Cylinder misfire

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SKEETER45

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Jan 10, 2021
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Nevada
Need some help. I have a misfire on #5 cylinder of my 88 4.2 . I replaced that plug (plug did look different than the rest) with another one and problem did not move. I then moved wires and no change either. Ran a compression test and all checked good . Replaced cap and rotor no change. I started it and can hear the spark jumping to the plug. Any thoughts? Thanks

Added. Took for a test drive. Ran great. Only will I slowed to idle was any sign of misfire. Also noticed when I shut it off something keeps making a noise under valve cover.
 
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SKEETER45 said:
Need some help. I have a misfire on #5 cylinder of my 88 4.2 . I replaced that plug (plug did look different than the rest) with another one and problem did not move. I then moved wires and no change either. Ran a compression test and all checked good . Replaced cap and rotor no change. I started it and can hear the spark jumping to the plug. Any thoughts? Thanks

Added. Took for a test drive. Ran great. Only will I slowed to idle was any sign of misfire. Also noticed when I shut it off something keeps making a noise under valve cover.
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What noise do you hear under the valve cover? Liquid/oil/coolant dripping or hissing like compressed air? Ticking?
 
You can hear the spark jumping to the plug? sounds fishy. Pop the hood in a pitch dark place and fire it up. You will see any arching taking place if its going on. Intake gasket leak on 5? try squirting some (pick your favorite) and hit #5 thoroughly and see if the rpm changes indicating a bad intake gasket. If you have good and even compression Im wondering if a bad cam would cause what you have. Guess you could use a dial indicator on the rockers and check for lift on each cyl.
 
Gilaguy23 said:
You can hear the spark jumping to the plug? sounds fishy. Pop the hood in a pitch dark place and fire it up. You will see any arching taking place if its going on. ....
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That's what I was thinking too except OP stated the wires were switched around with no change to the miss.

That being said, starting it in the dark is quick and easy and a good diagnostic test. Definitely do that before peeling the valve cover off.

I pic or description of the #5 plug would also be helpful.
 
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