P0301 - Looking for some help

[Brian McVickers]

Hi everyone,

1997 Jeep TJ Wrangler with about 65,000 miles on the 4.0L 6Cyl engine.

Engine light on
Code P0301
Code Meaning = Cylinder #1 Misfire detected

What has been done to remedy:
- New Plugs
- New Plug Wires
- New Ignition Coil

What has been done to check other possible remedies:
- Rotor Cap is in good condition and all 6 contacts are in good condition
- Moved the #1 fuel injector to another cylinder but the code did not follow.

Still getting P0301
Any ideas on what to do next to solve the issue?

Thanks!
Brian

 

[Deleted member 48574]

This may be an absolutely silly suggestion that you may have already tried, but have you reset the codes in the computer and THEN had it come back? An old GM of mine would trigger trouble codes and even post-repair, they'd stay on until I used my scanner to reset.

Again, might be foolish/I'm sure you've already tried it!

Other options would depend on the sophistication of the sensor. If you have fuel, air, and spark, you shouldn't have a misfire. If one of those is a little wonky it might trigger the sensor to report a trouble code. Some sensors trigger for just spark/just fuel/just air -- I'm not sure what would be the case here.

I'll sleep on it and see what else I can think of. Sorry I'm not much more help!

Regards
Craig

 

[irish1371]

Pull the injector harness plug off of #1 injector and see if you have fuel puddling in the socket around the pins. I had an injector start leaking on my 88 XJ and it was pushing fuel into the plug and causing a short creating the misfire condition.

 

[Brian McVickers]

I will check that but when I swapped the injectors around the other day I did not notice anything.
Also if it was a leaky injector it the code would have followed it to the new location when I moved them around.

 

[shortbus4x4]

I will check that but when I swapped the injectors around the other day I did not notice anything.
Also if it was a leaky injector it the code would have followed it to the new location when I moved them around.

It might take it a little while and your Jeep does not have the problem prone Renix injection. Are you 100% sure your dist. cap is good? Are there any vacuum line ports on the #1 runner or close to it? Have you done a compression test yet? Does it run poorly or have a miss and did it do it before you did any repairs? Also did you put the new parts on in an attempt to fix the code? What did the #1 plug look like that came out of it compared to the rest? And last are you clearing the code with a scan tool?

 

[Brian McVickers]

It might take it a little while and your Jeep does not have the problem prone Renix injection. Are you 100% sure your dist. cap is good? Are there any vacuum line ports on the #1 runner or close to it? Have you done a compression test yet? Does it run poorly or have a miss and did it do it before you did any repairs? Also did you put the new parts on in an attempt to fix the code? What did the #1 plug look like that came out of it compared to the rest? And last are you clearing the code with a scan tool?

- The Dist Cap has no cracks and all of the contact points look the same in ok condition
- Not sure on the vacuum lines
- Planning to do the compression test tomorrow after doing Mopar Combustion Chamber Cleaner tonight
- It has been running fine, drove it 1500 miles from the midwest to AZ and then about a week later the Engine light came on and threw P0301. There is only a very slight stumble at idle but runs fine with power even on the highway.
- All the new parts have been installed in an effort to fix the code
- All the plugs looked the same, just fine when I replaced them
- I am readin and clearing the code after each change using a ODBII reader tool

Thanks
Brian

 

[shortbus4x4]

Make sure you don't have a vacuum line off and also check and make sure your intake manifold isn't warped allowing a vacuum leak on #1. Use a can of carb clean with a straw, be careful as you can make a fire quickly with it. Idle should pick up when carb clean is sucked into engine via vacuum leak.

 

[1stDeuce]

It you've replaced the critical ignition stuff and swapped injectors, it's time to get that compression/leakdown test done, and look for a vacuum leak on the #1 cyl. Interesting that you say it doesn't seem to be missing. In my experience, the misfire was pretty noticable before the light would come on.
If you're clearing codes and it's coming back #1, it evidently thinks it's still misfiring. If compression and leakdown come out ok, you might try swapping spark plugs just for fun. I had a bad one that looked good, but it was a SOLID misfire once it warmed up. It was a new plug.

With mileage that low on a '97, it's possible that you might have a slightly sticking or stuck valve, particularly if the jeep has been sitting a lot. Or has it been run out of oil, or had very few oil changes? If run low or change infrequently, the timing chain will stretch and allow a bunch of slop in the distributor, which might be mis-interpreted by the diagnostic as misfiring. ??

Keep looking, you'll find it!
chris

 

[Brian McVickers]

Thanks everyone,

I tried to do a compression test last night but the tool that I borrowed from Auto Zone was broken, the gauge dial would only go up to 40! I got a 40 reading on #1 so I tired #5 and also got 40. The dial was rattling and about to fall apart so I need to try again with a properly working compression gauge.

I While I had the spark plugs out I swapped #1 and #5, I also Swapped the #1 plug wire with another. I cleared the codes and checked that they were clear.

It has had about 5 key cycles today and about 90 miles of driving from in town to up around 70 mph on the highway with no running issues.

I'm waiting for a code to come back but will still plan on compression testing this evening.

Regards,
Brian

 

[MattScott]

Are you using platinum or copper plugs?

 

[Brian McVickers]

Are you using platinum or copper plugs?

Not sure, I got the basic OEM matched plugs. Nothing fancy. For plugs I tend to go with the basics as the rest seem to be Hype!

 

[Brian McVickers]

Update,

I just compression tested all six cylinders and got readings of 150 (+/- 3) on all six cylinders including #1.
I repeated the test (3) three times with consistent results. All cylinders are at 150.

So to Recap:

1997 Jeep Wrangler TJ, 4.0L 6cyl, 69,000 miles.
- Sat for two years getting about 300 miles per year.
- Made a 1,500 mile drive from MO to AZ with no issues no lights no codes.
- About a week after 1,500 mile drive the CEL came on and code was P0301 (cylinder #1 misfire detected) engine seemed to run fine.
- Replaced plugs, cleared code, code came back
- Replaced plug wires, cleared code, code came back
- Replaced ignition coil, cleared code, code came back
- Moved cyl #1 fuel injector, cleared code, code came back for cyl #1 and did not follow injector so injector ok
- Also added two bottles of Chevron Tech fuel injector cleaner to fuel tank
- Tested all cylinders for compression and all read 150

Yesterday during first attempt at compression test I swapped spark plugs between #1 and #5 as they are next to each other and swapped plug wires with #1 and #6 as they are similar lengths. At this time I also cleared the code and double checked that it was cleared.

So far about 10 key cycles and close to 200 miles with no CEL and no codes.

I'm waiting for / expecting the CEL light to come back on with P0301 but I'm not sure why it is coming on to begin with.

 

[shortbus4x4]

You fixed it.

 

[Brian McVickers]

The CE Light has not come back on yet, a good thing!

Yesterday I:

- Added a can of Mopar Combustion Chamber Cleaner and there was no white smoke as a result

- Dropped the fuel tank and installed a new fuel pump (After the engine would sit off for over 30 minutes or so, it would require 3 key turns to start it, each key turn would allow the fuel pump to prime for 2 seconds. The pressure regulator on top of the old fuel pump was not holing pressure in the system and it would need to be re primed each time.) The new fuel pump assembly solved the issue and it starts fist try!

- Oil change

- New air filter