two cans Sea Foam, one bottle of O'Reilly's fuel injector cleaner, a can of Berryman's B-12 cleaner/treatment).
- Ran premium gas for a tank just to see.
- Performed an "Italian tune up".
- Ran >1/2 can of Sea Foam through the brake booster line and <1/2 can in oil crankcase. I've been monitoring my oil since adding it to the crankcase, it's a little dirtier than before but nothing to write home about.
- Tested the compression on each of the cylinders. If any are low, add a couple tablespoons of motor oil to that cylinder and retest - if the pressure goes up, you know it's a piston issue; if there is no change, then it's a valve issue. I didn't have low compression so I didn't bother adding motor oil to the cylinders.
http://troubleshootmyvehicle.com/jee...-compression-1
Results:
Cylinder 1 = 114 PSI
Cylinder 2 = 120 PSI
Cylinder 3 = 106 PSI
Cylinder 4 = 115 PSI
Cylinder 5 = 120 PSI
Cylinder 6 = 120 PSI
I performed this test on a cold engine so my values are a bit lower than what you'd see on a warm engine since there isn't any oil coating the walls of the cylinders to increase the compression numbers. The point here is that cylinder 1 (where my misfire is) isn't even my weakest cylinder. As a result, poor compression is ruled out as a cause of the misfire.
- Check if fuel injector #1 is functioning correctly - someone on one of the forums mentioned using a screw driver and touch the tip to the #1 injector and place your ear on the handle. You should hear a ticking noise. This is the injector opening and closing. Now listen to a couple of the others. Compare what you heard in #1 to the rest. If the ticking in #1 is faint compared to the others, or there's no ticking at all, you have an injector that's dirty or broken. Looking at fuel trims (if you have a scan tool) can help diagnose this too.
I found the first method (using a screwdriver) to be fairly ineffective since I couldn't hear a damn thing over the engine. Here is a better method: http://troubleshootmyvehicle.com/jee...uel-injector-1
The FSM says that fuel injectors should be 12 ohms +/- ~1.2 ohms at 68 degrees Fahrenheit.
My results:
Cylinder 1 = 13.2 ohms
Cylinder 2 = 13.3-13.4 ohms
Cylinder 3 = 13.2-13.3 ohms
Cylinder 4 = 13.3-13.4 ohms
Cylinder 5 = 13.3-13.4 ohms
Cylinder 6 = 13.3-13.4 ohms
Bottom line: All of my fuel injectors are pretty much within spec of one another so they are not the cause of the misfire either.
- Replace the fuel injector on cylinder 1 or swap it with another injector to see if the misfire moves.
Not necessary, resistance test showed that fuel injectors were not the issue.
- Performed a vacuum leak test using carb cleaner. The first time I did this I didn't find any leaks.

List of other ideas:
- Pull the throttle body and clean the idle air control sensor (reference: https://www.jeepforum.com/forum/f9/cl...mmies-1114849/)
- Buy a fuel pressure tester and test the fuel pressure coming from the fuel pump.
- Distilled water through the intake manifold.
- Recheck for vacuum leaks using carb cleaner.

Hopefully this helps some people by serving as a compilation of all of the regular P0301 misfire threads. If anyone has any suggestions for what I should do next, I am taking votes.



-Brandon


2000 Jeep Wrangler Sport -- it's green!

Last edited by KSBoilermaker; 10-15-2015 at 12:44 AM. Reason: Fixed some typos and added some links to write-ups. Also forgot to list vacuum leaks.
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post #2 of 30 Old 10-13-2015, 10:44 AM
BigJake87
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I've had no real luck fixing this myself either... Basically my current solution is having bought a louder radio and to pretend I have a camed out small block under the hood....
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post #3 of 30 Old 10-13-2015, 02:21 PM
crazyj23
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Man I having the same issues with nothing fixing it. I've done fuel pump, crankshaft position sensor, camshaft sensor, plugs, etc. and still getting error codes. I can't wait for someone to have an answer!
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post #4 of 30 Old 10-13-2015, 06:57 PM Thread Starter
KSBoilermaker
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Quote:
Originally Posted by crazyj23 View Post
Man I having the same issues with nothing fixing it. I've done fuel pump, crankshaft position sensor, camshaft sensor, plugs, etc. and still getting error codes. I can't wait for someone to have an answer!
Did you try testing your cylinder compression and fuel injectors? I included a couple links in my above list that explain how to do it.

The problem is that for those of us that have to pass emissions testing, the P0301 code makes us kind of dead in the water (unless you have family out of state that will let you use their address to register your car ).

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post #5 of 30 Old 10-14-2015, 12:01 PM
97wranglerD
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Hey It's me.. Vacuum leak guy again. I'm always the one telling people to check for vacuum leaks lol Anyways, it's possible that the intake port going to cylinder 1 is leaking? As you know cylinder 1 is the furthest back.

As we all know a vacuum leaks' symptoms get better when the RPMs are higher. Under idle is where problems usually show up since air is being VERY carefully metered to keep the idle smooth.

Once a small amount of air gets into the cylinder without hanging around the manifold for too long the MAP sensor just can't see this small change. AFR becomes lean and misfires start to happen. PCM sees the misfire and MIGHT go into open loop or will see the AFR go a little too rich due to unburned gases in Cylinder 1 and then misfires becomes worse under idle. Once the throttle is opened the path of least resistance now becomes the throttle body so the leak won't be as bad.

Pretty basic searching for a leak with either propane or carefully using brake clean

06 TJ, "Suicide Jeep", M62 Supercharged! 6PSI
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post #6 of 30 Old 10-14-2015, 03:08 PM
giles45shop
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Here's something to check - probably unlikely for most, but it happened to me on my 99. I was getting the misfire and open circuit codes on #6. I tried replacing injectors, plugs and wires, etc., even swapping computers. It would come and go intermittently, I thought I had fixed it at least three times but it kept coming back.

In the end the problem was that I had an aftermarket battery hold down and the edge of the hold down had rubbed into the wiring harness and the wires for the #6 injector because the wire loom next to the battery hold down had cracked and broken away, exposing the wires to the relatively sharp edge of the metal battery old down clamp. Over time the vibration caused it to go through the insulation into the wire and cause an intermittent short. When I looked very carefully at the wire bundle, several wires showed very slight abrasion of the wire insulation, and one green wire had a very tiny hint of copper showing. Guess which color the wire on the #6 injector was

I repaired the insulation on all the wires and added new wire loom and haven't had a problem since.
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post #7 of 30 Old 10-14-2015, 11:11 PM
crazyj23
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Quote:
Originally Posted by KSBoilermaker View Post
Did you try testing your cylinder compression and fuel injectors? I included a couple links in my above list that explain how to do it.

The problem is that for those of us that have to pass emissions testing, the P0301 code makes us kind of dead in the water (unless you have family out of state that will let you use their address to register your car ).

I did not check compression, but ohm wise the injectors are good. The jeep runs fine and very strong with only 76k. Just hard to start. It does show p0456 as well as p0306.
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post #8 of 30 Old 10-15-2015, 12:42 AM Thread Starter
KSBoilermaker
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Quote:
Originally Posted by 97wranglerD View Post
Hey It's me.. Vacuum leak guy again. I'm always the one telling people to check for vacuum leaks lol Anyways, it's possible that the intake port going to cylinder 1 is leaking? As you know cylinder 1 is the furthest back.

As we all know a vacuum leaks' symptoms get better when the RPMs are higher. Under idle is where problems usually show up since air is being VERY carefully metered to keep the idle smooth.

Once a small amount of air gets into the cylinder without hanging around the manifold for too long the MAP sensor just can't see this small change. AFR becomes lean and misfires start to happen. PCM sees the misfire and MIGHT go into open loop or will see the AFR go a little too rich due to unburned gases in Cylinder 1 and then misfires becomes worse under idle. Once the throttle is opened the path of least resistance now becomes the throttle body so the leak won't be as bad.

Pretty basic searching for a leak with either propane or carefully using brake clean
I forgot to mention this in my original post - I did perform a test using carb cleaner to identify any vacuum leaks. In retrospect, I might do this test again just to be sure. Thanks for the reminder! I'll update my original post now.

2000 Jeep Wrangler Sport -- it's green!
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post #9 of 30 Old 10-15-2015, 01:53 PM
97wranglerD
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Quote:
Originally Posted by KSBoilermaker View Post
I forgot to mention this in my original post - I did perform a test using carb cleaner to identify any vacuum leaks. In retrospect, I might do this test again just to be sure. Thanks for the reminder! I'll update my original post now.
I HATE vacuum leaks.

06 TJ, "Suicide Jeep", M62 Supercharged! 6PSI
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post #10 of 30 Old 10-23-2015, 04:34 PM Thread Starter
KSBoilermaker
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Quote:
Originally Posted by 97wranglerD View Post
I HATE vacuum leaks.
Updates:
- I retested for vacuum leaks and couldn't find one anywhere.
- I also retested using the "screwdriver test" that I described in my original post. I heard fuel injector #1 ticking identically to fuel injector #2 so I know it is firing.
- I also retested the fuel injectors to see if I could isolate the problem further to no avail.

Basically, I'm at a point where I'm just monitoring. The problem seems to be under control but isn't resolved yet (i.e. sometimes it will idle rough and throw the P0301 code but it has never stalled or left me stranded). If I ever find out what caused my issue, I'll update this thread. Unfortunately, I'm not there yet. If the problem gets worse I'll probably perform the following:
- Check for corroded pin at the ECU/ECM. Check the wiring diagram, clean the pin and replace the terminal on the wire. Apply dielectric grease to connections on fuel injectors and other electrical connectors. From the sounds of it, you have a bad injector, faulty wire, bad connector, or a bad ECM. Check the wire for continuity first, then the ECM pin-out, then swap the injector with another cylinder and see if it changes. THEN buy just one NEW injector.

In the next couple weeks I'll probably pull the throttle body, clean it out thoroughly and clean or replace the IAC sensor (reference: https://www.jeepforum.com/forum/f9/cl...mmies-1114849/).

Good luck to everyone chasing this same issue! Remember to share your final solutions when you figure out what caused it!

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post #11 of 30 Old 10-24-2015, 01:12 AM
thegerb50
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I have the same issue on my 97. I'm leaning towards valve springs. Parts are on order and I will be pulling the valve cover off as soon as they come in.
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post #12 of 30 Old 10-24-2015, 01:25 AM
rep-tile
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A vacuum leak will NOT cause a misfire over and over again on the same cyl.

What ohm value did you get when you tested the wire between the PCM connector and the injector connector for resistance? It should be very low and steady (even if you wiggle the wires). Any resistance in the wiring is like adding resistance to your injector and injection timing will be off. You might have to end up rewiring it with some 18 AWG wire and crimp connectors, it's a very easy fix.

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post #13 of 30 Old 10-24-2015, 05:20 AM
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I would go ahead and swap your cyl 1 injector with another. Resistance testing and listening are not conclusive for an injector that may just be sticking a little.

You could also inspect the cylinders w a bore scope. Might notice valve build up causing an intermittent problem that's not bad enough to be out on the compression test.

Great write up. Thanks.

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post #14 of 30 Old 10-25-2015, 06:43 PM
97wranglerD
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Quote:
Originally Posted by rep-tile View Post
A vacuum leak will NOT cause a misfire over and over again on the same cyl.
It will if the intake manifold gasket is leaking near that cylinder.

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post #15 of 30 Old 10-25-2015, 06:47 PM
97wranglerD
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Pull all the spark plug wires and do a dry start run, or pull the fuel pump relay. Listen for any irregularities in the cranking noises. Any irregularities in speed or noises could indicate valve problems. I know you did a compression test but try this in the morning, before any oil has gotten into the cylinders.

The way the Jeep senses misfires, I wonder if it could also be a cylinder 6 that is misfiring as well? Doesn't it sense a change in cam speed and since 1 and 6 fire at the same time, could be worth looking into?

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