Strange misfire>>>> please help!!

[Dcoy1283]

That sounds like bad grounds, and a wire shorting out?

We need some late year, 2000, O2 sensor wiring troubleshooting experts here now!!!!!

Try posting the measurements yuo have on the wires on that link I gave you below, to the long O2 sensor testing thread!!!! Be sure to post the year, etc!!!

Will do thanks. I wonder where they are grounded, because yesterday I took off all the grounds in the engine bay and cleaned up the conections, took paint off of the mating surface and put some EJC on them that we use at work on all of our electrical conections.

 

[Ecomike]

Some of the grounding is at and through the PCM.

 

[GHFCWheeler]

Just went through this on my 2001 in the last 30 days. Same issues. Evap leak, cylinder 3 misfire consistently and sometimes cylinder 4. Changed out a big list of things with no help. Finally had a leakdown test done that showed a head gasket leak between cylinders 2 & 3. Pulled the head off to change the gasket and have the head & valves done. When checked they discovered the head was cracked. If you've been fighting this for a while it would be worth doing a cylinder leakdown test just to eliminate a gasket or head problem. I replaced the head on mine and now have a trouble free Jeep with no mystery misfire issues, no evap leak, no problems!

 

[Dcoy1283]

So I think things are only going to get worse. Since this problem has started all I had to do is shut it off and restart it and it would be fine. Now living here in the wonderful state of PA in was 55 degrees the other morning and it took shutting it off 3 or 4 times before it would run good. So I can only imagine how its going to run when it drops down to 0 in a few months

 

[Dcoy1283]

Some of the grounding is at and through the PCM.

You have any ideas for me?

 

[Sidewinder CC]

The O2S heater elements are grounded at G101. G101 is located on the right side of the engine, near the alternator. G101 is/are two studs protruding from the engine block (this is where the earlier XJ ignition coil was mounted). This is where the O2S heater grounds are, as well as a battery ground cable, the PCM ground, TC Switch ground, and TCM ground.

Only the O2S heater elements are grounded here. Other O2S ground functions are in the PCM. As noted, the PCM is grounded at G101.

 

[Ecomike]

You have any ideas for me?

Mine are all 85-89 vintage, so I won't be much help, but some vehicles fire the injector by grounding the injector at the PCM through a solid state device in the PCM. Others send a voltage to the injector.

I had an idle miss for 6 years on my 87 that baffled me, and one day I stumbled onto the cause, a loose connector contact at the injector. Pull one way the miss stopped, push the other way the miss came back.

I have had new plug wires, new spark plugs, bad head gasket, leak, cracked head, bad injectors, cap, rotor, oil in cap from a leaking dizzy O'ring......and a whole string of things cause an engine miss.

 

[Dcoy1283]

Ok everyone, answer this............ I have made 3 trips that would normally make the jeep misfire, but I have been running my ac and it has not missed. I even ran it in the morning with 55 degree temps and the CEL flashed twice then stopped and it didnt act up. What would be different?

 

[Ecomike]

Vibration, physical movement of wires, harness, a short that comes and goes with movement.

A flashing CEL is very BAD, when it is flashing!!!!

 

[Dcoy1283]

Well everyone I figured out my misfire problem.........I replaced the valve springs and it has been running great ever since, so my guess is that one or both of them where weak and not sealing the valve good and tight.

 

[JohnX]

Thanks for the update. I was curious to see how it turned out.