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Generation III Internal Engine
1997-2006 LS1 | LS6
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Old 04-30-2012, 04:23 PM   #1
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Question Spark plugs causing bad gas mileage?

I used to get approximately 22mpg highway in my '00 LS1 Z28 but my friend and I changed my spark plugs to AC delco plugs I think and we gapped them to 45 and it made my gas mileage 14-15 so we took them back out and gapped them to 55 and I now get 16 mpg highway...am I gapping my spark plugs wrong or are AC delco plugs not good or what? If they arent very good plugs what should I get instead? NGK's or Bosch maybe?
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Old 04-30-2012, 06:01 PM   #2
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How reliable is your measurement of gas mileage?

The difference in spark plugs or gaps should make little difference in gas mileage, particularly when new. The gap specifcation published by GM and most plug manufacturers is 0.040 inch. The gap can be as high 0.060. The smaller gap makes the plug easier to fire and therefore, reduces the likelihood of misfires, especially later in the plug's life. The larger gap provdies a fatter spark, but is more prone to misfiring.

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Old 04-30-2012, 06:42 PM   #3
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Yeah gm recommends only a .030" gap for the gen 3/4. These motor are really fickle about plugs and the gaps you put on them.
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Old 05-04-2012, 08:54 AM   #4
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I have an 01 ss completey stock with 138,000 miles on it. I dont recall my mileage before but i just got a tune from East Coast Superchargers, and now im onoy getting 15 ,iles to a gallon. Something has got to be wrong? Hubby says plugs need to be changed but i dont really think this is everything to do with it.....any other sugestions?
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Old 05-04-2012, 10:07 AM   #5
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Originally Posted by my1st01ss View Post
I have an 01 ss completey stock with 138,000 miles on it. I dont recall my mileage before but i just got a tune from East Coast Superchargers, and now im onoy getting 15 ,iles to a gallon. Something has got to be wrong? Hubby says plugs need to be changed but i dont really think this is everything to do with it.....any other sugestions?
If the plugs have over 50k miles on them, change them out.
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Old 05-05-2012, 08:07 AM   #6
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are the plugs the correct heat range for your engine?
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Old 05-05-2012, 08:22 AM   #7
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Quote:
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Yeah gm recommends only a .030" gap for the gen 3/4. These motor are really fickle about plugs and the gaps you put on them.
Where are you getting the 0.030" gap? - all my sources for both GM and most spark plug manufacturers is 0.040".
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Old 05-05-2012, 10:55 PM   #8
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Guys I wish I could tell you, but I really don't know about any of that. I had some one put them in for me today so I guess we will see. Any other suggestions for poor gas mileage?
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Old 05-06-2012, 01:45 AM   #9
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Are you using 93 octane?
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Old 05-07-2012, 08:05 AM   #10
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My experience is always go for OEM items in this case. You can't go wrong because otherwise the manufacturer would of made them differently when designing the vehicle. Are you throwing any CEL's? Maybe something happened to the wires when you were changing out the plugs? Do you still have the old plugs? If you do it couldn't hurt to throw them back in and see what happens. At least you then could rule out if it is actually your plugs or not.
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Old 05-07-2012, 02:26 PM   #11
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Silver I didnt mean to walk into your thread and try to take over, but i started my own and no one answered me at all. So i apologize for that to you. The plugs were done the old ones looked good he said but y never know i guess. I run Super always and like i said before the tune i was getting better mileage, to me the car isnt shifting properly. When im on the parkway im at 2000 rpm at 75mph i mean is that normal i dont think so but i dont know either.
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Old 05-07-2012, 03:44 PM   #12
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Check tire pressure. Fuel filter air filter clean MAF. When was last time wires where changed I would check all these things
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Old 05-07-2012, 03:44 PM
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