Dershum Posted October 11, 2011 Share Posted October 11, 2011 I'll be the first to admit I'm a newbie on reading plugs, but this one's got me. Take a look at the right-hand plug...I'm going to go with way too lean, but why would the brown "crud" move that far down the plug? Quote Link to comment Share on other sites More sharing options...
Dillon Posted October 12, 2011 Share Posted October 12, 2011 well i believe the spark plugs stick out of the hole sorta speck.. so your gonna have carbon and stuff on that first little tread on the spark plug. and it looks like you could be running lean or/and over heating. Quote Link to comment Share on other sites More sharing options...
beermanpete Posted October 12, 2011 Share Posted October 12, 2011 The "brown crud" is rust. It looks like there was rust on the outside of spark plug (likely due to water puddling while cleaning the engine) before you removed it from the engine. The rust got on the tip and threads because you did not clean the area around the plugs prior to removal and you dragged the tip of the plug through the rust as you removed it. You are right about being lean. The right plug looks lean compared to the left plug. This is unrelated to the rust issue. Quote Link to comment Share on other sites More sharing options...
Dershum Posted October 12, 2011 Author Share Posted October 12, 2011 The "brown crud" is rust. It looks like there was rust on the outside of spark plug (likely due to water puddling while cleaning the engine) before you removed it from the engine. The rust got on the tip and threads because you did not clean the area around the plugs prior to removal and you dragged the tip of the plug through the rust as you removed it. You are right about being lean. The right plug looks lean compared to the left plug. This is unrelated to the rust issue. I kind of thought the shaft/nut area was aluminum or something else that wouldn't rust...that's why I didn't really think rust to begin with. Thanks for the diagnosis! That last cylinder (it's the plug from #6) is is a little strange. I (finally) did a compression test last night, and while cylinders 1, 2, 4, and 5 are reading around 120 psi (except 3 which is at 100...I'm guessing new rings are in order), that one is at 130 psi. I'm guessing that the lean condition is related to the fact that cylinder is performing "better" in relation to the overall air/fuel mix in the other two cylinders, but I've got the mixture fully enriched on that carb. I know the carbs need a rebuild, but I'm also wondering if there's something I might be able to do in the mean time (short of a rebuild) to get a little more fuel flowing through that carb. Quote Link to comment Share on other sites More sharing options...
beermanpete Posted October 13, 2011 Share Posted October 13, 2011 I kind of thought the shaft/nut area was aluminum or something else that wouldn't rust...that's why I didn't really think rust to begin with. Thanks for the diagnosis! That last cylinder (it's the plug from #6) is is a little strange. I (finally) did a compression test last night, and while cylinders 1, 2, 4, and 5 are reading around 120 psi (except 3 which is at 100...I'm guessing new rings are in order), that one is at 130 psi. I'm guessing that the lean condition is related to the fact that cylinder is performing "better" in relation to the overall air/fuel mix in the other two cylinders, but I've got the mixture fully enriched on that carb. I know the carbs need a rebuild, but I'm also wondering if there's something I might be able to do in the mean time (short of a rebuild) to get a little more fuel flowing through that carb. If you have a turbo model your compression is about right. For a normally aspirated model you should be getting around 170 PSI. The service manual for your particular model will provide the correct PSI specification. The variation from cylinder to cylinder indicates a problem, but not necessarily rings. You should check the valve clearance as a tight valve will give a low compression reading as well as other poor run symptoms. A cylinder leakage test should be done. This will tell you whether the rings, valves, or something else is the cause of the low compression. If you have weak rings or valves you will not be able to set the fuel/air mixture reliably due to those problems affecting the plug color and other indicators of the mixture. Quote Link to comment Share on other sites More sharing options...
Dershum Posted October 13, 2011 Author Share Posted October 13, 2011 If you have a turbo model your compression is about right. For a normally aspirated model you should be getting around 170 PSI. The service manual for your particular model will provide the correct PSI specification. The variation from cylinder to cylinder indicates a problem, but not necessarily rings. You should check the valve clearance as a tight valve will give a low compression reading as well as other poor run symptoms. A cylinder leakage test should be done. This will tell you whether the rings, valves, or something else is the cause of the low compression. If you have weak rings or valves you will not be able to set the fuel/air mixture reliably due to those problems affecting the plug color and other indicators of the mixture. Not the turbo, it's 71 240z...it's also not the original motor or at least the ID number on the block doesn't match the VIN, but it's an L24. I also can't vouch 100% for the compression gauge (I got it from harbor freight on sale), but at least it tells me that something is going on with cylinder #3 over and above the other five. I'm definitely going to pull the head and inspect everything, and at minimum replace the head gasket. We'll see how it goes from there I guess. Quote Link to comment Share on other sites More sharing options...
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