Strange misfire>>>> please help!!

[Dcoy1283]

Ok, so I have been trying to figure this misfire out for a while now, and im just so lost right now. I have a 2000 cherokee XJ with 143K on it and its giving me a p0304, cylinder 4 misfire, and sometimes one about an very small evap leak ( but not lately). Basicly it starts and runs alittle rough ( when cold ) then misses and runs real bad within a mile from where I leave from ( this is when I get the misfire code flashing) then if I shut it off and restart it will run fine. This only happens when the jeep sits for a few hrs, but never when its hot. I changed the plugs ( copper core) coil rail, cam sensor, crank sensor, TPS, the ECM, swapped injectors around, did a compression and fuel pressure test, ran seafoam through it, checked the index of the cam sensor ( was good) and a coolant temp sensor, looked all over for bad wires or loose conections. Still having problems. Please Help, Im lost

 

[Jnida63]

front 02 sensor is lazy, i fought the same problem for weeks, i just replaced both due to the mileage

 

[Dcoy1283]

front 02 sensor is lazy, i fought the same problem for weeks, i just replaced both due to the mileage

you mean the 2 before or the 2 after the cats?

 

[Ecomike]

The two before.

 

[Dcoy1283]

The two before.

Now I have unplugged them for a few days and still had the misfire problem. Wouldnt the computer just refer back to some sort of default setting if they're not getting any signal?

 

[Dcoy1283]

Also forgot to add in my original post that the missing only does it when I'm coasting down a hill with no load on the engine, and if I put it in neutral and keep the rpms up I can prevent it from happening .

 

[Ecomike]

That has been my experience with older ones. It does not prove the O2 sensors are good, but might prove you have another problem.

Could it be a cracked head, the 0331 head in 2000 Yr?

Checked compression yet? On #4?

 

[Dcoy1283]

That has been my experience with older ones. It does not prove the O2 sensors are good, but might prove you have another problem.

Could it be a cracked head, the 0331 head in 2000 Yr?

Checked compression yet? On #4?

Yeah, 140-155 psi with #4 where my problem is at 150.

 

[Ecomike]

Try some gumout, injector cleaner in the gas. Maybe some carbon build up, and lean.... carbon promoted missfire during deceleration, with no O2 sensor help to guide it.

Sounds like a tough one so far.

 

[Dcoy1283]

So far its had 2 cans of seafoam, a few tanks of 94 octain, just to had it running while spraying gum out in the throttle body, let it heat soak for a few hours then ran it at high rpms then let it cool then drove it down the road and the engine light started flashing ( same p0304 code ) but didn't feel the misfire, although I wasn't happy and was hard on the skinny pedal to return home.

 

[Ecomike]

Bad spark plug wire, or coil pack....what ever the 2000 used (Spark), bad injector (fuel), both at cylinder #4, or a head-headgasket-valve problem is a real possibility.


I did not understand the reply below in bold:

Yeah, 140-155 psi with #4 where my problem is at 150.

the 150 at the end throws me?

 

[Ecomike]

Oh, and try a different plug in #4, a new one, just be sure!!!! They are cheap!

 

[Dcoy1283]

Bad spark plug wire, or coil pack....what ever the 2000 used (Spark), bad injector (fuel), both at cylinder #4, or a head-headgasket-valve problem is a real possibility.


I did not understand the reply below in bold:



the 150 at the end throws me?

Ruled all out except the valve, but don't understand why shutting it off and restarting it would stop it from missing. No idea why the reply was bold, I'm on a tablet and it has a mind of its own sometimes.

 

[Ecomike]

Ruled all out except the valve, but don't understand why shutting it off and restarting it would stop it from missing. No idea why the reply was bold, I'm on a tablet and it has a mind of its own sometimes.

LOL, I added the bold, not you.

The PCM computer starts fresh after you power down and restart. It tries to use cyl #4, until it detects too many missfires (I forget the number, something 200 out of 600).

Once it detects too many missfires, the PCM turns off injector 4 until you restart the engine, to avoid damaging the Cat converters, but it makes a noticable engine miss from that pint on. The engine PCM detects miss fires you can not feel or hear, and once it trows the code, it shuts down that injector, then you feel the miss fire, a total lack of fuel. It is an OBD-II logic thing!

 

[Dcoy1283]

LOL, I added the bold, not you.

The PCM computer starts fresh after you power down and restart. It tries to use cyl #4, until it detects too many missfires (I forget the number, something 200 out of 600).

Once it detects too many missfires, the PCM turns off injector 4 until you restart the engine, to avoid damaging the Cat converters, but it makes a noticable engine miss from that pint on. The engine PCM detects miss fires you can not feel or hear, and once it trows the code, it shuts down that injector, then you feel the miss fire, a total lack of fuel. It is an OBD-II logic thing!

I see, in ever could understand that till now. I think its time to pull the valve cover and check things out, but can't figure out that if I have a bad valve or spring why does it only happen when the engine is cold, never when its hot. That's why I was always leaning towards some faulty sensor, and nothing mechanical.

 

[Dcoy1283]

I did a compression test on all cylinders and they all ranged from 140 to 155 psi, don't rememder which was which except that #4 was 150 psi

 

[Ecomike]

I see, in ever could understand that till now. I think its time to pull the valve cover and check things out, but can't figure out that if I have a bad valve or spring why does it only happen when the engine is cold, never when its hot. That's why I was always leaning towards some faulty sensor, and nothing mechanical.

Give serious consideration to a poor spark at just #4, or erratic fuel injection, clogged or sticky when cold fuel injector!!! Swap the spark plug, then the fuel injector, to see if the problem moves to another cylinder!

 

[Dcoy1283]

Give serious consideration to a poor spark at just #4, or erratic fuel injection, clogged or sticky when cold fuel injector!!! Swap the spark plug, then the fuel injector, to see if the problem moves to another cylinder!

did both and the problem stayed at # 4

 

[Ecomike]

That only leaves the spark coil /or wire to # 4(coil pack ??? in 2000?), or the wire to fuel injector, poor contact when cold. Except for the good compression, it sounds more like a bad head, #4....

Any sign of coolant issues?

 

[Dcoy1283]

Well I have a coil rail and its been replaced and I have ohmed out the injector wires and all was good. Coolant and oil both are clean. I did talk to a guy at work today about it and he has a "live data" scanner that he's bring in tomorrow for me to use, so I should get a better idea on what's going on under the hood.