Misfire - I need help!

[Gstar]

Hi Guys.

I need some help please...

I've been chasing down a misfire on my 2001 2.5 Sport and I don't seem to be able to crack it. Below is a rundown of what I've done to date and I would really appreciate any advice on steps / what to do next...

The misfire is apparent when trying to accelerate, sometimes I will get a hesitation and by backing off the throttle and re-applying gently, I can get the engine to increase revs. This is not ideal as I don't have confidence that the engine will do what I want it to do / when I want it to!!

There are no dash warning lights and it idles and revs fine when stationary.

Here's what I've done (in order):

1. Replaced the aftermarket fuel locking cap (it was flapping around and loose). Made no difference.
2. Replaced the plugs - seemed to improve things for a couple of days / approx. 50kms.
3. Replaced the plug leads, coil, rotor arm and distributor cap. No difference.
4. Replaced the CPS. Made no difference.
5. Replaced the entire distributor - now I have a more pronounced misfire. It's like someone is flicking the ignition off for a second and the engine kicks back. No dash lights come on though and after a second it runs as it should.

Any ideas on where to go from here? I seem to be throwing money at it without much joy. As much as I love driving it, I am starting to lose faith!

Thanks, G.

 

[Jerry Bransford]

Could be a bad upstream (closest to the engine) O2 sensor. Try unplugging just the upstream O2 sensor (just disconnect its electrical connector) and drive it around a couple hours like that and see if the symptom improves or goes away. If the symptom clears up, replace it with an NTK (only) O2 sensor. Do not use a Bosch O2 sensor which does not work well in Jeeps.

The downstream O2 sensor mounted on the catalytic converter has nothing to do with this issue, no need to replace it too if you end up replacing the upstream O2 sensor.

 

[ac_]

My guess would be that you have the distributor off a tooth or you have a couple wires crossed. I would go back under, and put the motor on Top Dead Center and make sure you have the distributer in correctly. And go over the firing order one more time. I am throwing that out there because you said it got worse after you replace the distributor. That would definitely make it misfire.

If all of that is fine then I would pull the plugs back out and see if you have an indication of where the issue is. If all of them are clean but one is black it could show signs of maybe a bad injector? Or a bad wire if it is not firing.

Check your battery terminals make sure they are clean, also I would check all of the grounds coming off of the battery and make sure they are clean also. Double check the connector that plugs into the distributor make sure the wires are nice and tight and the pins in the pigtail itself.

I know you replaced this already, but I ran into a little trouble with mine doing the same thing one day. and it turned out to be a broken rotor, so double check everything in side your distributor again.

 

[Gstar]

My guess would be that you have the distributor off a tooth or you have a couple wires crossed. I would go back under, and put the motor on Top Dead Center and make sure you have the distributer in correctly. And go over the firing order one more time. I am throwing that out there because you said it got worse after you replace the distributor. That would definitely make it misfire.

If all of that is fine then I would pull the plugs back out and see if you have an indication of where the issue is. If all of them are clean but one is black it could show signs of maybe a bad injector? Or a bad wire if it is not firing.

Check your battery terminals make sure they are clean, also I would check all of the grounds coming off of the battery and make sure they are clean also. Double check the connector that plugs into the distributor make sure the wires are nice and tight and the pins in the pigtail itself.

I know you replaced this already, but I ran into a little trouble with mine doing the same thing one day. and it turned out to be a broken rotor, so double check everything in side your distributor again.

Thanks. The plug leads are definitely in the correct order - I labelled them and made sure they were in the correct order after fitting the new distributor - so confident that's ok - it starts, ticks over and runs fine - until it misses!

Is it possible to have the distributor off by a tooth? I thought it was keyed. I did follow the instructions to align the timing marks and the oil pump to 10 O'clock. If it is possible I'll remove it and refit it and see if it helps.

The plugs were changed by a local shop and he commented that one of the plugs was clean (no. 3) and replaced the lot as he thought it was a bad plug. I've done probably 8000-10,000kms since then, so I can take a look at them and see how they look now.

 

[Chris]

Is it throwing any check engine lights? I always ask this, because sometimes knowing the code it's throwing can help others to diagnose it.

 

[Gstar]

Is it throwing any check engine lights? I always ask this, because sometimes knowing the code it's throwing can help others to diagnose it.

The check engine warning light is lit with just the ignition on but once the engine is running it doesn't come on - even when it misfires.

I can check the codes - do I need to do the ignition on/off 3 times to see them in the LCD display?

 

[Chris]

I can check the codes - do I need to do the ignition on/off 3 times to see them in the LCD display?

I believe that's the procedure, yes. Unless you happen to have a OBDII scan tool.

But checking the codes would at least help.

 

[ac_]

Thanks. The plug leads are definitely in the correct order - I labelled them and made sure they were in the correct order after fitting the new distributor - so confident that's ok - it starts, ticks over and runs fine - until it misses!

Is it possible to have the distributor off by a tooth? I thought it was keyed. I did follow the instructions to align the timing marks and the oil pump to 10 O'clock. If it is possible I'll remove it and refit it and see if it helps.

The plugs were changed by a local shop and he commented that one of the plugs was clean (no. 3) and replaced the lot as he thought it was a bad plug. I've done probably 8000-10,000kms since then, so I can take a look at them and see how they look now.

Honestly, I am not positive on the TJ, I didn't actually replace mine just the rotor, but my old 4.0 YJ did have a gear as far as I recall.

I am not sure what to say about one clean plug, from what you described I would have thought the opposite would be true, but what if your number three injector is frozen shut? I would imagine no gas would be getting in there, and I supposes the plug would be clean in that case, but I don't know how to check for that.

Seems like I may not be any help at all, I will stand by and let the real gurus chime in.

 

[Jerry Bransford]

I would definitely be looking at the upstream O2 sensor first before thinking the distributor jumped a tooth. A bad upstream O2 sensor will cause misfires/stumbling/poor idle/poor acceleration. Like spark plugs, O2 sensors eventually go bad and need to be replaced. Again, stick with NTK and avoid Bosch O2 sensors.

Re-read my post #2 above, you can temporarily disconnect the upstream O2 sensor and then drive the Jeep around for a while as a test to see if the symptoms improve with it disconnected.

 

[ac_]

I would definitely be looking at the upstream O2 sensor first before thinking the distributor jumped a tooth. A bad upstream O2 sensor will cause misfires/stumbling/poor idle/poor acceleration. Like spark plugs, O2 sensors eventually go bad and need to be replaced. Again, stick with NTK and avoid Bosch O2 sensors.

Re-read my post #2 above, you can temporarily disconnect the upstream O2 sensor and then drive the Jeep around for a while as a test to see if the symptoms improve with it disconnected.

as a side note. I didn't suggest it jumped a tooth, I suggested it was put in a tooth off only because in his original post he said that it got worse with the new distributor.

Carry on!

 

[KCsTJ]

Also u say plug #3 Is clean, that could indicate a loose intake bolt(s), making a lean condition, you might make sure they r all torqued @23

 

[Gstar]

Appreciate all the suggestions, I'll work through them.

1. Progress so far, codes checked - nothing displayed - just gives message done.

2. Upstream O2 sensor disconnected and when started the check engine light came on and stayed on.



Idled and ran fine, if anything seemed more responsive than usual. Then once it had warmed up it started to misfire at around 2000 rpm.

I've checked the earth connections from engine to body and they look fine.

I am heading back out now and I'll:

1. Check the new rotor arm and inside of distributor cap look good. Although I think these will be fine as it's the same misfire I've had since buying the jeep.
2. Disconnect and reconnect the battery terminals to check they are good.
3. Will take the plugs out and see what the current state is.
4. Will check bolts on intake manifold.

I'll report back!

 

[Gstar]

1. Rotor arm and cap look fine.
   
   
   
   
   
   
   
   
   
   
2. Battery clamps removed, cleaned and tightened.
3. Plugs removed. No. 3 is darker than the rest. (1,2,3,4 from left to right in picture).
   
   
   
   
   
4. A couple of the manifold bolts were loose and I could turn them a half a turn.

A few other pics of the engine bay in case anything leaps out:

Not sure if the exhaust manifold also has a crack from the picture below:

After all the above it seems to be running OK although I am not convinced anything I have done has really fixed the problem!

I'll run it a bit further than just around the block and see if the miss happens again.

Any ideas as to why no. 3 plug is fouled up more than the others?

 

[KCsTJ]

1. Rotor arm and cap look fine.
   View attachment 32804
   
   View attachment 32805
   
   
   
2. Battery clamps removed, cleaned and tightened.
3. Plugs removed. No. 3 is darker than the rest. (1,2,3,4 from left to right in picture).
   View attachment 32806
   
   
4. A couple of the manifold bolts were loose and I could turn them a half a turn.

A few other pics of the engine bay in case anything leaps out:

View attachment 32807 View attachment 32808

Not sure if the exhaust manifold also has a crack from the picture below:

View attachment 32809

After all the above it seems to be running OK although I am not convinced anything I have done has really fixed the problem!

I'll run it a bit further than just around the block and see if the miss happens again.

Any ideas as to why no. 3 plug is fouled up more than the others?

 

[KCsTJ]

Good, snugging up manifold bolts always helps and is good PM.
Don't worry about darker #3 plug, drive it a couple hundred miles, check again
Also you mention better response after 02 upstream sensor disconnect - don't ignore this... I just replaced (last week) an upstream 02 sensor, never a code, with NTK #23506. Solved the hesitation and coughing . So consider the 02

 

[Gstar]

Good, snugging up manifold bolts always helps and is good PM.
Don't worry about darker #3 plug, drive it a couple hundred miles, check again
Also you mention better response after 02 upstream sensor disconnect - don't ignore this... I just replaced (last week) an upstream 02 sensor, never a code, with NTK #23506. Solved the hesitation and coughing . So consider the 02

Thanks. I'll get a replacement O2 sensor ordered. Living on a small island, parts are a 'challenge'!