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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: greenmonster on July 19, 2010, 04:52:11 pm
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I had my plugs out today and one of them was carbon fouled.What whould cause that?
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oil through the valve seals or
oil through the piston rings or rippled cylinder walls
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It also misfires somtimes, even after a tuneup.So I know it has something to do with that cylinder but compression test was fine.
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does it misfire after you clean the deposits off?
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no misfires do not clean spark plugs. another thing you might check is if your timing is off just a little bit. if your rig is carbed, you might be dumping too much fuel into the cylinders and causing to foul up the plugs. so check your tune on the carborator.
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SgtDel is asking if it misfired after cleaning the plug...not if the misfire cleaned the plugs. He's trying to find out if the misfire was just from that fouled plug (I think). Here's a chart that might help you nail it down a little more closely:http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html (http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html). If its just one plug I don't think it has anything to do with the carb. or timing. Its something in that one cylinder like the things SgtDel mentioned or possibly a bad spark plug wire if its more of a dry soot than an oil buildup. Do you have blue/gray smoke in your exhaust at any point?
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The plug looks like #3 in the chart.I put a tuneup on and even tried a different distributor.It still misfires sometimes.Could a sticky or burnt valve cause that.
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Its hard to nail this kind of thing down sometimes. Your misfire may be unrelated, maybe not. Does your truck puff out a little smoke when you start it? What about under load or when you get on the gas sitting still? Are your plug wires (particularly that one) in good shape? If it really is carbon soot and not oil then I would say your likely not getting good spark to that plug. You might try the ol' rpm drop test. Start the engine and pull that plug wire off while its running and note the rpm drop. Pull one of the others and compare the rpm drop. If it doesn't drop much when you pull that wire then that cylinder's not doing its share.