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  Topic Review (Newest First)
05-18-2016 12:56 AM
aogmitch Have you pulled the valve cover and looked at what the valves are doing compared to the other cylinders?
05-17-2016 05:53 PM
MarioM0one I feel your pain. It is like shooting in the dark. Anyway, the one thing I don't see you having tried is to check the voltage of the O2 sensors, especially bank one. The voltage should be fluctuating fairly rapidly within a range of .20 to .90 volts.
I never thought bad O2 sensors had much to do with misfires but it did on mine. Make sure the voltages of the Map and TPS sensors display similar values. Good luck.
05-17-2016 05:25 PM
KSBoilermaker
Quote:
Originally Posted by KSBoilermaker View Post
Another update regarding the chronic P0301 misfire that I've been battling since last summer. The past couple weeks I started getting a rough idle again and finally today the CEL came on on my Jeep and it threw a P0301 cylinder 1 misfire again. I was anticipating this and was able to pick up a new IAC sensor on Amazon for $12.99. I installed it today. While the new part didn't seem to fit the electrical connector very well, I was able to get it on as far as I could. After installing the new IAC sensor, I'm not sure I've ever heard my Jeep idle this smoothly before but that could just be placebo effect. Perhaps it was always the IAC sensor that was bad and the cleanings only helped limp the old one all along.

Anyways, I'll keep monitoring and will continue to post updates here if any other work is required by me. If I do not post anything new, the IAC sensor resolved the issue.

-Brandon
Another update: The new IAC sensor did not fix the issue so I tried swapping fuel injector #1 with injector #4. I also cleaned them while I had them out. The replacement fuel injector that I picked up online didn't fit so I figured, while I had the fuel rail off, I might as well try swapping the injectors to see if the misfire moved. It didn't. CEL came back on over the weekend. I pulled the codes, still P0301. I then reset the system and I'm back to where I started - i.e. not sure what is causing it or how to fix it.

Fortunately, the CEL only comes on once every couple months so I'm able to ignore it fairly easily. Maybe I'll try the crankshaft position sensor next...
03-19-2016 08:46 PM
KSBoilermaker
Quote:
Originally Posted by KSBoilermaker View Post
Updates over the past six weeks - I continued my everything including the kitchen sink approach after logic and diagnostics failed:
- I tried using an entire can of Berryman's Motor Tune Up through the brake booster line (it's basically a competing product to Sea Foam). No dice there, not nearly as much smoke as when I used Sea Foam so there must not have been much carbon buildup left.
- I began a regimen of trying to clean out any engine sludge from my crankcase (IMO most of these products are simply snake oil, just like fuel injector cleaners, but I went against my better judgement to try them anyway). Basically, my plan was to try a different additive to the crankcase with each oil change as a part of my 100k mile clean-up of the engine. This started when I used 1/2 of a can of Sea Foam in the crankcase a few thousand miles ago (the other half went through the brake booster line as mentioned in my original post). Step two of this regimen was to try out put a quart of Marvel Mystery Oil in the place of a quart of oil when changing the oil. I also bought an additional quart of MMO to start using in the gas tank per the bottle's recommendation (i.e. 4 ounces for every 10 gallons of gas). Step three of this regimen is going to be a quart of Rislone with the next oil change in a couple thousand more miles. I'll still perform step three once the Jeep is ready for its next oil change.
- Last week I pulled the throttle body and cleaned out the IAC sensor, IAC housing, intake butterfly valve and MAP sensor breather hose hole (reference: https://www.jeepforum.com/forum/f9/cl...mmies-1114849/). All of the sensors looked fairly good without major issues or too much carbon buildup (i.e. the photos in the above link looked far worse than mine) so, at the time, I didn't think taking everything apart was necessary and couldn't be accomplished simply by spraying throttle body cleaner into the throttle body. During the cleaning process ​I had to create a new gasket out of RTV for the IAC housing since the old one crumbled and fell apart. Since I performed this cleaning over a week ago, I stopped having issues with the rough idle and have yet to throw a the P0301 code again. This work coincided with starting to add 4 ounces of MMO to the gas tank for every 10 gallons of gas so I cannot definitively say whether the cleaning of the intake sensors or the MMO in the tank were the solutions. Regardless, things seem to be running better now. Because it's a Jeep though, I'm sure in a month or two this problem will come back - if so, I'll post again. If you don't hear from me - this is what fixed my rough idle/P0301 cylinder 1 misfire.

For more information, here is another great thread to give you ideas if you're chasing a misfire: https://www.jeepforum.com/forum/f310/...guide-2212697/

Keep Tahoe blue.
-Brandon

Another update regarding the chronic P0301 misfire that I've been battling since last summer. The past couple weeks I started getting a rough idle again and finally today the CEL came on on my Jeep and it threw a P0301 cylinder 1 misfire again. I was anticipating this and was able to pick up a new IAC sensor on Amazon for $12.99. I installed it today. While the new part didn't seem to fit the electrical connector very well, I was able to get it on as far as I could. After installing the new IAC sensor, I'm not sure I've ever heard my Jeep idle this smoothly before but that could just be placebo effect. Perhaps it was always the IAC sensor that was bad and the cleanings only helped limp the old one all along.

Anyways, I'll keep monitoring and will continue to post updates here if any other work is required by me. If I do not post anything new, the IAC sensor resolved the issue.

-Brandon
01-24-2016 12:42 PM
T's '97TJ
Quote:
Originally Posted by JasonATC View Post
Update: I had the exact same issue and fixed it by replacing the crankshaft position sensor. I hope this helps somebody.
Ditto. Replaced this and all of my Misfires & Multiple Misfire codes were gone. Ordered from RockAuto ($28) and got it the next day. Awkward, but easy fix. Here's a great step by step.

I too have to pass smog this year and was getting desperate; glad I found this fix.
12-03-2015 11:06 AM
KSBoilermaker
Updates over the past six weeks...

Quote:
Originally Posted by KSBoilermaker View Post
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
Quote:
Originally Posted by KSBoilermaker View Post
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!
Updates over the past six weeks - I continued my everything including the kitchen sink approach after logic and diagnostics failed:
- I tried using an entire can of Berryman's Motor Tune Up through the brake booster line (it's basically a competing product to Sea Foam). No dice there, not nearly as much smoke as when I used Sea Foam so there must not have been much carbon buildup left.
- I began a regimen of trying to clean out any engine sludge from my crankcase (IMO most of these products are simply snake oil, just like fuel injector cleaners, but I went against my better judgement to try them anyway). Basically, my plan was to try a different additive to the crankcase with each oil change as a part of my 100k mile clean-up of the engine. This started when I used 1/2 of a can of Sea Foam in the crankcase a few thousand miles ago (the other half went through the brake booster line as mentioned in my original post). Step two of this regimen was to try out put a quart of Marvel Mystery Oil in the place of a quart of oil when changing the oil. I also bought an additional quart of MMO to start using in the gas tank per the bottle's recommendation (i.e. 4 ounces for every 10 gallons of gas). Step three of this regimen is going to be a quart of Rislone with the next oil change in a couple thousand more miles. I'll still perform step three once the Jeep is ready for its next oil change.
- Last week I pulled the throttle body and cleaned out the IAC sensor, IAC housing, intake butterfly valve and MAP sensor breather hose hole (reference: https://www.jeepforum.com/forum/f9/cl...mmies-1114849/). All of the sensors looked fairly good without major issues or too much carbon buildup (i.e. the photos in the above link looked far worse than mine) so, at the time, I didn't think taking everything apart was necessary and couldn't be accomplished simply by spraying throttle body cleaner into the throttle body. During the cleaning process ​I had to create a new gasket out of RTV for the IAC housing since the old one crumbled and fell apart. Since I performed this cleaning over a week ago, I stopped having issues with the rough idle and have yet to throw a the P0301 code again. This work coincided with starting to add 4 ounces of MMO to the gas tank for every 10 gallons of gas so I cannot definitively say whether the cleaning of the intake sensors or the MMO in the tank were the solutions. Regardless, things seem to be running better now. Because it's a Jeep though, I'm sure in a month or two this problem will come back - if so, I'll post again. If you don't hear from me - this is what fixed my rough idle/P0301 cylinder 1 misfire.

For more information, here is another great thread to give you ideas if you're chasing a misfire: https://www.jeepforum.com/forum/f310/...guide-2212697/

Keep Tahoe blue.
-Brandon
10-28-2015 08:48 AM
97wranglerD
Quote:
Originally Posted by Knuckelhead View Post
So far-fedged that its nonexistent on the TJ 4.0L engine because the serpentine belt only drives accessories that have nothing to do with the engine timing, cam and crankshaft positioning. The timing chain takes care of that...
well... yeah you might be missing the point. The misfire may or may not have anything to do with timing. The speed of the crankshaft could be changed by anything that puts an uneven load on the engine, that's a lot of possible things. To put it broadly

but yeah, I would guess highly unlikely as well. Just putting out the information seeing as OP has been very thorough in his troubleshooting.
10-28-2015 02:13 AM
Knuckelhead So far-fedged that its nonexistent on the TJ 4.0L engine because the serpentine belt only drives accessories that have nothing to do with the engine timing, cam and crankshaft positioning. The timing chain takes care of that...
10-28-2015 12:18 AM
rep-tile
Quote:
Originally Posted by 97wranglerD View Post
I never would have thought the belt could be have enough grip to change the crankshaft speed lol
My guess is because the belt puts a load on the crankshaft. If the belt is worn unevenly throughout its length, the load might fluctuate a bit. Since the crank sensor monitors variations in speed of the crankshaft to determine a misfire condition, I could see that happen. Very far-fedged but plausible.
10-27-2015 11:52 AM
97wranglerD another thing to note... "Anything that affects the speed of the crankshaft can cause a misfire DTC."

Just copying from some other Jeep FSM

- Visually inspect the engine for any of the following conditions.
- Worn serpentine belt
- Binding Engine-Driven accessories: A/C Compressor, P/S Pump, Water pump.
- Misalignment of the Water pump, P/S Pump and A/C Compressor pulleys
- Corroded PCM power and ground circuits.
- Improper CKP, CMP, MAP, and TP Sensor mounting
- Poor connector/terminal to component connection. i.e., CKP sensor, Fuel Injector, Ign coil, etc.
- Vacuum leaks
- Restricted Air Induction system or Exhaust system.


I never would have thought the belt could be have enough grip to change the crankshaft speed lol
10-27-2015 12:31 AM
MorganDan I had the same problem. Started with changing the spark plugs. Then swapped out injector 1 with with injector 4 and 6. Codes still read p0301. Fixed the problem with buying a new coil rail.
10-27-2015 12:00 AM
rep-tile
Quote:
Originally Posted by JasonATC View Post
Update: I had the exact same issue and fixed it by replacing the crankshaft position sensor. I hope this helps somebody.
You had multiple misfire codes. That qualifies as random. The crank sensor makes sense in your case. I hope you bought a Mopar sensor or you'll be doing the same job in a year or two.
10-26-2015 07:12 PM
JasonATC Update: I had the exact same issue and fixed it by replacing the crankshaft position sensor. I hope this helps somebody.
10-25-2015 11:25 PM
rep-tile
Quote:
Originally Posted by 97wranglerD View Post

It will if the intake manifold gasket is leaking near that cylinder.
The intake manifold feeds all cylinders. It would rather be ramdom. The injector wiring happens to be a known issue on 4.0 WJ s I've had to fix this myself.
10-25-2015 10:00 PM
JasonATC I'm having the exact same issues. 2002 4.0L. P0300 code Also had a P0301 and P0305 code previously that got reset and never came back. I'm running out of options.

I have replaced the spark plugs, coil rail, all 6 injectors, cleaned throttle body, replaced the IAC module, and the TPS... I feel like I am just throwing parts at it.

It ran well after replacing the injectors with the Five-O "4 hole" injectors. It was purring like a kitten. But a day later the code came back, this time just the P0300, but with the same rough idle.

The symptoms are just that it seems to have a rough idle and maybe a misfire. With the other two codes, the CEL was flashing and on the highway I couldn't go over 35. I pulled into a gas station (at 1am) and put seafoam into the oil and gas and let it idle for 30min. The flashing went away and I was able to drive home. That was the original problem. I have done all of this work since then.

The question is: what is my next step? I've heard about TJ valve spring issues causing a P0300, from JeepForum.com, but I'm no mechanic, I can change simple bolt on stuff like Plugs and injectors, but I don't wanna mess with internal engine repairs. I have an appointment at the Jeep dealership to get the CEL diagnosed, what should I tell them? I think I need to tell them about all of the stuff I replaced, but I also don't want to hint at valve springs if that isn't the issue. Can they isolate if the valve springs are weak with a compression test, or will that just point to needing all new valves? I want to avoid a $1,000 repair job if I can.

Any tips or recommendations on my next step would be appreciated, thanks in advance for the ideas.
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