MISSFIRE ON ONE CYLINDER ?

[cudaneil]

     Gave my car a run out today and it developed a missfire .   pulled off each plug wire one at a time and found it was #8 cylinder.

      Changed both the plug and wire and the missfire still remains      Removed the plug again, checked compression on cylinder -ok'

     checked the new plug was actually sparking- yes !        My distributor is a brown cap,with electronics,  so what next  ?

      Could it be the rotor arm or cap................or something more serious          


                                                                    Cudaneil.

[6packCuda]

How did the old plug that came out of that hole look?

[cudaneil]

How did the old plug that came out of that hole look?

             Just a little black with soot...............unburnt fuel ?



               There is a spark on that cylinder, but it seems a little weak !         My friend just sent me a text telling me to check if there is any wear in the distributor shaft,

               allowing side movement (away from the post in the distributor cap)    Hope not,that would mean a new distributor is required.

[6packCuda]

Boy, I don't know. Seems to me, if you've got spark, fuel, and compression there, it should be firing. Unless the spark isn't getting there at the right time. Check for cracks in the cap.

[cudaneil]

   Yes Dave,  a crack in the cap would cause the spark to be delivered randomly wouldnt it.       Good idea !


    Think i will put a new cap on anyway as it looks fairly old to me.     

[6packCuda]

The only other thing I can think of is a flat cam lobe. I hope that's not it. You could pull that valve cover off, crank the engine over, and see if the valves on that cylinder are moving as far as the rest.

[cudaneil]

The only other thing I can think of is a flat cam lobe. I hope that's not it. You could pull that valve cover off, crank the engine over, and see if the valves on that cylinder are moving as far as the rest.

   I hope its not that either Dave,  the motor was only rebuilt with new cams 2 years ago.   Wouldnt the cam be noisy(tapping) if that was the case ?       

[6packCuda]

Not always.

[cudaneil]

Not always.

    Ok thanks Dave,  I will go and do some poking around.........see what i can find !             Catch you later     

[moparmaniac59]

Also maybe check to see if the wire is arcing on the block or headers. I had that problem with a split boot. It looked OK from a distance until I pulled it and saw the crack. I know you said you replaced the wire, so it really shouldn't arc or ground out, but just a thought.


                                                       Matt B.

[Chryco Psycho]

so you have power brakes I assume , my guess is a vacuum leak into the booster causing the lean mix on # 8 cylinder try removing the hose & capping the nipple & see if the miss disappears

[cudaneil]

    Wow, thats a new one on me Chryco......................ok thanks, will check that one out also.


       Not just now though, we have just had about 10" of snow today  !!!!!         

[carguybradd]

so you have power brakes I assume , my guess is a vacuum leak into the booster causing the lean mix on # 8 cylinder try removing the hose & capping the nipple & see if the miss disappears

Wow! That is one incredible diagnosis! 

[Chryco Psycho]

I have seen it 4-5 times now

[72 cuda]

This may sound a little crazy but.I was having a similar problem on 96 chevy truck.
I asked my father (retired master mechanic) who said the insulation on the wires was old and breaking down.
I said bs, he said take a spray bottle whith a fine mist ,wait till it was dark and fog the wires.
I was shocked, it lit up like a christmas tree.
new wires no further problem.
Mark
ps don't you hate, more age and experiance.