Chasing down a P0301 misfire

[hear]

I haven't really ever had misfire problems before, so I'm going to do this scientifically (and probably get lots of useful tips along the way). This is on my 02. Jeep runs smooth, no hint of a miss that I can detect. But the code is there, CEL is on, and my inspection is due this month so here we go.

Step One: changed out the #1 spark plug with an iridium gapped at about 0.032". It's hard to move it by just 3 thou since you can't gap them like you would a copper plug, although I guess it is a 10% variance. I manually cleared the code, but it came back after a day or two of driving. TODO: move that plug to a different cylinder to see if the gap is part of the issue.

Step Two: Friend had a brand new Blue Streak coil on hand so we put that on last night. I didn't have my code reader with me so I did not clear the codes in the hopes that it would just clear on its own. It did not, at least not on the 15 minute drive home.

Step Next: I didn't realize the gap was actually 10% at the time (I mean, it's just 3 thou, right!?), because I would've moved the spark plug the same time I did the coil to see if the misfire would move. I will go out this morning and move the plug to save me a testing cycle.

Beyond that I plan to swap injectors to see if I can get the misfire code to move that way.


QQ on anti-seize: I used to have this tube of copper paste that I really liked. I got it from Autozone or something, but now all I can find in stores is this aluminum anti-size. Is aluminum anti-seize suitable for spark plugs? Or rather, is there anywhere where aluminum anti-seize should be avoided?

 

[Jerry Bransford]

Aluminum antiseize for spark plugs is fine since aluminum is a good conductor too. However, be VERY careful to apply only a very small amount of it on the threads and be sure it can't get onto the electrodes while installing the spark plugs. Antiseize will cause misfires if it makes its way to the electrode.

 

[JKP]

It's not the spark plug gap.

You can clear the codes by disconnecting the battery. Or get a bluetooth OBDII reader to use with your phone to read and clear codes.

Most of the plugs now are nickel plated and don't recommend anti-seize. But if you use it, just don't get it on the electrodes.

Check all the vacuum lines on the intake.

 

[hear]

Aluminum antiseize for spark plugs is fine since aluminum is a good conductor too. However, be VERY careful to apply only a very small amount of it on the threads and be sure it can't get onto the electrodes while installing the spark plugs. Antiseize will cause misfires if it makes its way to the electrode.

I know HOW to apply it, just wondering if this stuff was ok since I can't find the stuff I've used for years.

It's not the spark plug gap.

Check all the vacuum lines on the intake.

What makes you say it's not the gap? At what variance could it be the gap? I replaced the vacuum line at the rear of the intake a while back to fix an HVAC problem, but I'll check the other lines/plugs as well.

 

[JKP]

Your gap is slightly less than spec. Which means the spark can easily jump it. The coil rail will fire gaps nearly double the spec'd gap as well.

 

[Jerry Bransford]

Your gap is slightly less than spec. Which means the spark can easily jump it. The coil rail will fire gaps nearly double the spec'd gap as well.

X2. .035 is the older gap's size which went to .040 with the later coil rail ignition system.

 

[hear]

Your gap is slightly less than spec. Which means the spark can easily jump it. The coil rail will fire gaps nearly double the spec'd gap as well.

I know, I said that in my initial post, I even did the math. Then you said "it's not the spark plug gap" which I questioned. Now you're telling me why it could be the spark plug gap.

It's too early for this.

 

[JKP]

No. I repeated it isn't the gap.

 

[hear]

No. I repeated it isn't the gap.

Sorry, I interpreted both you and Jerry as saying it was too small and was able to jump the gap early or something.

 

[hear]

Update.

I swapped the #1 & #2 injectors, replaced some o-rings, and cleared the codes. I drove it long enough for the monitors to be ready & pass my inspection yesterday but this morning the P0301 came back.

Seems like it's not the plug, not the coil, not the injector. Somebody suggested a vacuum problem although I'm not sure (a) I buy that as a possible cause and (b) why would it only affect cylinder #1? The jeep seems to run perfectly, so the misfire is quite intermittent in my extremely amateur estimation.

 

[JKP]

If the vacuum leak is by the runner for cyl 1, it would lean out cyl 1.

Have you done a compression check?

 

[macleanflood]

I had random misfire codes and chased everything on my 4.0. Then I finally replaced the crank position sensor and everything was perfect. Go figure.

-Mac

 

[hear]

I had random misfire codes and chased everything on my 4.0. Then I finally replaced the crank position sensor and everything was perfect. Go figure.

-Mac

That will infuriate me. I'm going to pretend you didn't mention it.

If the vacuum leak is by the runner for cyl 1, it would lean out cyl 1.

Have you done a compression check?

I don't think there are any vacuum ports near the cylinder 1 path, but maybe I'm wrong. I know I have vacuum from when I fixed my HVAC stuff, but I can't rule out there isn't a leak, I don't think. If I manually pump up vacuum and it holds, would that rule out all leaks anywhere? I haven't done a compression test yet.

 

[97JadeTJ]

That will infuriate me. I'm going to pretend you didn't mention it.



I don't think there are any vacuum ports near the cylinder 1 path, but maybe I'm wrong. I know I have vacuum from when I fixed my HVAC stuff, but I can't rule out there isn't a leak, I don't think. If I manually pump up vacuum and it holds, would that rule out all leaks anywhere? I haven't done a compression test yet.

It just dawned on me. I had the same problem a couple years ago with my 97 2.5L and after I replaced everything that you have, I found it to be the distributor had play in it… Replaced it and it went away…

 

[Moglocker]

I haven't really ever had misfire problems before, so I'm going to do this scientifically (and probably get lots of useful tips along the way). This is on my 02. Jeep runs smooth, no hint of a miss that I can detect. But the code is there, CEL is on, and my inspection is due this month so here we go.

Step One: changed out the #1 spark plug with an iridium gapped at about 0.032". It's hard to move it by just 3 thou since you can't gap them like you would a copper plug, although I guess it is a 10% variance. I manually cleared the code, but it came back after a day or two of driving. TODO: move that plug to a different cylinder to see if the gap is part of the issue.

Step Two: Friend had a brand new Blue Streak coil on hand so we put that on last night. I didn't have my code reader with me so I did not clear the codes in the hopes that it would just clear on its own. It did not, at least not on the 15 minute drive home.

Step Next: I didn't realize the gap was actually 10% at the time (I mean, it's just 3 thou, right!?), because I would've moved the spark plug the same time I did the coil to see if the misfire would move. I will go out this morning and move the plug to save me a testing cycle.

Beyond that I plan to swap injectors to see if I can get the misfire code to move that way.


QQ on anti-seize: I used to have this tube of copper paste that I really liked. I got it from Autozone or something, but now all I can find in stores is this aluminum anti-size. Is aluminum anti-seize suitable for spark plugs? Or rather, is there anywhere where aluminum anti-seize should be avoided?

Had the same code earlier this year,on my 2002,dead miss zero compression on number 1,getting ready to buy a tupy head .used the top engine cleaner heat soak twice and no more codes,and compression is back,guess it was carbon?but I still think I have a valve problem on that cylinder (very slight vibration)at idle.

 

[Luke90]

hello Hear, what was the issue in the end? just got P0301 in my 03 4.0 and looking for shortcuts to solve. thanks!